If you have to frequently get up to talk to someone in your place, this intercom can save you the trouble. You can just switch it on from both sides to communicate. It can also come in handy if you have someone observing isolation at home due to Corona. Block diagram of the intercom is shown in Fig. 1.
Circuit diagram of the intercom is shown in Fig. 2. The simple, low-cost intercom is built around two condenser microphones (MIC1 and MIC2), four 2N2222 transistors (T1 through T4), and two LM380N audio amplifiers (IC1 and IC2).
A condenser microphone is like a capacitor (also called condenser) with two parallel plates placed close to each other. One of the plates is made of a very light and thin material that acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates, and thereby varying the capacitance. The resulting fluctuation in capacitance creates the corresponding electrical signal, which is amplified.
Most intercoms do not use a pre-amplifier circuit, because of which you get distorted sound output. This circuit employs pre-amplifiers to enhance the inputs for loud and clear sound from the loudspeakers.
As shown in Fig. 2, there are two identical circuits. In the lower circuit, R1 works as a current-limiting resistor for MIC1. The electrical signal produced by sound flows through coupling capacitor C1, which blocks any DC signal and allows only AC signals to pass through. This signal is boosted by the amplifier sections formed by transistors T1 and T2 and IC LM380N.
The signal is first boosted by the amplifier built around transistor T1 and further by the second amplifier built around transistor T2. The output at collector of T2 goes to LM380N (IC1) through its pin 2, via capacitor C5, and the pot VR1.
The 8-ohm speaker (LS2) connected to output of LM380N via capacitor C7 and resistor R12 reproduces the sound. A 9V regulated supply or 9V battery each is required to power the upper and lower sections of the circuit.
The upper section of the circuit is identical and works the same way. When you speak in front of microphone MIC1, your voice can be heard from speaker LS2. And when you speak in front of microphone MIC2, your voice can be heard from speaker LS1. Thus, 2-way communication becomes possible.
Construction and testing
An actual-size PCB layout for the intercom is shown in Fig. 3 and its component layout in Fig. 4.
Download PCB and Component Layout PDFs: click here
Assemble the circuit on the PCB or a general-purpose board. Cut the PCB into two parts (SET1 and SET2) along the dotted line marked on it. Enclose each PCB separately in a suitable box. Place SET1 in one room and SET2 in another. Connect LS2 in SET1 in first room to SET2 in the second room and LS1 from SET2 to SET1 in first room using a twin wire.
Fix MIC1 on the front side and LS2 on the rear side of the SET1 enclosure. Similarly, fix MIC2 on the front side and LS1 on the rear side of SET2 enclosure. Switch on S1 in SET1 and S2 in SET2 to start communication from both sides.
When you speak into microphone MIC1 in SET1, the person in the second room will hear you through loudspeaker LS2. Similarly, when he speaks into microphone MIC2, you will hear him through loudspeaker LS1.
Please note, there will be acoustical feedback between the microphone and loudspeaker if they are close and facing each other. So, keep distance between the microphones and loudspeakers while assembling and fixing them on the enclosures.
S.C. Dwivedi is an electronics enthusiast, circuit designer and technical article writer