Precision LED Blinker

Posted on Dec 3, 2012

The circuit allows a precision regulated drive current to be set to drive an LED, and in response to a TTL level signal, the LED is switched on and off with rise and fall times of less than 500 nanoseconds and less than 7% overshoot. This was designed for biological experiments to study the photo repsonse of cone cells rabbit eyes. This circuit may also be useful in systems that require a stable pulsed light source, such as those that measures light transmission (turbidity and densitomitry insturments, for example) and for the transmission of low speed optical data (to 200 kHz). The components are all easy to obtain and low cost.

Precision LED Blinker
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The circuit consists of an adjustable regulated current source (the op amp and Q2), a differential amplifier (Q3 and Q4) driven as a switch, and a level shifter (Q1) to shift the TTL input signal to levels necessary to drive the differential pair. The current from the current source is switched to either the +5 volt supply to the LED by the differential amplifier.

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