Found in a Popular Electronics from the 90s, this shield is activated by a 20 mA current. The inverter is an Open Collector TTL type. The power is 5 V.
This circuit, suggested by National Semiconductor, provides an output voltage proportional to the...
This powerful alarm circuit was found in the July 1975 issue of Radio Electronics magazine. The...
This two-transistor circuit was found in the January 1994 issue of Electronics Now magazine....