433 MHz transmitter

 Continuing from previous part.

Later addition: you can find sources here.

After I got receiver working - that is, successfully decoding received data - and played a bit with it, it was time to start thinking about transmitter part. I mentioned that I had neglected to order one, so I went looking for one from same large web store. This is where I hit a wall though.

My system is running at about 3.2v. As it turns out, most of the transmitter and receiver modules available are designed or at least sold to be used with Arduino. Therefore most had operating voltage specified to be 5v, with lower limit at 3.5v. Just a few transmitter modules listed 3.0v as lower operating voltage, but these had somewhat worrying review history and I chose not to risk it. (received I had ordered however was specced to work at 3v and had mostly good reviews so no issue there). All in all, no luck.

Then I realized I had transmitter already. Hand-held module works with 3v battery, and circuit was relatively simple (very close to this SAW-based one from this site). Main difference is that transmitter I had was passing supply voltage of RF through LED (so when LED is on, RF is on). Easy enough to hack, I thought.

So to testing. Left pin is enable (connected to LED series resistor input), other wires are modulation input from my PCB (and other for scope probing). At lower part of board were 3v supply and ground (battery inputs). I think I could have cut the lower part of board completely off but decided against that.

Getting waveform shape roughly right didn't take long (this I did without transmitter being connected, just looking at generated modulation data from scope), but final tweaks to get it just right for receiver to accept it took a bit longer, mostly getting start and end conditions to match original signal closely enough. That code is fairly trivial, just I/O pin switching and delays of various lengths in between, done with interrupts disabled so that they don't mess with timings. Nothing fancy required here, unlike receiver where a bit of timer trickery was needed to be able to receive data buried in noise as background operation (without interfering with other operations of system)