So I made a gadget that plays sounds using a LM386N amplifier chip and an ATTiny85 with a memory chip... it works well. Today I attached my PC oscilloscope board to it as usual to the ATTiny sound output pin, but when I touched the LM386 chip it started playing music that was not on the memory chip! Imagine my surprise when I found it was a local AM radio station, AM1030! Loud and clear! It sounds as clear as if it had been programmed onto the memory chip. I found that this only happens when I plug the usb cable into the oscilloscope board and is greatly enhanced when I plug the other end into the PC. Totally amazed at the clarity! What I don't understand is why that frequency. The wavelength for 1030kHz is 955 feet! The usb cable is about 4 feet long. Someone want to explain this?! Anyone want a good cheap AM Radio that only listens to AM 1030 Family Life Christian Radio ??
EDIT: This only happens when I touch one of the LM386 gain pins. ?
This is normal if you are close to the transmitter. You are in fact receiving all frequencies all the time only the signal is so small you can only actually hear the very strong ones.
What is happening is that the input of an amplifier looks like a diode and rectifies the AM and the input capacitance smooths out the RF leaving you with the audio signal.
In terms of blaming someone, if you report this to FCC or what ever transmission authorities you have in your country they will say it is your fault because your amplifier is acting as a radio receiver when it is not designed to be one.
I have even heard of some people picking up AM radio on tooth fillings, with the diode being formed by the metal enamel junction.
If you connect an antenna to an amplifier this is exactly what to expect. You are the antenna (or
part of the antenna system to be more accurate) in this instance.
The receiver you've accidentally made is not tuned in the slightest, its just that the strongest
signal is way above the level of any others.
The human body is a very good antenna for mains frequencies and strong broadcase stations, so touching
the input to an amplifier is expected to inject signals.
With a suitable 'scope (with low noise input and good frequency range), you can hold the probe tip
with the 'scope in FFT mode and see the local RF spectrum that you are picking up...
