With this circuit it is possible to excite direct current relays from an alternating signal, either from the low voltage network (secondary to a transformer) or audio. The resistor R1 depends on the voltage of the input signal, being 1k2 for signals up to 6 V. The maximum current of the relay is 100 mA and R2 depends on the signal strength, having values typically between 1k and 22k. The circuit is published by Newton C. Braga. For currents greater than 100 mA use the BD135 (up to 500 mA).
