Simple two-transistor circuit lights LEDs
Improve on a previous Design Idea with fewer parts
Barry A Tigner, Michigan State University, East Lansing, MI; Edited by Martin Rowe and Fran Granville -- EDN, April 9, 2009
A previous Design Idea describes a circuit that uses an astable multivibrator to drive an LED (Reference 1). The circuit in Figure 1 uses a simpler alternative approach. The circuit uses a 2N3904 NPN transistor and a 2N3906 PNP transistor, which operate as a high-gain amplifier.
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The 1-MΩ resistor supplies bias current. The 1-kΩ resistor helps linearize the oscillator waveform into one that is close to a square wave with about a 50-to-50 duty cycle. The capacitor supplies positive feedback from the output of the amplifier to the noninverting input. The frequency of oscillation depends mostly on the RC constant of the feedback capacitor and the input-stage impedance. The circuit oscillates at 91 kHz with a 48% duty cycle. You can use almost any common NPN or PNP transistors, as long as they have moderate forward-current gain of 50 or more and can handle 100-mA collector currents.
The LED connects across the output transistor because this approach lets the inductive kickback voltage add to the battery-supply voltage and makes the LED brighter. This circuit operates well from approximately 0.8 to 1.6V, which is the useful range of an alkaline battery. The LED-light output decreases as the supply voltage decreases from 1.6 to 0.8V.
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Good point made by Dick Cappels! The circuit similar to this I was using more that 2 decades ago...
Also, you could check my article in EDN going back to Dec 5 1996 "Single NiCd cells drives op amp", describing ultra low-voltage INDUCTORLESS DC-DC converter, powering LED and providing stable output to Op Amp. There is online version available - just search EDN archive: www.edn.com/archives/1996/120596/25di_07.htm
Best regards,
Alex Bell
Alexander Bell - 2009-18-4 16:16:00 PDT -
See U.S. Patent 6,603,342
Dick Cappels - 2009-14-4 05:15:00 PDT
