Signals ranging from a few millivolts to a few volts can be used to excite a relay with this circuit that has a gain between 1 and 100 adjusted by a potentiometer or trimpot. The circuit does not require a symmetrical source and the power must be supplied with voltage according to the relay used. The relay must have a maximum coil current of 100mA. From a book by Newton C. Braga.
