# 3 Intercom circuit using LM386 – OP-Amp ICs

Now I am going to show you 3 intercom circuit using LM386.

If you ever built a transistor intercom. It is good.

But you need louder and you want to try IC.

Sure we have a lot of IC, LM386, LM380, LM390.

Which is better?

What is intercom?

I will recall your memories. It is a private home communication.

Like a telephone between two points, such as upstairs and downstairs.

Convenient and safe, economical as well.

Sound good, doesn’t it? Ready to get started?

## How it works

### Basic Intercom Learning

See in the block diagram of a basic intercom. We notice it has a microphone too.

When we speak to a MIC1 of intercom 1. It will change your voice to an audio signal to a speaker (SP2) of intercom 2. And, this speaker will convert that signal to the voice is the same a sound for MIC1.

Figure1: Block Diagram Principle of great Intercom

In contrast, we speak to the MIC2 of intercom 2. Also, we hear the sound of intercom(SP1) with the same principle.

Because both intercom 1 and 2 are the same. So, I explain just only intercom 1.

What is more?

We have 3 choices. But I want to save you time. Now, LM386 is best. I recommend to use it better than others.

## LM386 intercom circuit operation

See the complete circuit of LM386 intercom.

In normal, S1 is opened to turn off the circuit. But when we communicate intercom 2. We just press S1 to turn on the circuit.

Here is step by step process:

The current of 9V battery flows through S1 to the circuit.

Then, R1 passes a limiting current to MIC1. It is a condenser microphone to transduce your voice to an electrical signal.

Next, C1 will pass this signal to amplify with IC1 at pin 3. And the stronger signal comes out of pin 5 through C3.

And, via a shielded cable (3 wires inside) to connect the speaker(SP2) without turn on intercom 2.

Now, At intercom2 will hear our voice.

Others parts function:

• Connect C4 across the power supply to make more stability.
• Indicate power-on of the circuit using LED1 and R3 is limit the current of LED1.
• Eliminate a noise signal by using C2 and R2

#### Parts lists

IC1, IC2: LM386 Amplifier

Electrolytic Capacitors
C1, C5: 0.47uF 50V
C3, C7: 100uF 16V
C4, C8: 1uF 50V

Ceramic Capacitors
C2, C6: 0.047uF 50V

0.25W Resistors, 5% tolerance
R1, R4: 30K
R2, R5: 10 ohms
R3, R6: 680 ohms

SP1, SP2: 8 ohms 0.25W speaker
MIC1, MIC2: Condenser Microphone
9V power supply circuit

Not only that.. See two circuit below. Some may have these parts in store.  Is it good, if you try them?

These are two simple Mini intercom circuit. They use IC-OP-amp as main parts.

## LM380 Small Intercom

It uses LM380 Op-amp just one. And works well. When we turn switch-S1 in a position “Talk” show in the circuit. And, switch S1 in circuit 2 to a position “listen”.

The loudspeaker of the First circuit will perform as a microphone. And this signal comes to a step-up transformer, T1. It causes a stronger signal up.

Then this signal flows into an input pin of op-amp LM380 amplifier. It will increase the sound up.

Parts lists
IC1: LM380 1W audio amplifier 14pin
C2: 0.1uF 50V Ceramic
C1: 100uF 16V Electrolytic
VR1: 100K Potentiometer
T1: audio transformer
SP1, SP2: 8 ohms 0.25W speaker

The voltage of power supply is 8-20 volts.

## LM390 Mini Intercom Circuit

This circuit is older. But if you have LM390 a lot. It may better to try it.

Also, it is a similar second circuit. This circuit speaker instead of a microphone.

Some said LM390 give louder than transistor intercom. Its power is 1wats only.

It requires voltage supply 6V to 9V.

Parts lists

IC1: LM390 1W audio amplifier 14pin
C1: 0.1uF 50V Ceramic
C2: 4.7uF 50V Electrolytic
C3: 100uF 16V Electrolytic
C4: 0.047uF 50V Ceramic
C5: 10uF 16V Electrolytic
R1, R2: 180 ohms
R3: 2.7 ohms
R4: 47 ohms

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