Two Line CB Telephone Exchange

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, since 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 both off hook, they are directly connected to each other, with the RLA4 coil and 1K 5W resistor providing the necessary impedance to allow 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 types, with a coil current of around 30mA. RLA1 and RLA2 are SPDT types. The line current is set to around 25-35mA by means of the 1K 5W resistor and RLA4's coil resistance. Higher current can be damaging to the carbon transmitters. If RLA4 has sufficient resistance, and a high enough voltage rating, the 1K can be reduced or even eliminated. For example, it should be possible to use a 48V relay with 2000 ohm coil on its own.

RLA3 is a 4PDT type, since it requires three sets of N.O. contacts and one N.C contact. (See 'Modifications' below to eliminate one N.O. contact set). A higher voltage relay can be used here by reducing the 1.5K 5W resistor. RLA4 is SPST with N.O. contacts.

While it is normal in the telephone world to use a positive earth, this unit was built with a negative earth. It makes no difference to telephone operation. Positive earth is used to reduce corrosion effects, and it is easy enough to adapt the circuit if it is felt this is preferable. As the circuit is 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:

Assume both phones are on-hook.
At both telephones, full DC will be present, by virtue of the 1K 5W resistor, the coil of RLA44, and the coils of RLA1 and RLA2.

Assume Ext. 1 goes off-hook. Now, current will flow through the 1K, RLA4 coil, and RLA2 coil, into the phone connected to Ext. 1.
RLA4 will be actuated, and via its N.O contacts, supply is fed to the ringing circuit via the N.C. contacts of RLA3.
RLA2 is also actuated, and Ext. 2 is now connected to the ringing supply via the N.C. contact of RLA2.

The ringing 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 from 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 RLA5 are used for this purpose, with its coil driven by the 556. RLA5 is a SPDT type, and 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, which discharge the capacitor.

At this point, Ext. 2 is now ringing. Because of the sharp current pulses and imperfect power supply filtering, the ring tone can be heard in the calling phone.

When Ext. 2 answers several things happen. Because an off-hook phone draws considerably higher current than when ringing, and that there is now a DC path through the telephone, the current through the 220R increases. This is sensed by a TIP32 transistor. By means of the 1K trimpot, the TIP32 is set to conduct only when the called phone is taken off-hook. This then turns on the TIP31, actuating RLA3. Its N.O. contacts latch it so it stays on. The 220R also limits the current if the line should be shorted, and also immediately when the called phone is taken off hook, before RLA3 actuates. Note that TIP31/32 transistors were chosen primarily for their voltage rating. Their current and power ratings are overkill for this application, but were easily obtainable.
At the same time, the supply to the ring generator is removed by RLA3's N.C. contacts, and additionally, the coils of RLA1 and RLA2 are short circuited so that audio voltage is not lost across them, as well as causing both coils to deactivate, so that the two phones are connected directly to each other via the N.C. contacts.

Now the two phones can talk to each other. At the conclusion of the conversation, when both phones are placed on-hook, current no longer flows through RLA4. This deprives RLA3 of current and so de-latches it. The circuit is now dormant, and draws no current.

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, some simplification could be had with the existing circuit, by replacing the TIP31 with an SCR. The N.O. latching contacts on RLA3 would be made redundant, along with the associated 1N914.

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, an audio amplifier IC, and a step up transformer. RLA5 would be deleted. The secondary of the transformer would connect between the N.O. contacts of RLA1 and RLA2, and where the N.C. contact of RLA5 was. The ring voltage would therefore be superimposed on the 50V DC supply.
The 556 could then be replaced with a 555, since it is only required to provide the ring cadence. The 555 output would switch the 17Hz oscillator. Some thought would have to be given towards the transformer and the DC supply to its driver.
Of course, such a modification will increase the complexity, and will be getting away from the present simple minimalist design.

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.
To improve current sharing, the coils of RLA1 and RLA2 could each have 1.8K-2.2K resistors in series with them, and the 1K in series with RLA4's coil deleted. A non polarised capacitor of at least 3uF would need to be connected between the N.C. terminals of RLA1 and RLA2. This is required to maintain the AC speech path between the two phones, since too much audio voltage would otherwise be lost across the 2.2K resistors.

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