Video intercom additional buzzer circuit

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.

See the video below and more on the blog.

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