Door intercom is very useful for home security. If you receive a door knock or door bell/alarm, you can push this intercom knob and ask the guest to identify his/her self, before deciding to open the door to welcome. Here is the schematic diagram of the circuit:
To install this door intercom, except for right side speaker, you should place all circuitry indoor. The right side speaker should be place outdoor near the door alarm knob, where your guest can hear from and talk to this speaker/mic. If there is someone knocking the door or pressing the guest alarm knob, you can switch on this circuit, pressing the DPDT switch to ask the guest to identify him/herself. After asking him/her, you have to depress the DPDT switch to hear the answer. As shown in the diagram, the default DPDT switch position is the outdoor as microphone and the indoor as speaker. Pressing the switch will interchange their function. Almost any type of small NPN transistor can be used for the transistor.