Electronic Candles HOWTO Electronic Candles HOWTO
The image above shows the circuit boards for four versions of the candles. For low volume production, I use printed circuit boards that are pre-drilled every 0.10 inch, with copper on one side. Use a black, fine-tipped Sharpie (lacquer marker) to draw the circuit, then dip the PCB into ferric chloride solution to etch away the exposed copper. The PCBs that are 8 holes wide fit into 3/4" PVC with some trimming of each long edge (the power busses), either version fits into 1" PVC. The 8-pin ICs are all 555 timers. The 14-pin IC is a 556 (dual 555) timer. Versions using lamps are made with tiny "grain of wheat" lamps rated for about 6 volts. Using lamps requires a transistor (2N2222 switching transistor) to handle the current. LED versions can be driven directly from the IC with a 100 to 150 ohm current-limiting resistor.
I normally use a small slide switch for the candle power. For the "2nd switch flicker" version I add a tiny, normally closed, round button momentary switch beside it. A Dremel with a small cutting wheel works well to make the rectangular hole needed by the slide switch. I mount the switch(es) about 1/2" above the 9V battery that sits at the bottom of the PVC candle body.
I cut the wires for the switch long enough to extend a couple inches from the top of the candle, and solder them last to the circuit board.
To eliminate the chances of the switch contacts short-circuiting anything on the circuit board, I slip a piece of 1" heat-shrink tubing over the completed circuit board and shrink it down.
A couple pieces of velcro at the top and bottom of the PVC candle body work well to keep the flame centered and both battery and flame retained within the candle body.
The chart below shows the timing resulting from various resistor values with a 0.1uF capacitor. I commonly use 2 2.2M-ohm resistors for the slow lamp, a 2.2M-ohm and a 1.0M-ohm resistor for the medium, and a 4.7K-ohm and a 1.0M-ohm resistor for the fast. This yields frequencies of 2.19, 2.67, and 7.20 flashes per second respectively giving a pattern that does not quickly repeat. Frequencies of 2.0, 4.0, and 8.0 would give a very obvious pattern to the flashes that would ruin the flame effect.
In my opinion, the LEDs give a less realistic effect due the their abrupt on/off switching. Someday I'd like to try a PIC chip version using a pseudo-random timing for each LED with a ramp-up/ramp-down duty cycle, a 10mm LED at the base, and three sets of two LEDs above it.
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