Encoding / Decoding solid '0' or '1' with IC (or discrete solution)

Question

I ordered a rf 434MHz transmitter and receiver for a hobby project.
It only needs to transmit a solid '0' or a '1' (this won't change more than once every 5s, probably much longer), so I thought I could get away with just the pair.

However it looks like it has trouble sending DC signals (they initially send properly but are quickly lost, I guess a low pass filter). I checked and it can send a ~8KHz square wave just fine (although varying duty cycle in the TX signal from 5%-95% still causes a nearly square-wave signal at the RX).

So I may need something like Manchester encoding, so that a solid '0' will still switch enough for the tx/rx pair.

It looked like the HT12E and HT12D would do exactly what I need. But, I can't find them anywhere except on Amazon for ~$10 a pop, or some site in a language I don't understand.

Are there any cheap IC's that would help me accomplish this (encode a 0 and 1 into some format with enough transitions, like Manchester)? Or a solution with discrete components that I am not thinking of?

Thanks

Answer 1

Your transmitter and receiver use ASK - Amplitude Shift Keying type of modulation. This means that when you input a "1" to the transmitter that it is producing the carrier wave at 434 MHz continuously. When you input a "0" to it there is no carrier wave produced.

You are likely correct that there is a filer involved, likely in the receiver, which is cutting off the constant carrier it is receiving when you are transmitting a "1", and the xmitter is therefore continually producing the carrier. This would be a high-pass filter, not a low-pass filter.

So, I think you can get around this problem by simply pulsing the transmitter on and off with something like a 555 timer set to an arbitrary low frequency - say 1,000 Hz - when you want to transmit a "1". Then on the receiving end merely half-wave rectify the output of the receiver with a low frequency signal diode (e.g. 1N914 or 1N4148), and a small filter cap (approximately 0.1 Mfd ceramic), and perhaps a bleeder resistor as shown in the schematic below. This will turn the 1,000 Hz ASK modulation into a steady high level, signifying the reception of a "1".

When you "transmit" a "0" the receiver will output approximately 0.0 volts, so the half-wave rectifier will output a steady 0.0 volts signifying reception of a "0". ( You are not really "transmitting" because there is no carrier!)

Since your "data" rate is so low (5 secs on, 5 secs off), the half-wave rectifier component values are not very critical. A little experimentation with different 555 frequencies at the transmitter end and half-wave capacitor values at the receiver end should yield workable results quickly.

^{simulate this circuit – Schematic created using CircuitLab}

If your receiver suffers from some reasonable degree of random carrier noise acquisition, you might be able to filter out the resulting output noise pulses by adding D2,D4,Q1,R2,R3 & R4 to the basic half-wave rectifier circuit. Random noise will produce an average DC voltage across C1 of about 1/2 Vcc ( presumably 2.0 - 2.5 volts). D2,D4 and Q1 form a crude comparator which will only switch on (creating a low output) when the real 1K signal comes along. The lower you make the 555 modulating frequency, the better will be your noise immunity.