Make yourself this special two-way intercom

Intercom sets are immensely useful and almost indispensable at places where a communication link between two or more persons is required. ITie installation of most of the commercially available intercoms involves the cumbersome process of laying the signal transmission wires, which results in increased expenditure and frequent failures (due to snapping of wires etc).

This article describes functional and constructional details of a simple, fail-safe, mains operated, integrated two-way intercom set. The circuit, in addition to being extremely versatile and low in cost, possesses a number of desirable features like in-built calling facility, self-contained solid-state power supply, activation indicator, APO (auto-power off) etc. As the circuit uses no transformers, transistors etc, it is almost zero) for sending and receiving signals.

Working

The circuit of the two-way intercom is shown in Fig 1. I x)w-cost op-amp IC 748 forms the heart of the circuit. Resistor R1 drops the mains voltage to a suitable level. This voltage is rectified, filtered and stabilised by Dl, Cl and D2 respectively. The 12-volt regulated supply thus obtained drives the rest of the circuit. Resistors R3 and R4 tap this voltage which in turn biases 7 the non-inverting input of IC1. Resistor R2 biases the condenser microphone. Capacitor C2 couples the same to the inverting input of IC1. Resistor R5 is used to attenuate this input signal. Resistor R6 determines the gain of 1C1. Capacitor C4 is used to couple the output signal to the speaker and to the other set/sets, while VR1 is used to attenuate the output signal to a suitable level.

The calling function is obtained by pressing switch S2. When S2 is pressed, capacitor C3 is introduced as a feedback element between the output and non-inverting input of IC1. Id thus ! begins to oscillate and an audio tone is generated at Uie output When switch S1 is pressed, the circuit gets deactivated and vice versa.

Construction

The PCB pattern of the circuit and the components placement on the PCB are shown in Figs 2 and 3. Soldering should be done using good quality soldering axe and flux. Extreme care should be taken in soldering the components, especially IC1. A socket is recommended for use with IC1. The pin configuration of the IC is shown in Fig.5.

After the soldering on the PCB has been done, excessive flux should be removed with the help of petrol. All connections with the PCB should be made using flat ribbon cables. The circuit may be housed in a suitable plastic cabinet. The circuit, being quite compact, can be enclosed in the handset cabinet itself.

Switch SI should be mounted on the cabinet in such a way that the circuit is switched on when the handset is lifted, and switched off when it is kept back in its original position. The speaker and mic. may be fixed inside the cabinet using a strong adhesive. The LED should be mounted on a LED holder. Finally, points A B and C on the PCB should be connected to a 3-pin mains plug using a 3-core colour coded cable.

Adjustment and installation

The adjustment of the circuit is very simple. Only VR1 has to be adjusted for the optimum level of output. Keep VRl in minimum resistance position which will produce a whistling sound from the speaker. Increase the in-circuit resistance of VR1 so that the whistling sound stops.

I’he circuit, if used between places having common live and netural mains wires, needs no extra connections except those with the mains lines. The connections between the two sets should be made as shown in Fig. 4. The completed intercom should always be kept facing downwards so that switch SI remains pressed and the circuit remains deactivated.

Precautions

The circuit may be irreparably damaged if any of the three mains wires (i.e.

live, neutral or ground) arc interchanged. It is advisable to colour code the 3-core input-output cable, using red for live, black for neutral and green for ground in all the sets.

No part of the circuit should be touched when the intercom is in use as it is directly connected to the mains.

Useful tips

1.The circuit may be used as a multichannel intercom by utilising digital or electromechanical switching techniques.

2. A battery back-up may be provided by connecting a 9/12-volt battery as shown in Fig. 6.

3. Use the smallest possible speaker for a compact assembly.

4. The circuit may be used as a door-answering system after increasing the value of R6 and replacing the small speaker with a larger one.

5. The call-tone frequency may be changed by changing C3.

The complete circuit (one side) would cost around Rs 30 only, excluding the cabinet.

Readers’ Comments:

In ‘Make Yourself This Special 2-way Intercom’ project in Jan. 92 issue, since the output of one intercom is connected to the speaker of the same set, how can we expect the output to reach the other end?

And, instead of IC 748C, can we use general-purpose op-amp IC 741 or any other easily available IC?

J. RAVINDRA Davangere

In our house we are having common ‘live’ and neutral mains wires but we do not have any ground wire in our electricity board. So how do we use this circuit?

MAHAVIR D. CHHAJED

Pune

Please clarify:

1. Can I use IC 741 for IC1?

2. Can a protection circuit be used, in case mains wires get interchanged?

3. If I use the circuit for a door-answering system. Is one-side circuit enough or both sides would be required?

4. Can I use loudspeaker as a mic.?

A. NAGABABU Rajahmundry

The components layout shows it as a carbon mic. while the parts list indicates a condensor mic.! Which one is to be used?

If IC 748C is not available, can it be replaced by )jA741?

The circuit as such seems excellent. This circuit was a much awaited one.

SUNISH ISSAC Calicut

The author, Mr Amrit Bir Tiwana, replies:

Its quite heartening to see a bundle of letters from EFY readers for my intercom design. Some common queries are replied below:

The output will reach the other end since the two speakers are connected in parallel.

As for IC 748 used, on practical verification it was found that IC 741, specifically makes |aA741 and LM741 in 8-pin DIPs, can be directly used instead. Results were not found to be as good with some other makes and with the TO-3 metal-can package and RAM741 and CA741.

Although this design will work best without additional wires, in absence of ground wiring one may run an extra wire, or run the circuit on batteries (9 to 12V) like a normal intercom.

No protection circuit may be feasible at a reasonable cost and is not needed—provided you take care in wiring and use a plastic enclosure for housing the intercom.

The terms condenser mic. and carbon mic. are invariably interchangeable. It is the button-sized component used in cassette recorders. The idea of using a speaker is not feasible. In any case, it would be costlier than using a Rs 7 mic.!

For using as a doorphone, you may use one side only if you wish to talk ‘one-way’. Otherwise, both sides are needed.

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