AC voltage probe (CB8185E)

This circuit for voltages up to about 20 V was found in a 90s Popular Electronics. The LED can be of any type or color.

Solar intercom (CB20503E)
This circuit can be used in demonstrations or science fairs. The circuit uses headphones and can...
Long-Range Wiretapping (CB19321E)
We found this wiretapping circuit in a publication from the 90s. The circuit allows the cable...
Tone activated Triac (CB7234E)
This circuit uses a resonant reed switch at the trigger frequency. The circuit is from General...