This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
The LM3911 from National Semiconductor (Now Texas) is a temperature sensor that provides an output...
Found in a 1990 Electronics Handbook, this circuit causes an LED to flash when the phototransistor receives...
This circuit was obtained in a Portuguese publication in 1979. It is a simple code switch that...