Electronic Birthday Candle

Posted on Feb 6, 2014

So the concept here is pretty simple. I wanted to mount a microphone on a stick with an LED and design a circuit that would react to a large amount of noise on the microphone input (blowing) to turn off the LED. As you might expect, the schematic for this thing is very straight forward. The device is powered by a CR2016 3V coin cell battery which

Electronic Birthday Candle
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always supplies power to the circuit (there`s no power switch). If left in, even when the LED is turned off, the battery will die, but I figure for something with such a limited one-time use, it was an acceptable requirement. The left side of the circuit is the familiar gain stage/envelope follower that you`ve seen in many of my sound reactive circuits. I left the low pass filter in (R6 and C7), though it`s not really necessary. The only difference this time is that there is no gain on the microphone. Typically, blowing on a microphone actually clips the mic which means you`re producing the largest electrical signal possible. I experimented with different gain levels until I realized that anything more than 0dB gain caused the candle to be too sensitive and made it blow itself out. Originally, I was hoping to flex my analog muscles and make a straight up FET gain stage, but I realized that with a CR2016 battery as my voltage source, there would be too much variability on the rail to set a good bias point. The second half of the circuit is a standard BJT flip flop. You probably saw a lot of these in my transistor clock posts. This is a "bi-stable" circuit which means it must always exist in one or two states. Specifically, either Q2 is on and pulling current down through the D2 (the LED) and keeping Q1 off, or Q1 is on which keeps Q2 off. If you try modeling a circuit like this, you might find that it can balance between...

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