Electrical – Simple AM receiver schematic

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I've been trying to make an extremely simple AM receiver (without even a tuner), however, it doesn't seem to be working.

As shown in my schematic, what I did was connect the antenna wire to the base of a BC547 transistor. The collector of that is connected to a 9V battery and the emitter is connected to the base of another BC547.

This BC547's collector is connected to the same 9V battery and the emitter is connected to a germanium diode (so that only positive voltage radio signals are received), and this diode is connected to a piezo buzzer, which is then connected to ground.

So basically, I'm using the transistors to amplify the antenna's signal so that the piezo buzzer can make noises as it receives AM radio signals.

When I connect the power, I hear a chirp initially, but nothing else. Even if it's extremely low power, I should still hear some small chirps/noises right? What am I doing wrong? Thanks.

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Best Answer

Seems an amplifier is needed, as well as a rectifier and current buffer.


simulate this circuit – Schematic created using CircuitLab

This is a low-input-capacitance gain of 300x amplifier (gm of Q1 at 0.1mA, times 47Kohm). Then we add a diode lightly biased on with 4uA, with the collector of Q3 sitting at 4 or 5 volts across the piezo.

Experiment with adding a small capacitor across base-emitter of Q3, for more sensitivity (like a PEAK_HOLD capacitor).