Mini AM Transmitter

This is the simple and low cost AM transmitter schematic diagram built using transistor BC109C.

AM Transmitter circuit diagram

The circuit is in two halfs, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500kHz to 1600KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit.

The 1nF capacitor C7, couples signals from the base to the top of L1, and C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal emitter resistance, re of Q1, and also increases the input impedance so that the modulation signal will not be shunted. Oscillation frequency is adjusted with VC1.

Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realising full gain of this stage. The microphone is an electret condenser mic and the amount of AM modulation is adjusted with the 4.7k preset resistor P1. An antenna is not needed, but 30cm of wire may be used at the collector to increase transmitter range.


4 thoughts on “Mini AM Transmitter

  1. Bryan

    To send digital (0 and 1) you’d need some way to modulate 0s and 1s… Simply switching it off and on wouldn’t be too effective. Do some research on Frequency Shift Keying (FSK)

  2. brendon

    hi,

    Do you know what the range is of this transmitter??

    And do you have the PCB layout of the circuit??

    Please attach to my e-mail

    thanks

  3. iula185

    i’m about to try your schematic but not with a microphone, but with a switch, I only need to send a signal not voice or anything else(something like digital). Can you please tell me if it would work with a switch and can you please direct me to a schematic that can receive this transmitter that would be also simple?
    thank you!

  4. jan jan villalobos

    hi.. can u provide me the pcb layout of this circuit? pls.. its for my project due on thursday.. pls pls pls.. pls attach it to my email. tnx

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