How to make a 27MHz AM radio transmitter from a radio remote control

The 27 MHz radio remote control can easily be converted into a radio transmitter and can transmit voice or other amplitude modulated signals, as well as simply Morse code or numbers. The output power, according to my estimates, will definitely be higher than 50 mW. This is enough to transmit a signal for hundreds of meters, and with a good antenna - for kilometers

I will make the transmitter from a simple remote control for a frequency of 27 MHz. It is not necessary to make it from the remote control, as in my photo, other remote controls will do, they usually contain similar cascades. So, first, let's consider the composition of the remote control. The command encoder is assembled on the popular TX2 microcircuit, but can be assembled on the YX4116 microcircuit or another. The TX2 microcircuit contains 5 commands, each command has its own encrypted signal. These signals are fed to the transistor of the master oscillator, which generates a stable signal at a frequency of 27.145 MHz. This frequency is specially allocated for radio control of models. Then this weak signal goes to the power amplifier, made on one transistor, for amplification. The signal from the amplifier goes to the antenna and is emitted

What needs to be done to turn the remote control into a radio transmitter? The bases of the transistors of the master oscillator and power amplifier are connected to the microcircuit via bias resistors. When the microcircuit is active, it sends an encrypted signal from pin 8 to the base of the master oscillator, which also unlocks the transistor and the generator starts working. According to the diagram, you can see that an LED is connected to pins 9-10. In radio remote controls, pin 10 is connected via a resistor to the base of the power amplifier and unlocks it when you press the command button on the remote control. All you need to do is disconnect these two base resistors from the microcircuit and apply bias voltage to them

This is approximately what the two base resistors look like that are connected to the chip on the remote control with the YX4116 chip. Disconnect them from the chip and apply bias voltage to them, then the generator and power amplifier will start working constantly.

I supplied the bias voltage from a separate 3.7 Volt battery, this voltage can also be taken from the 9V remote control power source using a resistor divider

Now the radio transmitter operates on a 9V supply and constantly emits a carrier signal into the air. To modulate this signal with sound or another signal, it is enough to feed the signal to the base of the power amplifier transistor through a capacitor

The signal emitted by the antenna can be checked using a high-frequency probe made on two diodes and a light-emitting diode. Touch the antenna with the lead of the high-frequency probe and the light-emitting diode should shine brightly. The transistor of the power amplifier will heat up and it needs a small heat sink in the form of a piece of foil attached to the case through heat-conducting paste. When powered by 9 volts, my radio transmitter consumed a current of 30 mA