The sensors of this liquid alarm are formed by two pieces of wire with the ends peeled off. The relay depends on the supply voltage that can be between 3 and 12 V. The transistors can also be the BC548.
This sample and hold circuit configuration was found in a manual from the 70s. The operational amplifiers...
This small MW transmitter emits its signals at a distance of a few tens of meters, depending on the...
Found in American documentation from the 90s, this circuit uses common FETs. It has a range drift...