RC AM senders/receivers: Changing the crystals, and using Servo PPM as a 1/0 switch

Question

I am trying to use a 27mhz RC car controller to send to an RC Car AM receiver and use that output (short 1-1.5ms pulse of a few mA) to trigger a 5V relay, with the signal stabilised via a retriggerable monostable-multivibrator I guess?

I thought this would be easiest and I have the junk bits lying around unused.

I have a few questions:

1. I am impatient so I replaced the 27mHz crystal in the sender with a 29.something because I had a matching one for the receiver. Given that it was a closed controller, is the sender circuitry likely tuned to ONLY the 27mHz crystal? I can solder the other back in when the new one arrives

2. The servo is a feedback circuit, but it is also self contained and does not send information back out. Will the Receiver signal output system likely work the same with the signal going to a transistor or mono-multivibrator and the power looping through a 5V relay coil? (instead of the servo setup) For instance will the sudden pull or jitter of the coil on the power supply mess with the signal?

Is there any easier way to do this? what if i put the coil inline with a mini servo +supply and it just picked it up like a load when the servo was "on"

(For reference, I just need to remotely trigger a solenoid/12Vbattery/relay hidden in Stage-props and for cheap. It is a heavy painting so I don't consider the servo an option)

Answer 1

1) Trading the crystal was probably illegal, and likely resulted in a somewhat detuned circuit. Further, it's not clear that you used a compatible crystal in the receiver, as the receiver crystal must be offset by the IF frequency from that of the transmitter - typically by 455 KHz (or 10.7 MHz for more recent dual conversion sets at VHF frequencies). Interchangeable RC receiver crystals are usually labelled on the overwrap with the corresponding transmit frequency, but other crystals tend to be stamped with the actual crystal frequency. Also a crystal needs to be cut for the load capacitance of the circuit and operating mode, so something out of your junkbox may or may not work at all.

2) An R/C receiver typically won't care what it is driving (as long as the signal is not excessively loaded). However, the signal to a servo does not turn on and off, rather it varies in duration from about 1-2 ms, with a very low duty cycle as this only repeats every 50 ms or so. Decoding that with analog electronics can be done but is finicky - most would use a microcontroller today. Although it sounds wasteful, those without a strong electronics background who want to use an RC channel to turn something on and off often resort to mechanically connecting a servo to physically operate a switch.