The control circuit that I designed for this project uses a 741 OP AMP to monitor the output of the receiver. Two series resistors (3.3kohm and 1kohm) form a voltage divider which sets a reference voltage that the OP AMP uses for comparison. These resistors may need to be changed to calibrate for your receiver. When the output voltage of the receiver goes above this reference voltage, the output of the OP AMP goes HIGH and activates the relay.
I am using an automotive relay so that it will be able to switch high currents (up to 40 amps). However larger relays like this also require more current to activate. The OP AMP can not output enough current to activate the relay by itself. So the output of the OP AMP is connected to a power MOSFET. This transistor is used to turn the relay on and off.
I added a diode, a capacitor and a resistor to the input of the control circuit to help filter out noise and to smooth out the pulses from the radio receiver. This prevents the control circuit from rapidly turning on and off.
The control circuit is powered by a 12-volt battery pack.
When assembling the circuit, I first prototyped it on a breadboard. To test the performance of the circuit, I connected an LED to the output. Once I was happy with how the circuit was working, I soldered it onto a printed circuit board.