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 circuit activates a relay when the water reaches the sensors. The circuit is from a magazine...
The noise comes from the PN junction of a common silicon diode. Since no gain is given by the...
We have here an interesting audio preamplifier, which can be useful to those who are looking to...