Two Line Central Battery Telephone Exchange.


Connect your vintage CB phones together with this simple exchange. Shown here are a 300 series and 800 series types of phone.

When a simple point to point telephone is required, magneto telephones are the first choice. No exchange or interface unit is required. The hand generator in each phone can ring the bell in the other directly. As magneto telephones are local battery, there is no requirement for a DC supply for the line between the phones.

The unit described here allows central battery telephones to be used for the same purpose; i.e., a simple point to point intercom. As manual telephone exchanges are no longer extant, except in museums, about the only thing that CB telephones can be used for, in the modern day, is as an extenstion on an automatic line, to take incoming calls. It should be noted that automatic (dial) telephones are identical to CB types except for the dial connections. In fact, the same chassis is used in both types, with the loop disconnect contacts in the CB version bridged out. It is of course possible to use an automatic telephone on a CB exchange, but the dial is not functional.

Because of the simplicity of the circuit, certain types of telephone equipment will not work. The phones must have a mechanical bell. Electronic ringers, or opto coupler circuits used in answering machines, modems, fax machines, etc. will not respond to the ring current produced by this exchange. However, given that all vintage CB telephones do use a mechanical bell, this is not seen as a limitation. The most common types of CB telephone in Australia are the series 300, 400, and 800 types. There is no reason the circuit wouldn't work with British 700 series and U.S. 500 series types, among others. It has also been tested with Ericofons, but the ringing buzz is slightly subdued compared to that from a normal exchange.
I built this unit in the late 1980's and it was used in a large house that I then lived in.

Design.
The exchange operates entirely off 40-50V DC. This could come from batteries, as there is no drain with both phones on-hook. Here, a 32V transformer provides the supply via a half wave rectifier.

The circuit comprises three sections; 1) the speech circuit, 2) the ring circuit, and 3) the off-hook-when-answered detector circuit.
When the phones are in use they are directly connected to each other, and so relay 4 coil and the 1K 5W resistor provide the necessary impedance to isolate the DC supply from the phones, whilst allowing the speech voltage to be developed across the line. There is no DC isolation between the phones when they are both off-hook and in use. All the relays are 9-12V SPDT types, with a coil current of around 30mA. The line current is set to around 25mA by means of the 1K 5W resistor. Higher current can be damaging to the carbon transmitters.
Relay 3 is the most complicated, requiring three sets of N.O contacts and one N.C contact. A higher voltage relay can be used here by reducing the 1.5K 5W resistor. Similarly, if relay 4's coil was rated for 40-50V and had a suitable coil resistance, the 1K 5W could be eliminated.

While it is normal in the telephone world to use a positive earth, this unit uses a negative earth. It makes no difference to telephone operation. Positive earth is used to reduce corrosion effects out in the real world, and it is easy enough to adapt the circuit if it is felt this is preferable. As the circuit is only earthed through the mains supply, it is simple enough to earth the positive rail instead.

How it works.
Now to describe the sequence of operation:


The ring circuit is based around a 556 timer IC. This is simply a dual 555. One section provides the ring frequency (2.2uF and 10K), and the other section determines the ring cadence (100uF and 22K). The 556 is powered at 9.1V by means of a zener diode and 1K 5W dropper resistor.
The method by which the ring current is created is unusual, but simple. It relies on the fact that CB (and automatic) telephones always have a DC isolating capacitor in series with their bells. If DC is applied to the phone, the capacitor charges up and in doing so, current flows actuating the bell striker once. If the line to the phone is now short circuited, the capacitor will be discharged and a current of opposite polarity will flow through the bell causing the striker to move in the opposite direction. If this charging and discharging of the bell capacitor is done rapidly enough, the bell will ring continuously. Incidentally, the familiar "bell tinkle" when telephones have been connected in parallel without correctly modifying the bell circuits is caused by exactly this effect.
The contacts of relay 5 are used for this purpose, with relay 5's coil driven by the 556. It operates as a low frequency vibrator with the contacts alternately charging and discharging the bell capacitor inside the called telephone. When the contacts are in the N.C position, the bell capacitor is charged via the 220R 5W fed from the 40-50V supply. When the relay is actuated, the telephone line is shorted by means of the N.O contacts.
 




Possible Modifications.

The two transistor circuit could conceivably be replaced by yet another relay. If such a relay had its coil replacing the 220R resistor, it could be made to detect when the called phone went off-hook. It may be necessary to shunt the coil with an adjustable resistor to set the sensitivity, and a capacitor might be beneficial across the coil also to prevent the ring current actuating the coil.

Alternatively, the TIP32 could be replaced by an SCR and the N.O latching contacts made redundant.

It could be possible to improve the ring generator with a proper 17Hz 90V inverter circuit. Such a circuit could consist of a sine wave oscillator, and audio amplifier IC and a step up transformer. The secondary of the transformer would connect in place of the redundant relay 5 N.C contacts. Doing this would allow the circuit to work with electronic ringers.

DC isolation between the phones is actually desirable because it prevents one phone sucking out all the current, depriving the other, if one phone should draw more current. This is not a problem with the stadard 300/400/800 types of phone, but it can be problematic when an exotic phone is used. At a quick glance, a non polarised capacitor of at least 3uF could be connected between the N.C contacts of relay 1 and 2, and the N.O contacts from relay 3 which bridge out coils 1 and 2 would be removed. The resistance of relay coils 1 and 2 would ensure a more equal current sharing between the phones.



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