a Humidity Sensor (CB25721E)

When the probes of this circuit meet water, the relay closes its contacts. The circuit is from a May 1994 Electronics Now magazine.

Crossing Signal for Railroad Modeling (CB3854E)
This circuit uses two sensors that are placed at the intersection of a miniature railway, causing...
Stereo Preamplifier (CB6035E)
This circuit with RIAA equalization is from 1985 US documentation. The circuit has a flat response...
Rocket Altimeter (CB25357E)
Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990....