Pulse Circuits for Infrared LEDs and Visible Diode Lasers

Posted on Feb 5, 2014    10888

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 i

Pulse Circuits for Infrared LEDs and Visible Diode Lasers
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nfrared 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. 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

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