My intention is to have multiple intermittent transmitters per receiver, so if I purchased 10 pair, I'd have 9 unused receiver modules. I thought I might be able to get my ATtiny85's to transmit a 433Mhz signal. I had read about spritesmod's FM transmitter hack, however 433Mhz is well beyond the 85Mhz maximum frequency spec for the t85's PLL. And the maximum square wave frequency output is half the PLL frequency - so even if I could overclock the PLL to 100Mhz, I could at best output a 50Mhz square wave.
I considered using multiple clock doubler circuits, however after some thinking I remembered it's still possible to generate a 433Mhz signal as a harmonic of a lower frequency square wave. After trying some differing multiples, I worked out that if I generated a 39.45Mhz square wave, the 11th harmonic would be very close to 433.92Mhz. That would require the PLL to run at 78.9Mhz, and the internal RC oscillator to run at 78.9/8 =9.86Mhz instead of the normal 8Mhz.
The RC oscillator frequency on the tiny85 is tuned by changing the value of the OSCCAL register. Figure 22-42 of the datasheet shows it can be tuned to over 14Mhz:
the code to enable the PLL, and output a square wave of 1/2 the PLL frequency and an ASK duty cycle of 30ms on and 50ms off. For an antenna, I cut a 17cm long piece of 24AWG copper wire from some cat5 cable and plugged it into my breadboard connected to pin6 (OC1A) of the t85. I hooked up my logic analyzer to the 433Mhz receiver's rx pin, and here's what I got:
So why didn't it work? I'm guessing that the ATtiny85 output drivers do not generate a sharp enough square wave to create strong harmonics at 433Mhz. An EDN article I found states, "most digital output waveforms follow a nearly Gaussian profile", meaning the transitions do not have significant high-frequency components.
I think there is still potential in the idea. I may purchase a 315Mhz receiver, or see if I can re-tune the 433Mhz receiver to 315Mhz (there is a screw on the board that looks like a variable capacitor). If I output a 35Mhz square wave, the 9th harmonic at 315Mhz will be much stronger than the 433Mhz signal. Running the PLL at 90Mhz would generate a 45Mhz square wave, and the 7th harmonic should be stronger than the 9th harmonic at 35Mhz. If that still doesn't work well enough, I might try fast swtiching mosfets or transistors that are rated >500Mhz, in order to generate a sharper output waveform. Comment if you have any other ideas.