With this circuit it is possible to activate relays with negative signals of very low intensity. Currents of some microamperes are sufficient to close the relay contact with coils up to 100 mA. For higher currents, the BD136 can be used at the circuit output. The circuit is suggested in a publication by Newton C. Braga. The resistor R3 depends on the intensity of the input signal and its value can be between 1k and 10k typically. Observe the polarity of the supply, with the positive to ground.
