Improved Electronic Birthday Candles Provide Better 'Blow Out' Simulation

摘要

An earlier Idea for Design described LED "candles" that you could blow out just like normal birthday candles ("Electronic Birthday Candles 'Blow Out' One At A Time," www.electronicdesign.com, ED Online 18653). A thermistor and heating resistor combination detected air blown over the thermistor. The control circuit incorporated an 8-bit shift register, a quad op amp, and driver transistors, allowing up to eight candles. For each gust of air blown at the thermistor, one LED was turned off, thereby mimicking the action of normal candles.rnThe improved circuit described here offers a more realistic behavior of the candles. It uses a simpler circuit that increases the number of candles allowed to 20.rnTo reduce complexity, we used an inexpensive microcontroller. Lots of options are available. We chose a device with one of the smallest footprints available-an ATtiny45 in an eight-pin package. Among otherrnthings, modern microcontrollers offer digital I/O and on-chip ADC channels. Both of these features are useful for this project. Although the limited number of I/O pins on the eight-pin package restricts the number of LEDs that can be controlled, an improved LED multiplexing scheme called Charlieplexing alleviates that problem ("Eight-Pin Microcontroller Handles Two-Digit Display With Multiple LEDs," ED Online 15512).