A Simple 1+9 Home Exchange


Home Exchange Rack

This simple exchange allows up to nine local extensions to interconnect with each other and allows them to make or answer calls on a BT line. Up to two connections can be made at the same time.
The equipment consists of nine line circuits, two linefinders with an allotter, two connect circuits which each handle a single digit, and a power control unit.

The Linefinders

Linefinders Linefinder Circuit

The line circuits are very simple as they consist of only one relay and some resistors, capacitors and rectifiers. When a caller loops his line the 100 ohm battery feeds around the loop to operate the linefinder ST relay and mark the caller's outlet on the linefinder bank. Which linefinder ST relay is operated is decided by the positioning of the allotter which always sits on a free linefinder, or the BT relay if both finders are busy.

ST1 connects the linefinder drive circuit, ST2 prevents the allotter from moving at this stage and connects the testing circuit to the T relay. The finder drives.

When the marked outlet is reached, relay T operates to the battery from the caller's loop. T1 disconnects the drive circuit and operates the H relay. T2 operates the TA relay, slowly. H5 operates the caller's K relay which in turn extends the caller's loop to the connect circuit associated with the linefinder. This removes the start and marking conditions and relay ST releases. Relay T holds until relay TA operates. Relay T then releases. T2 releases relay TA slowly. When TA1 restores, the H relay holds to the earth now being returned on the P wire from the connecting circuit.

With ST2 and TA2 now released and H2 operated, there is an earth placed on the allotter bank outlet which causes the allotter to drive to the next free linefinder marked by a "dis" on the allotter bank.

The Connect Circuits

A Connect Circuit Connect Circuit

The connect circuit is siezed by the caller's loop being extended from its associated linefinder on the - and + wires. This operates relays A and AA from the battery and earth via the barretter. A1 operates relay B. B5 returns a holding earth on the incoming P wire and earths the power hold lead. B1 operates relay CD. B4 connects dial tone to the 570 ohm winding of the AA relay which by transformer action returns the tone to the caller. B3 earths the ring start lead. B7 and CD3 operate relay E.

The caller now dials a single digit. The release of the A relay energises the uniselector magnet and the uniselector steps to the dialled outlet. The original operate circuit for relay C is disconnected as soon as the uniselector steps but each time A2 releases it provides a current in the second winding which holds CD. The 100 microfarad capacitor across the second winding makes relay C slow to release so that it holds during the dial train but releases when relay A stays operated at the end of the digit. When relay CD releases the following four actions may occur.

1) The outlet dialled is spare. In this case the release of CD2 operates the NU relay via the strapping on the bank 4 outlet. NU2 connects NU tone to the AA relay and the caller. Any further digits dialled are ineffective as relay CD has released and disconnected the magnet circuit. Relay E releases slowly but has no function if NU has operated.

2) The outlet dialled is allocated to a BT line call. When CD2 releases, relay EL is operated via the strapping on the bank 4 outlet. Relay EL switches the caller's loop to the exchange line via relay L which operates. L1 holds relay B. Relays A and AA release but are inoperative. Relay D is operated by EL4 and in turn contact D! disconnects the ring start lead. The caller is now connected to the BT line and can continue to dial out or answer an incoming call as appropriate.

3) The outlet dialled is a busy line. When CD2 releases it is unable to operate the PT relay as the P wire will be earthed. Relay E at E4 then disconnects the testing path for relay PT. E2 released connects busy tone to the caller.

4) The outlet dialled is a free extension. When CD2 releases, relay PT operates via the strapping on the bank 4 outlet to the K relay battery on the P wire. PT1 operates relay H, PT2 holds relay PT. H4 earths the P wire and operates the line circuit K relay, so removing the bridging apparatus. H2 returns ringing tone to the caller. When relay E releases, ringing and ring return battery are connected to the called extension's line.
When the called extension answers, Relay F operates to the direct current now flowing round the loop. F5 ensures that relay F stays operated. F1 and F3 connect the called extension loop to the D relay which operates. F6 removes the ring start condition.
The two extensions can now speak to each other.

Release : When the caller clears, relays A and AA release. In turn relay B releases. B7 causes the release of relays H, PT and F. The release of H causes the release of relay D and allows the release of relay K in the called extension's line circuit.
With all relays released, the magnet drive circuit is completed and the uniselector drives home. During the drive home, the incoming P wire is earthed to hold the linefinder until the connect circuit is available for another call. Similarly relays A and AA are not reconnected until the uniselector has returned home.

The Power Control Circuits

Power Control Circuit Power Control Circuit

When the exchange is taken into use either relay PS or relay PH is operated, or both. When only PH3 is operated the uniselector will drive back to its home position. PS1 or PH1 operates relay PW which then holds via PW2 to the home outlet of arc 3 of the uniselector. PW1 earths the PWR wire which in turn switches on the mains power. The mains power can be used to directly operate the exchange or to charge a 48 volt battery during any calls and for some short time after call completion. If a 48 volt battery is not employed then a 12 volt battery can be used just to "start" the exchange by operating relay PS to any calling loop.

When all calls have been completed relays PS and PH will be released. The PR1 earth will be connected to operate relays PA and PB via PS1, PH2 and PW4. Relays PA and PB interact slowly and via PA2 and PB1 step the uniselector magnet slowly. PB3 holds the slow release relay PW during stepping. On outlet 23 relay PR operates. PR1 disconnects the stepping circuit and energises the uniselector magnet. Relay PW releases and disconnects the magnet and the wipers then step to outlet 24. PR releases. PW1 switches off the mains. The uniselector will not step onto the home position until the next call starts.

Should a further call start during the release sequence, PH will operate and cause the uniselector to drive home before completing its switch off sequence.

The Power Circuits

Power Circuit

This shows the power arrangements on my home exchange. The ringing and tones are derived from a standard rotary BT ringing machine.


Page provided by John Bathgate

This page was last updated on
15th June 2010