In order to encourage us to play with electronics more, JWA and I have begun doing monthly challenges. For our inaugural challenge, we decided upon an LED Birthday Candle. This is my entry to that challenge.

LED Birthday Candle

You blow the center pole towards one side, and the LED shuts off. To turn it back on again, blow it the other direction. It’s a circuit known as a bistable multivibrator, or “flip-flop”.

I do like that it’s so simple and cheap, though.

My parts list is simply:

2 – 10k ohm resistors
1 – 1k ohm resistor
1 – 220 ohm resistor
2 – 2n3904 NPN Transistors
1 – LED
3 – AAA batteries (this thing’ll last forever!)

Not including the batteries, this thing costs less than 15 cents in parts. If I could work out the “blow” mechanism a little better, I think this would be a marketable product.

Okay, now for some photos:

On the breadboard:
LED Birthday Candle

Before being shoved in the pill bottle:
LED Birthday Candle

I had originally wanted to be able to shove this thing into a fat straw, like you get with a bubble tea, so I made it as small as possible, and soldered the pieces together straight rather than using protoboard:
LED Birthday Candle

Next: Brighter LEDs!

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