I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. 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. The 555 timer is configured as a pulse width modulator in a non-traditional configuration. So far this is exactly like an AM crystal radio. 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). 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 an old toilet paper roll, along with the 100pF variable capacitor. You may be wondering how all this is accomplished with a 555. The tuning is accomplished with an inductor and a capacitor, and the ICM7555 acts as an AM demodulator and class-D power amplifier to drive the speaker. The only active device (silicon, germanium, or otherwise) is the ICM7555. Turns out the fact that it is CMOS is important for this particular circuit… It uses an ICM7555, which is Maxim’s second source of Intersil’s CMOS version of Signetic’s original NE555 timer. Yes, it’s not really vacuum tube related, but I built an entry for the 555 timer contest. 555 Contest Entry Febru10:19 pm eric Projects
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