Design and Analysis of Half-duplex Intercom Circuit Using LM386

This intercom has low cost and simple circuit, and can be used for intercom between different rooms in the office, monitoring in the baby room, etc. The communication distance can reach 2Km.

How it works

The circuit principle is shown in Figure 1. This circuit can only perform "half-duplex" intercom, that is, only one party can speak and the other party can listen between the host and the extension, but both parties cannot speak and listen at the same time. Listening and speaking are switched by switch S1, which is set at the host. The position of S1 shown in the figure allows the extension to speak and the host to listen; if S1 is turned down, it becomes the host speaking and the extension listening. The extension has only one speaker BL2, which serves as both a microphone and a receiver. The same is true for the host speaker BL1. LM386 is used as an amplifier, with the signal input from pin ③ and the signal output from pin ⑤. The capacitors connected to pins ① and ⑧ can adjust the circuit gain and are not required. The power supply voltage can be from 4.5 to 9V.

Component selection and production

The component list is shown in the table below.
 No. Name Model Quantity R1 Resistor 10K1 C1, C3 Electrolytic capacitor 10u/16V2 C2 Electrolytic capacitor 100u/16V1 IC Power amplifier integrated circuit LM3861S1 Switch double-pole double-throw 1 BL1, BL2 Speaker 8Ω 0.25W2

It is best to use twisted pair wires for BL2 lead wires to reduce interference. If necessary, a power switch can be added to avoid wasting power when not in use. The disadvantage of this circuit is that the extension cannot call the host.