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.

Photoelectric Relay 2 (CB12929E)
This circuit is from an old publication but can be implemented with modern silicon transistors such as...
Simple Preamplifier (CB931E)
This circuit uses two JFET such as the BF244 or BF245 and has an excellent gain with small...
Generator 4mA at 20 mA with the XTR115 and PGA309 (CB12946E)
The presented circuit is suggested by Texas Instruments (www.ti.com) in its Application Report...