Davide Gironi looks to add a buzzer circuit on an existing intercom system.
On the external unit the button is connected to a 5V and a GND route. I suppose the 5V is a pullup connection to the unit microcontroller.
I decide to use a comparator. I find a 12V line from the external unit. I use the 12V to drive an op-amp (LM358) in comparator mode, comparing GND with the pullup line. Then the signal is inverted using a NPN transistor (2N3904). I use this signal to drive an opto-coupler (PC817).
On the other side of the opto-coupler I have to put the timer circuit. I want the rings to emit sound for a certain time after the unit button is pressed. I’ve used a common NE555 timer circuit. I’ve just to select the right resistor and cap to obtain a timer from less than 1 to more than 30 sec.
The buzzer are driven by a N-Channel Mosfet with the gate connected to the NE555 output.
This small circuit is now ready to be soldered on protoboard. It fits the enclosure of the external unit mounting.
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