Many Electronic circuit projects in simple ways of learning

Simple intercom using tree transistors

This is Simple intercom circuit transistor, using transistors so easy to build and cheaper than ICs. They perfect for a small home, and to use basic or demonstration projects, from easy to find materials.
It helped we can contact together simple in the house without have to walk a various room or shouting loudly, until may have a sore throat.

How it works
The simple circuit consists of tree transistors only and a few other parts. They serves as both a receiver and a transmitter to jointly.


Figure 1 Simple intercom circuit using tree transistors

When consider operation, we start with transmitter before, by toggle switch to NO.1. Which, it determines the circuit to works as the transmitter. By loudspeaker 8 ohms will serves as a microphone, send signal from its coil, that cause an oscillation when has sound wave come to it.
The 4.5-volts battery is connected that does not affect anything with the speaker or the circuit because is DC voltage, which the DC volts will is blocked by a capacitors-C2.

The AC signal from speaker will be send to C2 and resistors-R5, then into base of Q3,then both transistors Q3 and Q2 act as high gain amplifier circuit. Which they will send signal to drive Q1, to control current that flow from the Intercom another one, and will must flow also through the speaker of that intercom.
The speaker of receiver will also emits the sound signal. Which can get from the speaker of the intercom receiver.

If you do not understand. I explain the confusion. You try to write the intercom circuit another one, connected to the same circuit. But change position switch-S1 is at position 2, it is enough to understand even more.

The 4.5-volts battery of receiver circuit is a power supply of transmitter circuit, and speaker that connected wire together only.

The frequency operation of this circuit, is designed for frequency range, on the sound speak of human, in between around 200 Hz up to 3 kHz

The building for application
Before build this projects, You must understand that it has less sound , but enough for a quiet home.

This project use a few parts so can assemble on the Perforated board. But can make a PCB as Figure 2 is Printed circuit board copper foil track layout of this project. (actual-size). Then, the wiring for circuit wiring and various components can view of the example in Figure 3 is the component layout for the PCB.

Figure 2 Actual-size,Single-sided PCB layout

Figure 3 Component layout for the PCB

In the assembly circuit, Starting equipment lowest first to Beautiful and easy to the assembly, Start from resistors and sort of high continuously.

For the device has various polarity should be careful in the assembly circuit. Before placing these components will must set the polarity at PCB and the part match together because If you put backward, may cause equipment or circuit damage. How to check polarity and input device.

The soldering iron is less than 40watts. And Use of lead solder containing lead and tin in the ratio of 60/40. Including the need also to have a flux inside the lead.

If the wiring between the receiver and transmitter very far, and measure the resistance of both wires about 0.25 ohms. Can also add voltage of 6-volts battery and D sized battery will cause can use longer time than the AA-sized battery only slightly. Or if you want to save energy using a 1.5-volts battery 2 pcs. It still works.

The components list
C1_______0.001uF___Ceramic of Mylar capacitors ___= 1 pcs.
C2_______0.1uF_____Ceramic of Mylar capacitors ___= 1 pcs.
C3_______100uF__16V___Electrolytic_Capacitors___= 1 pcs.
Q1_______BD140________ 80V 1.5A PNP Transistor______= 1 pcs.
Q2, Q3___BC549_________45V 100mA NPN Transistor____= 2pcs.
R1_______1 ohm__+5% ½W___Resistors__________= 1 pcs.
R2_______150 ohm___+5% ½W___Resistors_______= 1 pcs.
R3,R5____1K___ +5% ½W___Resistors__________= 2 pcs.
R4_______2.2M +5% ½W___Resistors___________= 1 pcs.

The switch; single pole double throw (SPDT) (3 pole two strokes.)
The wires, Speaker 8 ohms 0.25 watts, Battery with battery carrier
Remember to buy two sets for the second machine to complete.

This Post Has 6 Comments

  1. i need equivalent full circuit of “simple intercom using tree transistor”

  2. Is it possible to increase the amplification by changing values of some resistors?

  3. Hi blaise yuo-b,
    Thanks for your feedback.
    Here is full circuit, but it old.

  4. Hi Chris
    Thanks for your feedback.
    Yes, you can increase sound with changing the resistors.
    But it not well because it low voltage supply.

  5. pls sir, the two wires going to the others,where exactly am I going to connect it in the other circuit

  6. and also, ½w resistors are not available in my place. can I use ¼w instead? and what’s the effect

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