Puzzled because 12v relay board seems to work with 3v Arduino signal

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1

I have a 3v Arduino and accidentally bought a 12v 4 channel relay board http://www.ebay.com/itm/261827538339 (I intended to buy a 5v 4 channel and use a Logic Level Converter for my application). I was going to return but for the kicks of it gave it a shot and it seems to be working but I am not sure how. Everywhere I read, I see that I need to provide a 12 volt signal. Here is what a got.

  1. 12v separate power from a wall wart to relay board VCC and Ground.
  2. 3v separate power from a wall wart to arduino.
  3. Common Ground connected between 1 & 2 above.
  4. Pin D2 going to CTR1 (I used a multimeter and can confirm 3V at CTR1 when D2 is HIGH).

Using a simple sketch I am doing D2 HIGH and LOW and the relay clicks and works as expected. I then connected D2 to all 4 controls points CTR1, CTR2, CTR3, CTR4 and they all clicked at the same time and worked as expected.

I do not understand why this is working from a 3V signal instead of 12v?

Any ideas?

2

You 3V3 is probably switching the 12V with a transistor somewhere on the board.

Maybe that's what the small black IC between the 102 and 103 is?

3

I asked the seller if they can point me to the datasheet but I doubt I will get it.

kenwood120s:
You 3V3 is probably switching the 12V with a transistor somewhere on the board.

Maybe that's what the small black IC between the 102 and 103 is?