This circuit was found in a 80's Popular Electronics. The sensor can be any phototransistor. The circuit is indicated for an audio link modulated in pulses.
This simple amplifier stage with a transistor has a linear response from 25 Hz to 120 kHz. The...
The output transistors can be the TIP41 and the BD135 and BD136 drivers. TR1 can be a BC558 and...
The general-purpose thermostat shown in the figure uses a high-resistive thermistor as a sensor....