AM radio built around 555 timer chip. The
only active device (silicon, germanium, or otherwise) is the LM555. The
tuning is accomplished with an inductor and a capacitor, and the LM555
acts as an AM demodulator and class-D power amplifier to drive the
speaker. You may be wondering how all this is accomplished with a 555.
Here’s how the circuit works: The AM radio signal is tuned by inductor
L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made
out of a paper roll, along with the 100pF variable capacitor. One end
of the parallel configuration of L and C connects to an antenna
(surprisingly long!) and the other end connects to a ground wire which
is tied to the AC outlet ground (old books tell you to ground it to a
water pipe).
So far this is exactly like an AM crystal radio.
The 555 timer is configured as a pulse width modulator in a
non-traditional configuration. If I used the standard approach and
connected the input to the CV pin, the low impedance of the pin would
prevent the circuit from receiving any radio signals. I had to invert
the circuit and tie both high impedance analog pins, Threshold and
Trigger to the radio signal input. This is the reason why the CMOS
version of the 555 timer performs much better than the standard bipolar,
which has higher input bias current.
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