There is a component called infrared receiver module, or words similar to this. Typically this component has three terminals: V+, gnd, and Vout, and typically (V+ - gnd) = 5 VDC. The ouput Vout is a logic type output. It swings high (to +5 V) instantaneously whenever a modulated IR signal is detected, and is low (0 V) the rest of the time. Or I might have remembered that backwards, but thats something that's not too hard to puzzle out once you've got the thing wired up. The encoding for the different buttons is done with pulses and timing, but that does not matter if you only want to do one thing. I am sure the play button on your greeting card is also a logic type input. Not sure if you have to pull it high or low, but that's something you can figure out experimentally. Also it would be neat if the greeting card toy ran on something close to 5 V, same as the supply wanted by the IR module. The last one I took apart (years ago) ran on nominally 6 V = 4 x 1.5V cells, and that's close enough. The attached picture (from http://www.goldmine-elec-products.com/prodinfo.asp?number=G12958 ) shows what these IR receiver modules typically look like. If you have some broken piece of electronics, like an old TV, or satellite reciever, you might already have an IR receiver module, and you just need to unsolder it from the board and install it in your thing. This component is pretty easy to find in the marketplace too. I think even RadioShack(r) might sell one, if you have one of those near where you live. If you are faced with a choice of what carrier frequency the module detects, you probably want something near 40 KHz. 38 KHz will probably work too. Ideally this is the same carrier frequency used by the IR remote.
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How "slim" does it have to be?You need an IR sensor, enough circuitry to reject IR "noise" and respond only to a remote control (more circuitry if you want it to respond only to THAT remote control, more if you want it to respond to a particular button on that control), and some form of output (transistor, relay, whatever) to replace the simple switch in the card as shipped.Even with surface-mount components, I suspect that's going to be builkier than you're looking for. But that's a guess since you haven't defined the problem very precisely.