Basic Pulsed Circuits for
Infrared LEDs and Visible Semiconductor Diode Lasers

M. Gallant 1/24/2008
This article demonstrates basic circuits for pulsing infrared LEDs and low power visible semiconductor lasers using components which are inexpensive and fairly readily available. Many interesting and useful applications can be found in the references cited here, as well as several online web pages. This article focuses on the basic circuits.

The infrared LEDS fabricated from GaAs or GaAlAs discussed here are PN semiconductor junction diodes fabricated from GaAlAs or GaAs and typically emit at wavelengths in the range 850 - 950 nm (with 880 and 915 nm being two readily available choices). Uses include infrared remote controls for consumer appliances. Typical specifications are DC optical power output ranging from 1 to 10 mW at DC forward currents of 20 to 100 mA with forward voltages from about 1.3 to 1.7 V. Considerably higher peak power outputs can be achieved if the LEDs are pulsed with short pulses in the range of 1 us to 100 us with low duty cycles of 1 to 10%. Optical pulse rise/fall times available from these LEDs range from ~ 500 ns to 20 ns (corresponding to bandwidths on the order of 1 MHz to 20 MHz).

Visible semiconductor lasers found in common laser pointers emit at about 650 +- 20 nm and are commonly in the Class IIIA Laser Product category, emitting less than 5 mW under DC bias conditions from 4.5 to 6.0 V powered by three or four 1.5V button cells. (Visible LEDS at this wavelength are also commonly used in audio digital optical output interfaces (S/PDIF) but power levels are only about 30 microWatts).Evidently the laser chips in most low cost laser pointers use no protection or biasing circuitry at all and the chips are reported to have modulation bandwidths of several hundred MHz extending the range of useful application considerably beyond that available with pulsed LEDS. Laser pointers can often be found at retail outlets at steep discount prices (often less than $5.00).



Many such pointers can easily be disassembled and pressed into pulsed service as demonstrated here. However, due to the nature of the semiconductor laser device (high power density at the laser chip facet), these emitters are susceptible to catastrophic damage due to transients current "glitches" in electronic pulser circuits and care should be taken to study the electronic driver pulse signal, using suitable low value resistors, before connecting the visible laser.

Photodectors suitable for studying pulsed infrared LEDS and visible diode lasers include high-speed Si PIN diodes with fast rise and fall times of less than 10 ns. Si phototransistors offer high sensitivity (with current gain) but typically have considerably lower speed, typically in excess of 200 ns.

The picture below shows 2 diode lasers (upper) extracted from laser pointers, two infrared LEDS (lower left) and a Si phototransistor (TIL 99) :


High Current Infrared LED Driver

Many infrared GaAlAs LEDS can be driven at currents approaching 1000 mA provided that the duty cycle and pulse width is short enough to keep the average power dissipation low enough. Using a standard 555 timer IC and a 2N2222a NPN transistor, it is possible to create a drive pulse with width down to a few microseconds and amplitude to about 800 mA. The 555 timer chip has a rise/fall time of about 100 ns and current drive capability of 200 mA. The transistor output driver raises the output drive capability by about a factor of 4.
The diagram below shows an implementation of this approach. The 555 timer circuit uses 1N914 switching diodes to enable complete control of the on/off time for the pulse. Despiking and power supply decoupling capacitors of 0.1 and 5 uF were used. A 220 ohm series resistor in the 2N2222a base circuit is used to set the base drive level. A 10 ohm collector resistor provides suitable current limiting. The LED in the emitter circuit is shunted with a parallel 100 ohm shunt resistor to speed up the fall time of charge drain. The 555 and transistor were biased at the same adjustable level with a common power supply (22-121 Micronta Adjustable Dual Tracking DC power supply rated at 1 Amp and 15 V).



The LED drive pulse and the PIN photocurrent were monitored using a Syscomp Electronic Design DSO-101 dual channel 2 MHz (~ 100 ns risetime) compact USB oscilloscope:



The optical pulse was detected using a reasonably low capacitance (50 pF @ -3 V bias) Si PIN detector amplified with a National LM4562 opamp current-to-voltage converter circuit. With a 55 MHz GBW and a slew rate of 2V/100ns, a feedback resistor Rf of 10 kohm provides good signal amplitude with a just sufficient 3dB closed-loop bandwidth of 4 MHz (88 ns tr/tf) for optical detection. A feedback shunt capacitor of 5 pF across Rf suppresses gain peaking. (Rf of 1 kohm enables 16 MHz BW with a 10 pF peaking capacitor; Rf of 220 ohm enables 28 MHz BW with a 20 pF peaking capacitor). Care should be taken in the component layout to minimize wire lengths to reduce stray capacitances, particularly around the feedback resistor. In addition, this opamp can be used with power supplies as low as +_2.5 V. In this work for convenience 4-AA batteries were used to provide +_ 3.0 V with clipping at 2.5 V. The detector was used in photoconductive mode with -3V applied to lower capacitance and improve linearity.



Using a SEP8703-001 880 nm LED (Radio Shack/The Source # 276-143a), the following LED drive current results are achieved:

Vcc= 5.0 Ic = 250 mA
Vcc=10 V Ic = 600 mA
Vcc= 14.5 V Ic = 820 mA
For drive currents less than about 200 mA, the 555 alone can be used, with a suitable current limiting resistor.

Scope display showing collector voltage (blue) and PIN photocurrent at Vcc=10 V Ic = 600 mA. LED pulse rise/fall times are on the order of 500 ns:



Scope display showing Vbase of 3.2 V at Vcc= 10 V Ic=600 mA. The corresponding Ve (LED) voltage level is about 2.0 V :



If higher current pulses are required (1000 - 2000 mA) but less control over the pulse shape can be tolerated, then a very simple discrete solution is a 2 transistor flip-flop circuit using a Si and Ge transistor (e.g. 2N2222a and 2N1305). Some infrared LEDs can be driven to 2 Amps with 1% duty cycle in this way. See Forrest Mims Circuit Scrapbook for a good discussion of this approach. The image below shows an example mounted in a dual C cell holder:


Visible Diode Laser Low-Speed Driver

For longer pulse widths and with rise/fall times no shorter than 200 ns, the 555 timer chip can be used to directly drive a visible diode laser at the nominal current level (usually 20 - 30 mA) with a current limiting series resistor. The drive pulse shape should be examined carefully using a load resistor simulating the laser to ensure that drive spikes from the 555 timer are not excessive. Wiring from the 555 to the laser should be short to minimize inductive effects which can cause ringing. The result below shows an example using only the 555 part of the circuit above with the same 220 ohm series resistor to directly drive the laser removed from a Nexxtech 6311060 Mini Laser Tool Set. The lower blue trace is the laser voltage (6V peak drive value). The upper red trace is the Si PIN/opamp voltage signal. The much faster optical rise/fall time compared to the infrared LED is obvious. A slight overshoot on the drive pulse is evident in the fast optical response:



The voltage overshoot of the 555 can be used as a crude way of obtaining an optical pulse of about 200 ns in width based on the steep I-V curve. If the laser is pulsed to just slightly above threshold, the overshoot will generate a narrow pulse as shown below:



The setup below shows the visible laser mounted in a box (background). The beam is directed to a tilt-mount mirror about 1 meter in the forground (not visible) and reflected back onto the pin photodiode (in forground):



To obtain an optical pulse from a visible diode laser significantly shorter than a few microseconds and with rise/fall times faster than 100 ns, faster drive electronics must be used. A simple example of one approach is shown below.

Visible Diode Laser Medium-Speed Driver

Assuming a typical visible diode laser can be digitally modulated at rates of several hunded MHz, then optical pulse risetimes of nanoseconds should be possible. Designing fast driver electronic circuits in this range requires a good understanding of the laser response characteristics and high-speed electronics. It is possible however to produce drive pulses in the 20 to 100 ns range fairly easily using standard TTL or CMOS circuits. The following circuit shows one simple approach using a 555 timer chip (~100 ns switch time) and a TTL inverter (~ 10 ns switch time):



The 555 timer generates pulses of several microseconds in duration with ~ 100 ns rise/fall time. The standard TTL 7404 hex inverter has a switch time on the order of 10 ns. In the circuit above, 1/4 7404 is used as a "half monostable multivibrator" in which the inverter is triggered only on the falling edge of an input TTL level pulse (see TTL Cookbook).The width of the inverter output pulse is determined by the two resistors and input capacitor of the inverter. In the example above, a pulse of about 200 ns is created and the rise/fall time should be on the order of 10-20 ns. Since the output voltage of the 7404 is limited (slightly less than 5 V in TTL) and visible diode lasers can typically require 5 - 6 V drive, a 2N2222a drive transistor is used again. However, the laser must be placed in the collector circuit due to the higher voltage requirement of a visible laser (compared to an infrared LED). Also, it is important to determine the correct base drive level in order to just achieve the required transistor switch saturation level; otherwise the 2N2222a drive pulse with be widened due to charge storage effects. The 2N2222a has tr/tf of 20-50 ns but potential charge storage effects can easily cause pulse broadening to hundreds of ns. A transistor biasing pulsed simulation programme such as MicroCap helps in determining a good operation point for fast pulse generation. The diagram below shows a simplified model of a CE 2N2222a circuit with a pulsed input voltage source (representing the 7404 output). The laser is crudely modeled as a fixed resistor of 220 ohm in the collector circuit corresponding to a laser with Vf=4.5 V at If=20 mA:



The transistor biasing conditions (voltage level and resistor values) can be adjusted in the simulation to determine the sufficient drive to produce the best shaped collector voltage pulse:



Overdriving the transistor in this configuration leads to pulse lengthening and a degraded fall time as shown in this simulation:



With proper biasing a reasonably fast 100 ns pulse with tr/tf ~ 20 ns is achieveable. The scope trace below shows measured results for the 555-->7404--->2N2222a circuit with transistor voltage-divider biasing resistor values of 220/100 ohms. In this case, a 650 nm visible diode laser from a laser pointer with a nominal DC bias of 4.5 V at 20 mA was used. The upper trace (blue) is the collector voltage showing a just resolved sharp 200 ns drive pulse with 4.5 V swing. (The time response in these results is limited by the scope bandwidth). The lower trace shows the Si PIN photodiode signal showing a similarly fast optical pulse.



Using a modest commercial function generator and a 100 MHz scope, the trace below shows a pulse width at least as short as 80 ns (limited in this case by the 40 ns tr/tf of the detector/opamp combination) can be achieved using an ordinary visible diode laser:



Visible Diode Laser High-Speed Driver

To obtain laser drive pulses on the order of a few nanoseconds, ECL fast logic or other series high-speed digital logic are possibilies. At these speeds, very careful layout design is critical and switching transients must be carefully suppressed to prevent laser damage.

References