I found tis circuit in na old documentation from 1958. The circuit uses germanium transistor availble that time and hard to obtain today. The circuit is indicated for phono applications using variable reluctance transducer.
Old multimeters used a 22.5 V battery for high resistance measurements. If you have one, you...
This circuit has an opposite action to the previous one, triggering when light falls on the...
This circuit was found in a Popular Electronics from 1991. The transistor could be the BC548. The...