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Modified frog, LED question

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Fishplate42:
Recently I bought a silly croaking frog from my local pound shop. when I dismantled it I found a small circuit board inside with a black blob on it. I posted a picture here and you guys kindly explained what it is. on that tread somebody posted the fact that some of these novelty manufactures use sound devices from greetings cards to drive LEDs to provide a random flashing (with rhythm!). Curious, I decided to see what would happen if I connected my croak to an LED. Well to my surprise it worked and the led flashed in time with the croak.

But now I have another question. I expected the LED to only light if connected in one direction but it lights if connected either way ???  Can somebody tell me why?

Here is a picture of my experiment. I Have replaced the LRD with a reed switch so the blooming thing does not go off every time I move!



It is amazing just how much fun you can have with a plastic frog! The rest of the story can be found on my blog: http://ralphsworkshop.blogspot.co.uk/

Ralph ???

0xdeadbeef:
Moving the coil in a loudspeaker always involves alternating current - that's why the LED lights up in both direction.

Fishplate42:

--- Quote from: 0xdeadbeef on May 27, 2012, 04:50:26 pm ---Moving the coil in a loudspeaker always involves alternating current - that's why the LED lights up in both direction.

--- End quote ---

Of course! If I had thought about it from basic principles I may well of got there myself. Sometimes I seem to make things far too complicated in my own head and need to stand back and THINK!

Thanks for your help!

Ralph :-[

SeanB:
Very simple. The output to the speaker is connected by a capacitor, and the one side is grounded. A transistor provides charge from the battery into the capacitor, and while it is charging, the speaker has voltage across it, and starts to make a sound. When the transistor is switched off the capacitor discharges via a resistor  and the speaker. This generates a reverse voltage across it and completes the other part of the sound waveform. This has now made a complete cycle of the sound, and is repeated thousands of times until the complete croak has been sent out of the speaker. The led thus has alternately positive and negative voltage voltage applied to it, and will light up when it becomes forward biased, the other half of the wave it is reverse biased and does not emit light.

If you look carefully you will find it lights up slightly brighter in one direction than the other. This is due to the transistor being able to deliver more current into the led, while the other way the only source of charge is the capacitor discharging via a resistor and the speaker.

Zero999:
I notice there's no resistor in series with the LED. The output resistance of the IC is obviously enough to limit the current but it isn't the right way to do it. The waveform going to the speaker will be clipped by the LED so the sound will be distorted. Not that poor sound quality will be noticeable in this case with such a cheap plastic speaker and low sample rated.

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