Singing Birthday Candle

Have you ever got tired of sing happy birthday at someone's birthday. Well then I have the perfect thing for you, a candle that sings only when you light it. Blow it out and it stops singing!

• A Candle
• One of these lotus candles
• One LDR
• A NPN transistor, something that has a high gain, I used the bc547c
• A potentiometer 100k 1turn
• some perf board
• A soldering gun
• Hot glue

Start by dismantling the lotus candle and extracting the music board from within. Light the candle, after it has gone out pull out the plastic petals followed by the spring in the middle holding it together and now you can get the musical board out. You don't have to use a candle like this, plenty of little things at the birthday shop have little musical boards like these in them, and all of them are pretty much the same. For example this greeting card that sings happy birthday when you open it uses the exact same board.

Now follow the circuit diagram shown in the picture above. All you are creating is a basic voltage divider that actuates the transistor. When the candle is lit the resistance of the ldr goes down causing the candle to sing. Blow out the candle, the resistance of the ldr goes up and the speaker turns off. The pot helps account for the fact that everyone's candle might be slightly different and allow varying amounts of light in. It is important to note that these cheap Chinese boards can be tricky to solder and it might help to gently scratch the surface to help solder adhesion. You also want to be super quick with your soldering. Also remove the battery from the music board before soldering anything. You should use really thin wire here something like 7x32.

The first step is to source a candle. If you go to the store and buy a candle make sure it is wide enough to be able to fit the music board inside. Get a small spoon and scoop out enough of the candle to be able to fit the musical board. using a lighter to soften the candle might help. I chose to make my own candle. I just took a paper cup, put a tiny hole in the bottom and run some twine through the hole, this will act like the wick of the candle. Then pour in some hot wax and let the candle set in a cool room or the fridge. You want to cool the candle quickly. The benefit of this method is that as the candle cools the wax shrinks creating a natural cavity into which the board will fit.

Put the board into the bottom of the candle making sure the LDR is pointing straight up. Light the candle then blow it out and see if it works. If it doesn't adjust the pot, increase the resistance if it makes a sound while the candle is off and decrease the resistance if it doesn't turn on when it is lit. Once you are satisfied with how it works use some hot glue to keep it in place

Now decorate the candle as you like. I'm better at electronics that I am at making things look pretty so I'm going to leave this candle quite plain.