This circuit with zener diodes was found in documentation from the 90s. The zener diodes determine the lighting point of the LEDs. The circuit is for 12 V automotive batteries.
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...