With this circuit we can trigger a relay from very weak signals of audio or alternating current. We can use it in a sound alarm or with a sensor.
The relay can be any sensitive type from 5 to 24 V depending on the supply voltage and the equivalent transistors. The circuit can be installed in a small box and built on a printed circuit board.
The capacitor of 100 nF can be increased according the signal frequency so vibrations of the relay contacts do not occur. In Figure 1 we have the complete circuit of the relay.
In the figure, the relay shown has only one normally open contact, but the RL series types are reversible.
Material list:
Q1- BC548 - NPN transistor
Q2 - BC558 - PNP transistor
D1 - 1N914 - silicon diode
K1- 6 or 12 V - Sensitive relay
R1 - 4k7 - resistor - yellow, violet, red
R2 - 47 ohm- resistor - yellow, violet, black
C1- 100 nF- ceramic or polyester capacitor
Miscellaneous:
Printed circuit board, power supply or batteries, assembly box, wires, welding.
