Two way Simple Very Small Telephone Exchange

This is a very small telephone exchange attached only two ordinary Tele phones. Fully bi-directional facility with an intercom system. When a phone is in off hook condition , Exchange will detect it & send ringing pulse to other phone. When it will be picked up , the connection will be established between both phones. This will be taken place vice-versa.

Micro Controller = PIC16F84A
Firmware version = 1.00
Power Supply = 230 v

Two way Simple Very Small Telephone ExchangePower Supply Unit

The Power Supply  Unit.
This is used normal step down transformer (400ma) for generating three type of deference voltages & 100 AC volt for rigging.

It doesn’t provided a PCB pattern for the power supply unit. So a Vero board can be used to build the unit.

Parts List :

1.Step down transformer ( 230 V to 30 V , 15 V  & 100 V )
2.Two bridged 1 A rectifiers
3.7805 – 5V regulator
4.7812 – 12V regulator
5.1000 uf – 35 V
6.1000 uf – 16 v
7.150 ma fuse & it’s holders.
8.Necessary Wires & a wire code for AC supply.

The controlling unit ( Exchange )

This circuit is base on PIC16F84A microcontroller. Two slimier SLIC are connected to The controlling unit. It needs to identify following connections on the controlling unit circuit diagram.


One SLIC connections should be marked as,

1. TN_SLIC_1
2. SD_SLIC_1
3. RG_SLIC_1

Other SLIC connections should be marked as,

1. TN_SLIC_2
2. SD_SLIC_2
3. RG_SLIC_2

These connections should be connected as on the controlling unit circuit diagram.

Two way Simple Very Small Telephone Exchange SchematicParts List :


1. 2SC945 x 3


1. 1N4007 x 1
2. 1N4148 x 2
3. LED x 1

Capacitors ( 16 V )

1. 4.7 uf x 4
2. 22 pf x 2


1. 10 K x 4
2. 4.7 K x 4
3. 330 x 1

Other Parts

12 V telephone rely x 1
Necessary wires
PIC16F84A with the firmware


For more detail: Two way Simple Very Small Telephone Exchange

About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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