With this circuit it is possible to control a relay up to 100 mA with an extremely low input current, in the order of a few microamperes. The resistor R2 depends on the input voltage and the transistors admit equivalents. Different loads can be controlled and with the use of a BD135 the current can reach 500 mA. R2 can be between 1k and 10k typically and must be obtained experimentally.
