The Illumination Source: The Light That Makes Reading Possible |
Executive Summary |
This article provides a comprehensive exploration of the illumination subsystem in barcode readers---the critical first active element in the signal chain that projects light onto the target barcode and enables the photodetector to capture reflected light. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Symbol Technologies, Texas Instruments, Microchip Technology, Quarton, and others. We examine how these companies have solved practical challenges like constant current drive, pulsed versus continuous operation, automatic power control for lasers, power efficiency, thermal management, and multi-mode illumination strategies. The article covers both LED-based readers and laser-based scanners, with special attention to the practical trade-offs between cost, performance, reliability, and power consumption. Throughout, we emphasize that the illumination source is not just a simple lamp---it is a carefully engineered subsystem whose design decisions affect every subsequent stage of the signal chain. The closing summary synthesizes the key lessons and offers practical guidance for designers. |
|
Chapter 1: The Illumination Source - More Than Just a Light Bulb |
The illumination source in a barcode reader is the first active element in the signal chain, but calling it a 'light bulb' would be a grave understatement. The LED or laser diode at the heart of the reader is a carefully engineered component that must produce enough light to be reflected back to the photodetector while maintaining stability, efficiency, and safety. Its design and drive circuitry directly affect the reader's reading range, decoding accuracy, power consumption, and reliability. |
At its most basic level, the illumination source provides the light that reflects off the barcode and returns to the photodetector. The reflected light carries the information about the barcode's pattern of bars and spaces. But the quality of that reflected signal depends entirely on the quality of the illumination. If the light is too dim, the signal will be lost in noise. If the light is unstable, the signal will fluctuate. If the light is poorly matched to the photodetector's spectral response, much of the illumination will be wasted. |
The requirements for illumination are demanding and sometimes contradictory. The reader must illuminate barcodes at distances ranging from a few centimeters to several meters. It must handle barcodes with high contrast and those with low contrast. It must work in bright sunlight and in dark warehouses. It must be efficient enough to run on batteries for a full work shift. And it must be safe for human eyes. |
This chapter begins our journey through the design of the illumination subsystem, examining how engineers at companies like Symbol Technologies, Texas Instruments, and Microchip have addressed these challenges through ingenious circuit designs and careful component selection. |
|
Chapter 2: The Light Emitting Diode - The Workhorse of Barcode Reading |
The light emitting diode, or LED, is by far the most common illumination source in barcode readers. LEDs offer several advantages: they are inexpensive, reliable, efficient, and available in a wide range of wavelengths. They can be pulsed at high currents for short durations to provide intense illumination, or they can be operated continuously at lower currents. |
In imaging-based barcode readers, LEDs are typically arranged in an array around the camera module. This arrangement provides uniform illumination across the field of view, ensuring that the entire barcode is evenly lit. The number and placement of LEDs are determined by the required illumination pattern and the reading distance. |
The operating wavelength of the LED is an important consideration. Red LEDs, with wavelengths around 630 to 660 nanometers, are common because they match the peak sensitivity of many silicon photodiodes. However, some readers use white LEDs or other colors to improve contrast on different colored barcodes. |
As described in a patent from Symbol Technologies, the illumination provided by LEDs is often required to maximize the reading range of the scanner . The illumination can be either high intensity pulses of short duration, or continuous illumination at a much lower intensity . This flexibility allows the reader to adapt to different operating conditions. |
|
Chapter 3: The Laser Diode - When Precision is Required |
For applications requiring longer reading distances or higher resolution, laser diodes are often preferred over LEDs. A laser produces a coherent, collimated beam that can be focused to a very small spot. This allows the reader to read barcodes from greater distances and with finer detail. |
Laser diodes, however, are more complex to drive than LEDs. The laser's output power must be carefully regulated, and the diode must be protected from electrostatic discharge and overcurrent. Most laser readers include an automatic power control, or APC, circuit. |
The principle of APC is simple: a monitor photodiode inside the laser package senses a portion of the laser's output light and provides feedback to a control loop. This loop adjusts the drive current to keep the output power constant, regardless of temperature or aging . |
Texas Instruments has published reference designs for implementing APC using their programmable DAC devices. The TPL1401, for example, provides an 8-bit programmable DC bias and integrates a precision internal reference. This device can be used to program a laser diode APC circuit, and typical applications include barcode scanners and laser distance measurements . |
Quarton inc. offers laser modules with integrated APC driver circuits, such as their green cross-line laser modules. These modules, used in consumer-grade barcode readers and other applications, include the laser diode, collimating lens, and APC driver circuit in a compact package. The APC driver circuit enables the laser power output to remain safe and constant . |
|
Chapter 4: Constant Current Drive - The Foundation of Stable Illumination |
The most fundamental requirement for any illumination source is that its light output remains stable. In LED-based systems, light output is directly proportional to the forward current through the diode. Therefore, driving the LED with a constant current is essential for stable illumination. |
A constant-current source is a circuit that delivers a predetermined current regardless of the load's voltage drop. This is in contrast to a constant-voltage source, which delivers a fixed voltage and allows the current to vary with the load. |
The reason constant-current drive is preferred over constant-voltage drive is simple: the forward voltage drop of an LED varies with temperature and from device to device. If you apply a fixed voltage, the current will fluctuate as the LED warms up or as manufacturing variations cause differences in forward voltage. A constant-current source, on the other hand, forces a precise current through the LED, ensuring consistent light output. |
A simple constant-current driver can be built with a transistor and a resistor, but many commercial designs use dedicated LED driver ICs that provide features like overcurrent protection and thermal shutdown. |
Microchip Technology has published extensive documentation on LED driver circuits. Their MCP1650 Multiple White LED Demo Board, for example, uses a PIC10F206 microcontroller to provide user button interface and generate the PWM control signal for the switch-mode power supply IC . This approach allows the LED current to be precisely controlled while the microcontroller handles other tasks. |
|
Chapter 5: The Buck Regulator - Efficient Power for Pulse Mode |
One of the most efficient methods of controlling the high currents required for pulsed LED illumination is a switching Buck regulator. A Buck regulator is a DC-to-DC down converter that reduces the input voltage to a lower output voltage with high efficiency. |
As described in the Symbol Technologies patent, a Buck regulator runs at a fixed frequency and controls the output power by adjusting the duty cycle of its switching waveform . This type of regulator is particularly efficient for pulse illumination because it can deliver high currents without dissipating excessive power as heat. |
However, Buck regulators have a limitation: when the current requirement drops below a specific threshold, the duty cycle becomes too small for the regulator to operate in continuous mode. It then switches to discontinuous mode, such as pulse skipping or burst mode. In these modes, the switching frequency changes randomly, which can create power supply noise that interferes with the sensitive imaging electronics . |
The Symbol Technologies patent solves this problem by using a combination of a Buck regulator and a linear regulator. When the driving current is above a threshold value, the Buck regulator provides the current. When the current is below the threshold, the linear regulator takes over . This hybrid approach ensures that the regulator always operates at a fixed frequency, minimizing noise and improving the performance of the reader. |
|
Chapter 6: Torch Mode and Flash Mode - Two Ways to Illuminate |
Barcode readers often operate in two distinct illumination modes: flash mode and torch mode. Flash mode provides a brief, high-intensity pulse of light for capturing a single image. Torch mode provides a lower-level, continuous illumination for aiming and for reading barcodes in challenging conditions. |
The patent from Symbol Technologies describes this dual-mode operation in detail. A high level discharge current is followed by a low level current . This pattern can be repeated after a short re-charge time. |
The drive circuit uses an energy storage capacitor to provide the high current pulse for flash mode. The capacitor is charged by a DC-to-DC boost circuit to a voltage of 10 to 16 volts. When the LED is triggered, the capacitor discharges through the LED, producing a brief, intense flash. After the flash, the circuit can enter torch mode, where the boost circuit maintains a lower current through the LED . |
This approach has several advantages. The energy storage capacitor allows the LED to be driven at currents far higher than the battery could sustain continuously. The pulsed operation reduces average power consumption, which is crucial for battery-powered operation. And the torch mode provides a steady illumination for aiming and for low-light operation. |
|
Chapter 7: The Capacitor-Based Flash Driver - A Practical Example |
The capacitor-based flash driver is a classic design for handheld barcode readers. It provides a high-current pulse to the LEDs without requiring the battery to deliver that current continuously. |
In this design, the drive circuit includes a DC-to-DC boost circuit, a capacitor, and a programmable current source. The boost circuit charges the capacitor to a high voltage. When the reader is triggered, the capacitor discharges through the LEDs, producing the flash. |
The programmable current source allows the controller to adjust the discharge current, providing flexibility for different operating conditions. As described in the patent, the controller can adjust the discharge current in response to a signal, allowing the reader to adapt to the specific requirements of the barcode being scanned . |
The drive circuit can also maintain a low-level boost voltage across the LED after the flash, entering torch mode. In this mode, the controller adjusts the DC voltage at the input to the boost circuit to maintain a specified current at the output . |
This design is efficient and flexible, but it does require careful management of the charging and discharging cycles. The controller monitors the capacitor voltage to determine when charging is completed and when the flash event has ended, allowing it to maximize the number of flash events in a given period . |
|
Chapter 8: Programmable Current Source - Adapting the Illumination |
The programmable current source is a key element of the capacitor-based flash driver. It allows the controller to adjust the LED current on the fly, adapting to different operating conditions. |
One suitable programmable current source is described in the patent: the circuit is coupled to a digital-to-analog converter that forms part of the control circuit and receives control signals from the host microprocessor or controller . The analog output from the DAC provides an analog input to an amplifier, which drives a transistor that controls the current through the LED. |
The current is determined by the voltage from the DAC divided by a resistance. As long as a minimum voltage headroom is maintained across the programmable current source, the current is well-controlled . This allows the controller to set the LED current to any value within the operating range. |
The programmable current source is also used to control the output current of the boost converter. The boost circuit has a control input coupled to the controller, and the controller sends signals that control the output voltage as well as the peak input current that the boost converter will draw from the host system power supply . |
This level of control allows the reader to optimize its power consumption for different operating modes. In flash mode, the current is set high for maximum illumination. In torch mode, the current is set lower to conserve power. |
|
Chapter 9: Microcontroller-Based LED Control - Adding Intelligence |
Adding a microcontroller to the LED drive circuit brings a new level of intelligence and flexibility. The microcontroller can manage the user interface, communication, battery status monitoring, and temperature measurement, all while controlling the LED current. |
Microchip Technology has been a leader in integrating microcontrollers with LED drive functions. Their PIC12F675, for example, includes an on-chip comparator that can be used to implement a switch-mode LED driver . This integration eliminates the need for a separate controller IC, reducing the component count and simplifying the design. |
The PIC16F785 takes integration even further. This 20-pin device integrates analog peripherals for switch-mode power supply applications and an 8-bit MCU. It has two on-chip op-amps, two on-chip comparators, two analog PWM modules, and an adjustable voltage reference . These peripherals can be digitally configured to implement a wide variety of SMPS circuit topologies. Once configured, the analog control circuitry can run independently of the MCU, freeing the MCU for other tasks such as communications and status monitoring . |
This level of integration is particularly valuable in handheld readers where space and power are at a premium. A single chip can handle both the LED drive and the overall system management. |
|
Chapter 10: The MIC2298 - A Practical Boost LED Driver Example |
The MIC2298 from Microchip Technology is a practical example of a dedicated LED driver IC designed for boost converter applications. This chip is suitable for driving LEDs in applications requiring high current, such as barcode readers. |
The MIC2298 evaluation board is designed to provide 500 mA nominal current into two series-connected power LEDs. The board includes jumper settings for different current levels: a 50% position for 500 mA, a position for 1A (flash), and a 20% position for 200 mA (torch) . |
The MIC2298 supports multiple dimming methods. The LED current can be set by varying the DIM voltage, the BRT voltage, or the BRT resistance. It can also be controlled by PWM signals on the enable pin or the BRT pin, allowing the microcontroller to adjust the brightness by varying the duty cycle . |
The evaluation board includes plots of the LED current versus the dimming control parameters, providing designers with practical data for implementing their own designs. The board is designed for high efficiency, with peak efficiency exceeding 87% at full load . |
This example demonstrates how a dedicated LED driver IC can simplify the design of the illumination subsystem while providing the flexibility needed for different operating modes. |
|
Chapter 11: Thermal Management - Keeping the LEDs Cool |
LEDs, especially when driven at high currents, generate significant heat. This heat must be managed to prevent damage to the LEDs and to maintain their light output stability. |
As the temperature of an LED increases, its forward voltage decreases, which can cause the current to increase if the drive circuit is not well-regulated. This creates a positive feedback loop: higher temperature leads to higher current, which leads to even higher temperature. Without proper thermal management, the LED can enter thermal runaway and fail. |
The solution is to design the drive circuit so that the current is well-regulated regardless of temperature. A constant-current source provides this regulation. Additionally, the PCB should be designed to conduct heat away from the LEDs, and the housing should allow for air circulation. |
Some LED driver ICs include thermal shutdown protection. The TLC6C598-Q1 and TLC6C5912-Q1 from Texas Instruments, for example, include a thermal shutdown function that enhances system reliability . When the junction temperature exceeds a threshold, the device shuts down to prevent damage. |
The MIC2298 also includes overvoltage protection. If the load is not present, the overvoltage protection limits the output voltage to prevent damage to the IC . |
|
Chapter 12: Bright Field and Dark Field Illumination - Two Optical Strategies |
The optical design of the illumination system is just as important as the electrical design. Different lighting angles and distributions can affect the quality of the image and the reader's ability to decode certain types of barcodes. |
Bright field illumination is the conventional approach. The light source is positioned close to the optical axis, and the light reflects directly from the barcode back to the camera. This approach works well for barcodes that are relatively flat and printed with high contrast. |
Dark field illumination is an alternative approach. The light source is positioned at a low angle relative to the surface, so the light reflects at a grazing angle. This approach enhances the contrast of surface features, such as raised or indented markings, and reduces the effect of specular reflections from glossy surfaces. |
A patent from 2017 describes a barcode reader with multiple illumination systems: a bright field illumination system and a dark field illumination system . The bright field illumination system is designed to illuminate a target area located relatively far away from the reader, while the dark field illumination system is designed to illuminate a target area located relatively close to the reader . |
The patent describes how the dark field illumination may be optimal for reading a barcode that is located within a close zone of the field of view, which may begin at the face of the reader and extend outward . The dark field illumination is directed into the field of view at a non-zero angle relative to the plane perpendicular to the optical axis, reducing the amount of light that reflects directly back into the camera. |
This multi-mode illumination strategy allows the reader to adapt to different barcode types and distances, improving the reading performance across a wide range of conditions. |
|
Chapter 13: Diffuse Illumination - Smoothing the Light |
The quality of the illumination is not just about intensity; it is also about uniformity and diffusion. Diffuse illumination reduces shadows and hotspots, producing a more evenly lit image that is easier to decode. |
A barcode reader patent describes an optical substrate through which illumination is projected and extracted as diffuse illumination . The optical substrate may have extraction features and reflective coatings that cause the illumination to propagate between the front major surface and the back major surface and be extracted through the front major surface. |
This approach creates a uniform, diffuse light source that illuminates the barcode evenly. The diffusion characteristics can be tailored to the specific application, such as by changing the intensity, wavelength, or angle of the illumination . |
The patent also describes how the diffuse bright field illumination may be generated by white LEDs, while the tertiary light source (dark field) and the secondary light source may be red LEDs . By using different wavelengths, the reader can adapt to different barcode colors and surfaces. |
|
Chapter 14: Multiple Illumination Systems - Adapting to the Environment |
A reader with multiple illumination systems can choose the best illumination for any given situation. The illumination selection circuitry captures test images with different illumination configurations and then analyzes the results to select the most suitable illumination. |
The barcode reader patent describes a method where the photo sensor array captures at least one test image. The illumination selection circuitry then evaluates the test image to determine which illumination system provides the best performance . |
The multiple illumination systems can be operated independently, allowing the reader to use the illumination system that provides the best illumination for reading the barcode. This flexibility is particularly valuable when reading barcodes that have been damaged, are printed on unusual surfaces, or are located at varying distances . |
The patent also describes how the photo sensor array can be operated in global shutter mode or rolling shutter mode. In global shutter mode, all photo sensors within the array are exposed at the same time. In rolling shutter mode, the rows are exposed sequentially. This flexibility allows the reader to adapt the exposure to the specific requirements of the scan . |
|
Chapter 15: Addressing Power Supply Noise - A Hidden Challenge |
Power supply noise is a hidden challenge in barcode reader design. The switching of the Buck regulator, the pulsing of the LEDs, and the operation of the digital circuits all create noise that can interfere with the sensitive imaging electronics. |
The Symbol Technologies patent describes this challenge in detail. When a Buck regulator operates in discontinuous mode, the regulator frequency changes randomly, which can introduce power supply noise that corrupts the image . The power supply is tuned to operate at a frequency that minimizes interference with the rest of the system. If the power supply does not run at a fixed frequency, the system becomes more susceptible to power supply noise . |
The solution, as described in the patent, is the hybrid driver circuit that uses the Buck regulator when the current is high and the linear regulator when the current is low. This ensures that the regulator always runs at a fixed frequency, minimizing noise . |
The patent also notes that sensitive imaging electronics are susceptible to corruption from power supply noise. This is particularly true for imaging-based readers, which capture a full image of the barcode and are sensitive to any variation in the illumination during the exposure . |
|
Chapter 16: The LT1618 Boost Converter - A Practical Component |
The Linear Technology LT1618 is a practical example of a boost converter IC suitable for LED drive applications. This integrated circuit provides constant-current/constant-voltage operation and can deliver the high voltages needed to drive multiple LEDs in series. |
The patent describes the LT1618 as a typical boost circuit for the capacitor-based flash driver. The circuit has a feedback pin that can be used to set the output voltage by selecting values for a resistor network. A second pin is controlled with a DC voltage to control the output current of the voltage step-up converter . |
The controller sends signals to the LT1618 that control the output voltage as well as the peak input current that the boost converter will pull from the host system power supply. This allows the controller to manage the charging of the capacitor and the peak current drawn from the battery, optimizing the power consumption . |
The LT1618 is just one example of the many boost converter ICs available for LED driver applications. Other manufacturers, such as Texas Instruments and Microchip, offer similar devices with various features and specifications. |
|
Chapter 17: Stacked LEDs - Maximizing Efficiency |
To maximize efficiency, some barcode readers connect multiple LEDs in a series stack. When the capacitor discharges, all the LEDs in the stack are activated simultaneously, producing more light for the same current. |
The patent describes this approach as an option for the exemplary system. Stacked LEDs in a series arrangement are all simultaneously activated to produce light when the capacitor is discharged . |
This approach is efficient because the same current flows through all the LEDs in the stack. The voltage across the stack is the sum of the forward voltages of the individual LEDs, so the boost converter must provide a higher voltage. However, the total power is the same as driving the LEDs individually at the same current, and the current from the capacitor is lower. |
Stacked LEDs are particularly useful in applications where space is limited, as they allow multiple LEDs to be driven with a single current path. |
|
Chapter 18: Real-Time Voltage Monitoring - Optimizing Performance |
Real-time monitoring of capacitor voltage allows the reader to optimize its performance. The controller can monitor the voltage on the capacitor to determine when charging is completed and when the capacitor is ready to be discharged in flash mode . |
The patent describes how the controller can feed these voltages back through an analog-to-digital converter. The controller can then evaluate these values in software to determine how much charge is available on the capacitor. The controller is then able to more aggressively charge and discharge to deliver more flash events in a given period, and to avoid unnecessary recharging delays . |
This is particularly useful when a full charge is not needed to expose a partial frame, and where it is determined that the capacitor has enough energy to deliver the required pulse for the job . |
The ability to monitor and respond to real-time changes in capacitor voltage produces a more responsive and aggressive barcode reader. The system does not need to be designed for the worst case, avoiding the conservative approach that would reduce the number of flash events that can be delivered . |
|
Chapter 19: Wavelength Selection - Matching the Light to the Sensor |
The wavelength of the illumination source is an important design consideration. The photodetector has a spectral response, and the illumination is most efficient when it is matched to the peak response of the detector. |
Red LEDs and laser diodes with wavelengths around 630 to 660 nanometers are common in barcode readers because silicon photodiodes have a peak sensitivity in this range. The BPW34 photodiode, for example, has a spectral response that peaks near 850 nanometers but has good sensitivity at 660 nanometers. |
However, some applications benefit from other wavelengths. Green lasers, for example, are used in some readers because green light is more visible to the human eye, making the aiming pattern easier to see. Green lasers also penetrate certain types of packaging better than red lasers. |
Quarton inc. offers green cross-line laser modules with wavelengths around 505 to 530 nanometers. These modules are designed for consumer-grade barcode readers and other applications where a visible green aiming pattern is beneficial . |
|
Chapter 20: Spectrum for Different Surfaces - Adapting to Barcode Color |
Different barcode surfaces have different reflectance characteristics. A barcode printed on white paper reflects a wide spectrum of light, while a barcode printed on a colored background may reflect only certain wavelengths. |
To handle different barcode colors, some readers use multiple illumination sources with different wavelengths. The patent describes how the diffuse bright field illumination may be generated by white LEDs, while the tertiary light source (dark field) and the secondary light source may be red LEDs . |
By using different wavelengths, the reader can adapt to different barcode colors and surfaces. If the barcode is printed on a surface that reflects red light poorly, the reader can use white light instead. If the barcode is printed on a glossy surface that causes specular reflections, the reader can use dark field illumination at a grazing angle. |
This flexibility improves the reader's performance across a wide range of barcode types and surfaces. |
|
Chapter 21: Laser Safety - A Critical Requirement |
Laser safety is a critical requirement for any barcode reader that uses a laser diode. Lasers can cause eye damage if the power is too high or if the beam is improperly directed. |
Laser products must meet international safety standards. The Quarton products, for example, meet FDA Class I, II, IIIa standards for the USA, PSC (JQA) for Japan, and EU60825, CE, and TUV-GS for Europe . |
Class 1 lasers are considered safe under all conditions of normal use. Class 2 lasers are safe for accidental exposure but can cause eye damage if stared at for extended periods. Class 3a lasers are safe for accidental exposure but can cause eye damage if viewed with optical instruments. |
The APC circuit is essential for laser safety. The APC circuit monitors the laser output and adjusts the drive current to keep the power constant. This prevents the laser from exceeding its safe power limit and ensures that the laser remains within its safety classification. |
|
Chapter 22: Automatic Power Control for Lasers - Maintaining Safe Power |
The automatic power control, or APC, circuit is a critical component of any laser-based barcode reader. The APC circuit maintains a constant optical output power, compensating for variations in temperature and aging. |
The principle of APC is simple: a monitor photodiode inside the laser package senses a portion of the laser's output light. This photodiode generates a current that is proportional to the optical power. The APC circuit compares this current to a reference and adjusts the drive current to keep the optical power constant . |
Without APC, the laser output power would vary with temperature. As the laser diode heats up, its efficiency decreases, requiring more drive current to maintain the same optical power. If the drive current is fixed, the optical power will drop as the temperature rises. Conversely, if the laser cools, the optical power can increase, potentially exceeding safe limits. |
The APC circuit ensures that the laser power is constant regardless of temperature. This is essential for safety and for consistent reading performance. |
Texas Instruments has published application notes on implementing APC for laser diodes using their programmable DAC devices. The TPL1401, for example, provides an 8-bit programmable DC bias and can be used to program a laser diode APC circuit . |
|
Chapter 23: Integrated Laser Modules - A Convenient Solution |
For designers who want to simplify their designs, integrated laser modules provide a convenient solution. These modules include the laser diode, collimating lens, and APC driver circuit in a compact package. |
Quarton inc. offers integrated laser modules for barcode readers and other applications. Their green cross-line laser modules, for example, include the laser diode, collimating lens, wavy lens, and APC driver circuit in a brass housing. The APC driver circuit enables the laser power output to be safe and constant . |
These modules are available with different specifications, including different operating voltages, output powers, and line widths. The VLM-520-29 series, for example, is optimized for short-distance applications and provides a green cross-line pattern . |
The modules are designed for high reliability, with a mean time to failure (MTTF) of 10,000 hours at 25C for some models. They also feature a patented solid brass structure for shock resistance and better heat transfer . |
Chapter 24: Low-Cost Laser Modules - For High-Volume Applications |
For high-volume consumer applications, low-cost laser modules are available. These modules are designed for mass production and offer a cost-effective solution for barcode readers and other consumer products. |
According to an industry publication, low-cost laser diode modules are available with integrated APC electronics that allow the laser diode to be operated automatically, giving the desired output power and increasing the life expectancy of the module . These modules can be operated directly with 3 VDC without additional electronics. |
The smallest modules have a diameter of 3.3 mm and a length of 7 mm, making them suitable for compact devices. Typical applications include consumer products such as CD/DVD players, laser printers, and barcode readers . |
These modules are reliable and have more than 10,000 hours MTTF, making them suitable for applications where reliability is important but cost is a primary concern . |
|
Chapter 25: Shift Register LED Drivers - Driving Multiple LEDs |
For readers with multiple LEDs, shift register LED drivers provide an efficient way to control many LEDs with a few microcontroller pins. These drivers shift in serial data and latch the outputs, allowing the microcontroller to control the LEDs with just a few I/O pins. |
Texas Instruments has published application notes on their TLC6C598-Q1 and TLC6C5912-Q1 shift register LED drivers. The TLC6C598-Q1 is an 8-channel LED driver, and the TLC6C5912-Q1 is a 12-channel driver . |
These devices offer several advantages over earlier designs. They have a maximum breakdown drain voltage of 40 V, allowing them to be connected directly to a 12 V car battery. They support a wider VCC operating range from 3 V to 5.5 V, supporting both 3.3 V and 5 V power supply rails. They have an optimized slew rate that improves EMI performance, and they include a thermal shutdown function that enhances system reliability . |
The TLC6C598-Q1 is pin-to-pin compatible with the earlier TPIC6C596, making it easy to upgrade existing designs . |
Chapter 26: Dot Matrix Displays - Using Shift Registers for LED Control |
Shift register LED drivers can be used to create dot matrix displays, which are sometimes used in barcode readers for status indication or user feedback. |
The Texas Instruments application note describes how a single TLC6C5912-Q1 can drive a maximum of 36 LEDs when scanning columns and rows. With an 11-row by 1-column structure, the device can drive 11 LEDs. With a 6-row by 6-column structure, the device can drive 36 LEDs . |
The application note provides a schematic for a 35-LED (7x5) dot matrix driver solution controlled by four GPIOs of the MCU. This is a simple implementation with minimum MCU GPIO resource. The supply voltage can be connected up to 40 V directly . |
The application note also provides timing guidelines for the dot matrix. Normally, a scanning frequency of 200 Hz or above is needed to avoid flickering . |
|
Chapter 27: Cascading LED Drivers - Scaling the Illumination |
For readers with a large number of LEDs, multiple LED driver ICs can be cascaded. The serial data is shifted through the chain, allowing the microcontroller to control all the LEDs with the same three or four I/O pins. |
The Texas Instruments application note describes how multiple TLC6C598-Q1 devices can be cascaded. The serial data output (SER OUT) of one device connects to the serial input (SER IN) of the next. The shift register clock (SRCK) and register clock (RCK) are common to all devices . |
This approach is particularly useful for applications that require a large number of LEDs for illumination. The reader can have LEDs distributed around the camera module or across the field of view, and the cascaded shift register drivers provide efficient control. |
Chapter 28: Fault Indication with LEDs - A Diagnostic Tool |
LEDs can also be used for diagnostic purposes, providing visual indication of system status and faults. |
The Texas Instruments application note describes a real-time fault status indicator using the TLC6C598-Q1. In this application, the shift register receives a serial fault diagnostic byte directly from a FET predriver with diagnostic capability. When a load becomes open, shorted, or when the battery voltage is out of range, the fault terminal transitions low . |
The serial data output of the predriver is connected directly to the serial input of the TLC6C598-Q1. After the fault data is transferred, the microcontroller sends a register clock to the TLC6C598-Q1 to transfer the fault data to the storage register and to the LEDs, providing visual indication of the fault condition . |
This approach provides the user a very quick and easy way to see which load has a problem without the need for additional diagnostic equipment . |
|
Chapter 29: The Illumination Driver Circuit - Hybrid Buck/Linear Approach |
The illumination driver circuit described in the Symbol Technologies patent is a good example of how different design requirements can be combined. The circuit provides a driving current for the illumination source with a Buck regulator when the driving current is above a threshold value and with a linear regulator when the driving current is not above the threshold value . |
This hybrid approach addresses the problem of power supply noise. When the Buck regulator operates in continuous mode, it runs at a fixed frequency, minimizing noise. When the current drops below the threshold, the Buck regulator would otherwise enter discontinuous mode, where the frequency changes randomly. By switching to the linear regulator, the circuit maintains a fixed frequency and reduces noise . |
The threshold current value is substantially close to the smallest current the Buck regulator can provide when operating in continuous mode. This ensures that the Buck regulator always operates in continuous mode when it is active, and the linear regulator handles the low-current operation . |
Chapter 30: The Selection Switch - Choosing the Regulator |
The hybrid driver circuit includes a selection switch that connects the illumination source to either the Buck regulator or the linear regulator. When the switch makes a connection with the Buck regulator, the illumination source is electrically connected between the Buck regulator and the linear regulator. When the switch makes a connection with the linear regulator, one terminal of the illumination source is electrically connected to ground . |
The selection switch allows the circuit to seamlessly transition between the two regulators. When the current requirement is high, the Buck regulator provides the current with high efficiency. When the current requirement is low, the linear regulator provides the current with low noise. |
The selection is controlled by the controller, which monitors the current requirement and switches the regulators as needed. This automatic switching ensures that the illumination subsystem is always operating in the most appropriate mode. |
|
Chapter 31: Buck Regulator Operation - Continuous vs. Discontinuous Modes |
A Buck regulator can operate in continuous mode or discontinuous mode. In continuous mode, the inductor current never falls to zero. In discontinuous mode, the inductor current falls to zero for part of the switching cycle. |
The Symbol Technologies patent explains that when the Buck regulator operates in continuous mode, it runs at a fixed frequency, which minimizes interference with the rest of the system. When it operates in discontinuous mode, the regulator frequency changes randomly, which can introduce power supply noise . |
The threshold current value for switching between the Buck regulator and the linear regulator is set to the smallest current the Buck regulator can provide when operating in continuous mode. This ensures that the Buck regulator always operates in continuous mode when it is active . |
By maintaining continuous mode operation, the power supply runs at a fixed frequency, minimizing noise and improving the performance of the imaging electronics. |
Chapter 32: Pulse Modulation - A Method of Dimming |
Pulse modulation, both PWM and PFM, is a common method of dimming LEDs. By varying the duty cycle of the signal that drives the LED, the average current can be adjusted. |
Microchip's MIC2298 supports dimming by PWM signals on the enable pin or the BRT pin. The LED current is proportional to the duty cycle, allowing the microcontroller to adjust the brightness by varying the duty cycle . |
The MIC2298 also supports analog dimming by varying the voltage on the DIM pin or the resistance on the BRT pin. This allows the microcontroller to adjust the brightness with a simple analog signal . |
The evaluation board for the MIC2298 includes plots of the LED current versus the dimming control parameters, providing designers with practical data for implementing their own designs. |
|
Chapter 33: System-Level Integration - The MCP1650 Demo Board |
The MCP1650 Multiple White LED Demo Board from Microchip is a good example of system-level integration. The board uses a PIC10F206 microcontroller to provide the user interface and generate the PWM control signal for the switch-mode power supply IC . |
The PIC10F206 has an internal oscillator and reset circuit, so no external circuitry is required. The device can also be used to linearize the brightness control or monitor battery status . |
This level of integration is particularly valuable in applications where space and cost are at a premium. The same microcontroller that provides the user interface can also be used for system monitoring and control, eliminating the need for additional ICs. |
Chapter 34: PIC16F785 - Integrating Analog Peripherals |
The PIC16F785 from Microchip is a more advanced device that integrates analog peripherals for switch-mode power supply applications with an 8-bit MCU. |
The PIC16F785 has two on-chip op-amps, two on-chip comparators, two analog PWM modules, and an adjustable voltage reference. These peripherals can be digitally configured to implement a wide variety of SMPS circuit topologies . |
Once configured, the analog control circuitry can run independently of the MCU. This frees the MCU for other tasks such as communications and status monitoring . |
The PIC16HV785 device adds a shunt voltage regulator to reduce external component count, making it even more compact . |
|
Chapter 35: Current Sense Resistor - Measuring the LED Current |
The current sense resistor is a critical component of many LED driver circuits. It provides a voltage that is proportional to the LED current, allowing the control loop to regulate the current. |
The MIC2298 uses a current sense resistor to set the LED current. The 100% current level can be set by the equation: ILED100% = 0.2 V / R1 . By choosing different resistance values, the designer can set the maximum current to the desired level. |
The current sense resistor should be carefully selected to minimize power loss while providing an adequate signal for the control loop. A smaller resistance reduces power loss but provides a smaller signal, making the current regulation more sensitive to noise. A larger resistance increases the signal level but also increases power loss. |
Chapter 36: Evaluation Boards - A Starting Point for Design |
Evaluation boards are a valuable resource for designers. They provide a working reference design that can be used to evaluate the performance of a device and to start the development of a custom design. |
Microchip provides evaluation boards for many of their LED driver ICs. The MIC2298 evaluation board, for example, provides a complete LED driver circuit that can be used to evaluate the performance of the MIC2298 in a variety of applications . |
The evaluation board includes jumper settings for different current levels, test points for measuring voltages and currents, and a circuit diagram. This allows the designer to quickly set up the board and evaluate its performance. |
The evaluation board also provides a starting point for custom designs. The designer can use the evaluation board's circuit diagram as a reference and modify it to meet the specific requirements of the application. |
|
Chapter 37: Thermal Shutdown - Protecting the Circuit |
Thermal shutdown is a common feature of LED driver ICs. It protects the IC from damage if the junction temperature exceeds a safe limit. |
The TLC6C598-Q1 and TLC6C5912-Q1 from Texas Instruments include a thermal shutdown function that enhances system reliability . When the junction temperature exceeds a threshold, the device shuts down to prevent damage. |
The thermal shutdown function is particularly important in LED driver applications because the LEDs and driver IC can generate significant heat. If the heat is not dissipated effectively, the IC can overheat and fail. |
The thermal shutdown function requires careful PCB layout and thermal management. The IC should be placed on a PCB with good thermal conductivity, and the PCB should be designed to conduct heat away from the IC. |
Chapter 38: Overvoltage Protection - Preventing Damage |
Overvoltage protection is another important feature of LED driver ICs. It protects the IC from damage if the output voltage exceeds a safe limit. |
The MIC2298 includes overvoltage protection that limits the output voltage to prevent damage to the IC. If the load is not present, the overvoltage protection limits the output voltage to prevent damage to the MIC2298 . |
Overvoltage protection is particularly important in boost converter applications. If the load is disconnected, the output voltage can rise to dangerous levels, potentially damaging the IC and other components. The overvoltage protection prevents this by limiting the output voltage. |
|
Chapter 39: ESD Protection - Safeguarding the Inputs |
ESD, or electrostatic discharge, protection is essential for any electronic device that will be handled by users. The inputs to the LED driver IC must be protected from ESD events. |
The TLC6C598-Q1 and TLC6C5912-Q1 have an ESD rating of 2000 V . This rating indicates the level of ESD that the device can withstand without damage. |
ESD protection is typically provided by internal clamping diodes that shunt the ESD current to ground. These diodes protect the internal circuitry from damage by limiting the voltage at the input. |
Chapter 40: Slew Rate - Controlling EMI |
The slew rate of the output drivers can affect the EMI performance of the reader. A high slew rate can generate high-frequency noise, causing interference with other circuits. |
The TLC6C598-Q1 and TLC6C5912-Q1 have an optimized slew rate that improves EMI performance . The optimized slew rate reduces the high-frequency content of the switching waveform, reducing the amount of radiated and conducted noise. |
Controlling the slew rate is a trade-off between EMI performance and switching speed. A faster slew rate provides faster switching but generates more noise. A slower slew rate reduces noise but also reduces the switching speed. |
|
Chapter 41: Quiescent Current - Minimizing Power Consumption |
Quiescent current is the current that the IC consumes when it is not driving a load. In battery-powered applications, minimizing quiescent current is essential for extending battery life. |
The TLC6C598-Q1 has a quiescent current of 0.1 microamps when the output enable pin (G) is high, and 16 microamps when G is low . The low quiescent current in standby mode allows the reader to remain powered on for extended periods without draining the battery. |
The MIC2298 also has a low quiescent current in standby mode, making it suitable for battery-powered applications . |
Chapter 42: Voltage Range - Compatibility with Power Supplies |
The voltage range of the LED driver IC determines the power supplies it can be used with. The TLC6C598-Q1 and TLC6C5912-Q1 have a VCC range of 3 V to 5.5 V, supporting both 3.3 V and 5 V power supply rails . |
The wide voltage range provides flexibility in system design. The reader can be designed for 3.3 V operation for lower power consumption or for 5 V operation for compatibility with legacy systems. |
|
Chapter 43: Evaluation Board Testing - A Practical Example |
Testing the performance of the LED driver circuit is an essential step in the design process. Evaluation boards provide a convenient way to test the performance of the IC and the circuit design. |
The MIC2298 evaluation board includes plots of the LED current versus the dimming control parameters. These plots provide valuable data for designing the dimming control circuit . |
The evaluation board also includes stability plots that show the gain and phase margins of the control loop. These plots are essential for ensuring that the control loop is stable and does not oscillate . |
Chapter 44: Design for Manufacturability - Practical Considerations |
Designing for manufacturability is essential for any product that will be produced in high volume. The design must be robust, consistent, and easy to manufacture. |
The LED driver circuit should use components that are readily available and have good supply chain stability. The PCB should be designed with standard manufacturing processes in mind. The circuit should be tolerant of component tolerances and variations. |
The Symbol Technologies patent describes a robust design that addresses these considerations. The hybrid Buck/linear regulator circuit uses standard components and is designed to work reliably over a range of operating conditions . |
|
Chapter 45: Summary - The Illumination Subsystem in Perspective |
The illumination subsystem of a barcode reader is far more than a simple light source. It is a carefully engineered system that must provide stable, intense, and adaptable illumination while minimizing power consumption and noise. |
We have seen how different companies and technologies have approached the challenges of illumination: |
Symbol Technologies developed a hybrid Buck/linear regulator circuit that combines the efficiency of a switching regulator with the low noise of a linear regulator. This approach addresses the problem of power supply noise that can corrupt the imaging electronics . |
Texas Instruments has developed shift register LED drivers like the TLC6C598-Q1 and TLC6C5912-Q1, which provide efficient control of multiple LEDs. They have also published application notes on implementing APC for laser diodes . |
Microchip Technology has integrated LED drive functions with microcontrollers, providing compact and efficient solutions. The PIC16F785, for example, integrates analog peripherals for switch-mode power supply applications with an 8-bit MCU . |
Quarton inc. offers integrated laser modules with built-in APC driver circuits, providing a convenient solution for laser-based barcode readers . |
The patent literature reveals a wealth of innovative solutions, from capacitor-based flash drivers to multi-mode illumination systems. |
|
The key lessons from our exploration are: |
Constant-current drive is essential. The LED current must be regulated to ensure stable light output. Variations in temperature and manufacturing tolerances can cause significant changes in light output if the current is not well-regulated. |
Efficiency matters. In battery-powered readers, the illumination subsystem can be a significant power consumer. Efficient switching regulators and well-designed drive circuits are essential for maximizing battery life. |
Noise is the enemy. The switching of the regulator and the pulsing of the LEDs can create noise that interferes with the sensitive imaging electronics. Careful circuit design, PCB layout, and filtering are needed to minimize noise. |
Lasers require APC. The laser output power must be regulated to ensure safety and consistent performance. An APC circuit with a monitor photodiode is essential for any laser-based reader. |
Multi-mode illumination is valuable. Different barcode types and surfaces benefit from different illumination techniques. Bright field, dark field, and diffuse illumination all have their place in a well-designed reader. |
Integration is the trend. The trend in barcode reader design is toward greater integration, with more functions being combined into single chips. This reduces component count, simplifies the design, and reduces cost. |
In the end, the illumination subsystem is a testament to the ingenuity of engineers who have turned a simple light source into a sophisticated system. The choices made in the design of this subsystem affect every subsequent stage of the signal chain, from the photodetector to the decoder. And the art of the barcode reader lies in the integration of all these stages into a reliable and efficient whole. |
cs@easiersoft.com