Intercom Circuit

Intercom Circuit is like smart walkie talkie for home use, no need of button it work hands-free with electronics.

We talk in mic and it changes voice to electric signal and amplifier make signal strong.

Furthermore, one part stop our own speaker sound, so there is no echo or noise and same signal goes to other unit and get loud again which come from their speaker.

Hence, with this, we both can hear clearly and also knobs help control volume and reduce extra sound.

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors (All resistors are 1/4 watt unless specified)	47k	2
	2.2k	2
	10k	2
	2.7k	2
	1k	2
	100k	4
	Potentiometer 20k	4
Capacitors	Ceramic 100nF	2
	Ceramic 10nF	2
	Ceramic 22nF	2
	Electrolytic 10μF	6
	Electrolytic 22μF	2
	Electrolytic 100μF	4
Semiconductors	IC LM386	2
	Transistors BC547	2
	Electret Mic	2
	8Ω Speaker	2

To begin with, this intercom use one transistor for mic boost and LM386 IC for sound power and both mic and speaker are always ON so it work hands-free.

To stop feedback noise Larsen mic part flip signal 180 degree and signal come out from collector flipped and emitter normal.

Furthermore, these two signals mix with 10uF and 22uF capacitors like 2.7k resistor and 20k knob and mixing cancel big part of own signal so speaker stay quiet with no self-noise.

Hence, the collectors of both units connect together so the inverted signal drives the other unit more strongly and clearly, as a result, this makes the sound loud and clear at the other speaker.

Moreover, it work both ways and talk in any mic with other side hear clear sound.

Also, user turn 20k knob to make own speaker more quiet and other sound more loud and second 20k knob is for volume control.

Formulas:

The LM386 IC amplifies audio to make sounds louder.

Voltage Gain:

How loud it get depends on parts resistor and capacitor between pin 1 and 8.

Formula:

Gv = 20 × Rf / Rin

• Gv is the voltage gain
• Rf is the resistor between pin 1 and 8 which controls gain
• Rin is the input resistor at pin 3

Power Gain:

Power gain is how strong sound signal becomes.

Formula:

Gp = Gv²

Hence, these formulas show the sound boost, making it easier to understand how transistors amplify audio and how to build the circuit; also the final working depends on right values of resistors, capacitors and other parts.

How to Build:

To build a Intercom Circuit follow the below mentioned connections steps:

Connection of IC LM386:

• First, pin 1 of LM386 connect through resistor R7
• Next, pin 2 connect to ground
• Then pin 3 connect to P2 potentiometer
• After that, pin 4 connect to ground
• Now pin 5 connect to speaker (8 ohm) through capacitor C9 and other side of speaker connect to ground
• Also, pin 6 connect to +12V supply and pin 8 connect to one side of capacitor C8

Transistor T1:

• Now T1 base connect to capacitor C1, collector connect to +12V through resistor R4 and then emitter connect to ground through resistor R5

Mic Connection:

• Also, one side of electret mic connect to +12V through resistor R1 and other side connect to ground

Capacitors for Filtering:

• Capacitor C5 one side goes to +12V and other to ground and capacitor C6 one side goes to +12V and other to ground

Special Connections:

• Also, use shielded cable for orange line between upper and lower circuit and then, ground of both circuits should also connect using shielded cable

Safety Tips:

• Always check all wires and parts before switching ON, solder carefully and do not overheat or short and follow circuit diagram fully.

Conclusion:

To conclude, this Intercom Circuit give hands-free talking.

One transistor boost mic sound and LM386 IC make sound strong for speaker, also this design avoid feedback noise.

Finally, this circuit is safe with low power and with clear sound.

References:

Datasheet LM386