Wednesday, June 27, 2012

Musical candle teardown

While in an office supplies store I saw a display rack full of "Singing Candles" and thought it would be interesting to take one apart to see how it works.

I've already taken off the white end cap when I took the picture.

There's a pre-made hole in the candle through which the thin silver strip threads and out the top of the candle. 

The MCU is in the black glob of course. The red ceramic capacitor is 10nF. The negative terminal of the button battery is soldered to the pcb. 

An S9014 NPN transistor drives the speaker. Base current limiting resistor is 4.7K 1/4-watt.

This is the speaker.

Other end of the speaker. It doesn't show in the pics but there's a coil of enamel wire if you peer into the holes.

Tinned pcb tracks after the retaining clip, rectangular metal bridge and metalized strip have been removed

From left to right: retaining clip, rectangular metal bridge, metalized strip, pcb

The narrow silver strip consists of two electrically conductive metalized outer layers and a nonconducting layer sandwiched in between. Measured resistance at DMM probe-to-probe distance of 1cm is 9 ohms, and ~60 ohms at 7 cm.

The following is a stylized diagram showing how the strip is positioned on the pcb tracks. Thickness of the strip and tracks are exaggerated of course just for illustration purposes.
The rectangular nickel-plated bridge sits on top of the strip and one of the tinned pcb tracks, providing an electrical pathway from the top layer of the strip to the other track. However, because of the insulating layer, the track on which the strip rests is isolated from the other track. A retaining clip secures the bridge and strip to the pcb.

As you might have seen in the first couple of images above the blister pack has a "press here" sticker that allows the would-be buyer to test the candle before purchasing. After opening the package I took a look at what they had under the sticker and found there's a 1-cm diameter aluminum foil glued beneath the plastic film and another on the cardboard. Ohmmeter check confirms they're zero ohm. There's one or two millimeters of airspace between the top and bottom foil. When the user pushes down on the plastic film the two foils make contact. At the same time they pinch the exposed silver strip. This causes electrical contact between the two outer layers of the strip and immediately wakes up the MCU thus causing the music to start playing. Only a momentary contact is necessary and the MCU will play the song three times.

Here's the musical candle in action.

Here's what I think happens: When the candle is lit and the flame begins to burn the strip, carbon forms and provides a relatively low resistance between the metalized layers, thus causing a switching action which wakes the MCU up (via an interrupt-on-change of sorts). As long as the candle is lit the MCU plays the song over and over.

Interestingly when the flame is snuffed out the MCU stops playing (not immediately but only after playing the song three in a row after the last check of the pin monitoring the "strip switch"). This implies that the resistance of the carbon and other substances in the burnt strip markedly increases when it cools down. Reigniting the wick quickly brings the temperature back up and causes the resistance to decrease once again thus triggering the MCU.

Pics of strip after candle had been lit

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