Pulse-width modulated LED testPosted: 2005-February-28
I played with a pulse-width modulated LED. The circuit wasn't pretty, but it worked … essentially a transistor amplifier that drives one differential input on an operational amplifier and the other is through a variable resistor set somewhere between the rails. The op-amp output drove an NPN transistor's base and the emitter was tied to ground. The collector went through a 1-ohm resistor through the LED to a 5-volt source — essentially, when the transistor was on, current followed a path through the LED, resistor, and the collector-emitter junction.
The peak-to-peak voltage on the 1 ohm resistor was 190mV, so in theory, I could have driven the LED to 200 mA average. The pulse-width modulator had a period of about 2.05 ms. The LED was barely on with a duty cycle of 14 µs which correlates to an average of 0.14 mA. It was "dim" around 50 µs or 4.9 mA. I considered it "fully on" at 134 µs which correlates to 13mA and "overdriven" by about 335 µs or 33mA. It appears the circuit works perfectly (and it doesn't appear to blow out the LED's.)
I also picked up some gray ribbon cable which I'll use inside the tubing to wire all the LED's.
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