Simple Two-Way Intercom
This Two-Way intercom can be a time-saver when you frequently need to communicate with someone in your office without getting up. To initiate communication, activate it on both ends. Additionally, it can be valuable if you have a family member at home dealing with a COVID-19 situation. Figure 1 illustrates a block diagram of the intercom system.
The circuit diagram of the intercom is presented in Figure 2. This straightforward and cost-effective intercom comprises two condenser microphones (MIC1 and MIC2), four 2N2222 transistors (T1 through T4), as well as two LM380N audio amplifiers (IC1 and IC2).
A condenser microphone resembles a capacitor, featuring two closely spaced parallel plates. One of these plates acts as a diaphragm, constructed from an exceedingly thin and lightweight material. When sound waves impact the diaphragm, it vibrates, altering the separation between the two plates, consequently adjusting the capacitance. This fluctuation in capacitance produces an electrical signal, which is subsequently amplified.
Most intercoms lack a pre-amplifier circuit, resulting in distorted sound output. In this circuit, pre-amplifiers are employed to enhance the inputs, ensuring loud and clear sound from the speakers.
There are two identical circuits, as depicted in Figure 2. In the lower circuit, R1 functions as a current-limiting resistor for MIC1. The electrical signal from sound passes through coupling capacitor C1, which blocks DC signals while allowing AC signals to pass. The signal undergoes amplification in the stages led by transistors T1 and T2, culminating in amplification by IC LM380N.
The signal initially experiences amplification in a circuit based on transistor T1, followed by further amplification in a second circuit based on transistor T2. The output from T2’s collector is directed to LM380N (IC1) via pin 2, capacitor C5, and potentiometer VR1.
Sound reproduction occurs through an 8-ohm speaker (LS2) linked to the LM380N’s output via capacitor C7 and resistor R12. Both the upper and lower sections of the circuit require a 9V regulated power supply or a 9V battery.
The upper part of the circuit operates identically and functions in the same manner. When you speak into microphone MIC1, your voice can be heard through speaker LS2, and similarly, speaking into microphone MIC2 results in your voice being audible through speaker LS1. Consequently, two-way communication is attainable.
Construction and testing
Assemble the circuit using either a PCB or a general-purpose board. Proceed by cutting the PCB along the dotted line into two distinct sections, denoted as SET1 and SET2. Place each PCB section into its respective enclosure. SET1 should be positioned in one room, while SET2 should be located in another room. Establish a connection between LS2 in SET1 from the first room and SET2 in the second room using twin wires. Additionally, connect LS1 from SET2 to SET1 in the first room.
Securely affix MIC1 to the front of the SET1 enclosure and LS2 to the rear. Likewise, attach MIC2 to the front and LS1 to the rear of the SET2 container. To initiate communication on both sides, activate S1 in SET1 and S2 in SET2.
When you speak into MIC1 within SET1, your voice will be transmitted through LS2 in the second room. Conversely, when the other person talks into MIC2, their voice will be audible through LS1.
It’s essential to note that if the microphone and loudspeaker are in close proximity and face each other, there is a risk of acoustic feedback. Therefore, when constructing and positioning the microphones and loudspeakers on the enclosures, ensure a sufficient distance between them to prevent this issue.