educational BFO circuit

[Max]

Hi,
look at this simple project:

http://bobsdata.com/metaldetector/index.html

it's very easy (and so also few really usable or sensitive on the field) BFO detector using just a bunch of transistors: useful for people how wanna understand how old BFOs work.

Best regards,
Max

[Geo]

It remember me something like an old Popular Electronics BFO detector.
Or the most BFO are about the same

[Rumcajs]

Hi,

here is an other edu-BFO with one CMOS IC:

http://itl7.elte.hu/bscalab/S/epites_femdetektor.pdf

Brg

Rumcajs

[Pepe LeMiau]

Many years ago I built something similar using TTL schmidts. It wasn't that bad but the thermal stability was poor, so poor that after power-on I had to wait a couple of minutes for the treshold to stop drifting.
It was also very sensitive to environment temperature changes. I guess that could be at least partially fixed by using two capacitors, one with positive and the other one with negative temperature characteristic just like it was done in old radios. I remember that simply moving finger close to the RC circuit produced tone after a while, when the capacitor slightly warmed up from body heat.