RF2Audio

Broadband AM radio with audio amplifer, for sniffing out sources of interesting AM from things like soda machines, swipe card readers, etc.

PDF of schematic:

The magic is in U2, TA7642 AM detector with built-in AGC.

I swiped the component values from here:

https://www.futurlec.com/Others/TA7642.shtml

then added a simple audio amp.  Two of them, with independent outputs so the students could listen (with sensitive headphones, not ear buds) from one jack while recording with the other.

The coil is roughly 200 turns of wire-wrap wire on a Fair-Rite 4078377511 ferrite rod.  I wound the core on a bit of (unshrunk) heat-shrink tubing which allows the coil to be positioned near the end of the middle of the rod, affording a bit of tuning.  The more turns, the more likely the coil will pick up local RF sources over AM radio.

I deliberately didn't add a tuning capacitor.  You can adjust the sound by holding the coil, moving the coil on the rod, or touching the alligator clips connecting the PCB to the coil.

Power is from 3 AA cells – should last a long time.

The TA7642 is long obsolete, but available from surplus houses.

I've notice an infant mortality issue with the TA7642 – every tenth one or so will work for a few minutes, then die.  Not sure what that's all about, but I kept some spare sniffers to replace the ones that died during the workshop.