An led not only lights when power is applied, but also generates a tiny voltage when light is applied. A PICAXE microcontroller can be used to monitor this voltage. I will show you how to make a electronic candle that will flicker an led when it gets dark.

Step 1: Assemble the "candle"

The candle part of the circuit is a piece of perf board with a green led, 220 ohm resistor, diode,capacitor and a M08 picaxe. I used molex connectors to make the perf board detachable from "power stalk" made from twisted 18 gauge enameled wire. You can use insulated solid wire such as building wire for the stalk. You can also use a yellow or red led. White and blue leds don't work well as light sensors. The diode protects from reverse polarity and allows using alkaline cells by dropping the voltage to the picaxe a bit. Nicad cells work well with picaxes.

Step 2: The Power Source

For the power source use 4 AA cells in a battery holder that has a 9 volt style snap connector on top. I took apart a depleted 9 volt battery and used the snap connector to mount the power stock. You can make the stock straight or add a "coil" to it. You can even wrap it with green florist tape and make a flower out of it. When done, place it in a vase to hide the battery.

Step 3: Program Your Picaxe

As you picaxers know, you will need to program the picaxe. Below is the program:

'picaxe program to flicker led like a candle when dark

start:
input 1 'make pin 1 an input before reading with ADC
pause 200
w0=0

main:
low 1 'discharge led before reading
input 1
random w0 'get numbers to set flicker rate
readadc 1, b2
if b2 < 10 then dark
goto main

dark:
'led will flicker depending on value of b0 and b1
high 1
pause b0
low 1
pause b1
goto main

Cut and paste the program into your latest copy of the free picaxe programmer software. For newbies with the picaxe go to this site for info about the picaxe.
http://www.rev-ed.co.uk/picaxe/

For a simple programmer use this circuit to build a programmer. Program the picaxe and remove to place the picaxe in the candle.

Step 4: Front Side of Candle

The candle circuit is simple enough to use perf board for the construction. Front side showing the Picaxe mounted on a socket.

Step 5: Backside of Candle

Construction using tinned solid wire to make the connections on the backside of candle.

Step 6: You Candle Is Done.

Feel free to modify the program. You can also make the candle do the reverse. Run this program to make it go on only when its bright.

' led goes out in the dark

main:

low 1
input 1
readadc 1, b0
if b0 > 10 then dark
goto main

dark:
high 1
pause 100

goto main

Have fun with the idea.

Step 7: Parts List

List of Parts:

Candle part:

Piece of Perf board about 5/8 by 7/8 "
Picaxe M08
8 pin IC socket
1N4005 diode
.1uf ceramic capacitor
220 ohm 1/4w resistor
Green,red or yellow led
Bare tinned copper wire
Molex header 22-23-2021 (Digikey WM4200-ND)

Power stalk and Battery:

Molex housing 22-01-2027 (Digikey WM2011-ND)
2 feet of solid insulated wire or #AWG 18 enamaled wire
9 volt battery snap connector
4-AA battery holder (Radio Shack, Frys)

Programmer:

9 pin D connector, female
22k 1/4 watt resistor
10k 1/4 watt resistor
200 ohm 1/4 watt resistor
Piece of perf board to mount components (Radio Shack, Frys)
8 pin IC socket