Uniselector circuit

[TrevorG3VLF]

I would like to switch a co-ax cable to various aerials and thought that a uniselector could give me 10 options. The pulses would go down the co-ax with the RF. Has anyone got a circuit to drive the uniselector from a dial telephone?

Thank you,
Trevor

[Sean Williams]

As far as I can tell the dial on the phone sends a number of pulses - a decent fast relay being driven by the dial will then be able to manage switching the uniselector coil.

I have to admit the thought of a Uniselector being used at RF fills me with horror - probably ok for low power and low frequency, but a liability anywhere else....

[Amraduk]

Uniselectors were used in some (at least) vinatge TV O/B vans for switching video signals!

Regards,

Dave.

[pmmunro]

Uniselectors were also used in studio centre video routing matrices, e.g. Broadcasting House, Glasgow.

PMM

[val33vo]

Trevor it will give you 25 options with 5 gangs as I remember, uniselectors had more contacts than group or final selectors but of course only had one level of bank access rather than 10

I would imagine a transistor like a 2n3055 would be more than up to the job of pulsing a uni from a phone dial, ( remember to connect a diode across the coil to stop the back emf,s killing the transistor ) a thought springs to mind why not use a multi position rotary switch to mark the desired contact on one of its banks and get the uni to self drive to the required position via its interruptor

You do know these things are designed to run off 50v ? , there were several types of uni, the large ones about 9ins x 9 ins were usually fitted in the subscriber line circuits, the smaller type ( about 3ins by 3ins ) in register translators and register access relay sets and the largest type the dreaded MUG ( motorised uniselector ) in trunk access equipment due to their much faster hunting capabilities

what a trip down memory lane

regards

val33vo

[brianc]

Originally Posted by Amraduk

Uniselectors were used in some (at least) vinatge TV O/B vans for switching video signals!

Regards,

Dave.

In the telecinearea in the Lime Grove studios, the BBC used motor uniselectors for routing telecine machines to studios. They were big brutes with about 20 banks of 50ish ways. These switched Video, return video, sound, cue sound, telephone, talk back, buzzer and tally. For the video circuits, every other bank was grounded (screens) and every other way was also grounded - two steps required for each "way". Thus a quasi-coaxial switch was formed which measurements showed to have a close approximation to 75 ohms. The system in Lime Grove was used for a short time only as soon after installation, a nearby brick wall was demolished which made it very unreliable!

[TrevorG3VLF]

Thanks for the replies, I was intending to use the uniselector to switch the aerial directly at low frequencies but of course it could be used to drive co-ax relays at high frequencies.

The intention was to use a normal dial to generate the pulses but I do not know how to reset the uniselector to its home position. I have a single layer uniselector somewhere (mislaid), if I remember right, it would work on 24V. I still have a dial phone in my house, I asked them to leave it so that I still had a bell. This works, presumably with 50V, at about 3 miles from the exchange.

The alternative scheme is to use a transformer, two changeover relays and a handful of diodes to give four outputs.

Thanks again.
Trevor

[Dave Moll]

If I remember correctly, the working voltage off-hook is about 10V, though 'phones are not fussy about the voltage. In fact, I have run many mechanical 'phones (as opposed to newer, electronic ones) on as little as 3V (as long as you don't replace the carbon granule transmitter with an electronic one - though these seem to be happy from 4.5V upwards).

I've never tried with as much as 24V, so I'll leave it for others to confirm whether there are any problems associated with doing so.

[Pellseinydd]

As mention before - most uniselectors run off 50 volts DC and take something like 1 amp. The circa 10 volts line voltage (at the phone) is used to operate a pulsing relay at the exchange contacts of which step the uniselector. A few smaller internal exchanges had 24 volt uniselectors but they are not that common.

[TrevorG3VLF]

The pulsing relay sounds right. So how did the exchange reset the uniselector to its home position when the phone was put down?

Trevor

[Sean Williams]

Er I think there are a pair of contacts used on the uniselector that would be used as a reset position when the line rings clear the drive relay is stepped around to this "holding pair"?

Just guesswork really - cant find my copy of Telephony to tell you for sure.

[Station X]

Uniselectors weren't stepped in response to dial pulses. When the phone was taken off hook the uniselector "hunted" round its outlets for a free first selector. The customer's line was connected to the first selector which returned dial tone. At the conclusion of the call the uniselector self stepped to one of two homing positions.

[Sean Williams]

Duh! teaches me for thinking out loud!

However, my idea might well work in this application.

[mawdsjeff]

The pulsing circuit is a normal relay (1000 ohms on 50V) in series with the dial and an on/offr switch. If you use an old phone, you don't need this switch! Call this relay "A", and one make contact operates "B". This relay must keep in during pulsing and GPO ones had a copper "slug" at one end of the coil, so eddy-currents circulated and kept the armature in for a while. You can slug this relay with a 250 MF 100V electrolytic instead. So now, when you dial, A relay follows the pulses, but \b should stay in during the" contact of pulsing. You connect a "break" contact of A and and a "make" contact of B in series with 50V and the uniselector. This will then pulse the same number of times as the dial contact, because you have converted the break pulse of the dial into make pulses to the uni coil. A further addition is the homing of the uni after use! Use a "Break" contact of B in series with 50V, the uniselector coil, the break interrupter contacts on the uni a spare wiper and the homing arc if there is one, (a solid arc of contacts). If not, make one by strapping all the contacts together except the home contact that it will rest on. It would be easier with a circuit to have explained this! I first did this 50 years ago as a lad of 14 with an ex-govt surplus shop uni. Now I have cord switchboards, automatic exchanges and a telephone cord braiding machine. Geoff Mawdsley

[Pellseinydd]

Originally Posted by Station X

Uniselectors weren't stepped in response to dial pulses. When the phone was taken off hook the uniselector "hunted" round its outlets for a free first selector. The customer's line was connected to the first selector which returned dial tone. At the conclusion of the call the uniselector self stepped to one of two homing positions.

A number of GPO/PO/BT exchanges used uniselectors to receive dial pulses from the callers telephone including PABX 5 and 6's and the Island Automatic eXchange No5's that served some remote Scottish Islands. Many smaller PAXes also use uniselectors to receive dial pulses.

[val33vo]

Uniselectors were not ( usually in my experiance ) stepped by a rotary dial they self hunted for a free group selector by testing the p wire,s on all the availlable group selectors, The voltage at the subscribers telephone would have been 50v unloaded ( on hook ) supplied by the L relay associated with the uni, as far as I can remember when the sub lifted the phone off hook the L relay ( probably a few hundered ohms coil resistance ) operated via the loop applied by the off hook telephone, The L relay would have connected the exchange battery to the drive magnet of the uni via its interruptor contacts, when a free group selector was found its P ( private wire )wire would have had 50v on the bank contact it was wired too ( a busy group selector would have had its p wire earthed ) If I remember correctly there was a C relay associated with the uni also and it would have operated ( due to 50v being on the contact ) and cut the drive to the uni,s magnet when a free group selector was found

The way they self homed was via the interruptor contacts connected in series with a homing bank ( ie all contacts apart from one wired to earth ) probably connected in series with either a contact off the L or C relay ) but its 23 years ago since I worked in a strowger exchange so my memory is growing dim

regards

Val33vo

[Pellseinydd]

Use as 'Linefinders' was but one of the many uses for uniselectors. In larger exchanges, they were used that way but I can think of a number of exchange systems which used uniselectors to respond to dial pulses - I'm sat two feet from one now that did just that - the GPO exchange that used to serve the Island of Foula http://www.foulaheritage.org.uk/index.htm until 13th July 1995. You can hear it clicking away in the background in an excerpt from a BBC Radio 4 programme about telephones that can be found on a Lerwick phone number (01595 70 8222) - it is a geographic phone number so may be included in your 'free calls' allowance?

Numerous Private Automatic eXchanges by various manufacturers used uniselectors to receive dialled digits to step uniselectors and the circuitry mentioned by Geoff Mawdsley is straight from one of those PAXes.

Have you come across uniselectors that could be stepped in either direction? The GPO used those as well - I've a couple of those amongst the various ex-GPO exchanges and ex-GPO equipment I've got.

[Brian R Pateman]

I've discovered this thread rather late in the day.

Many years ago a power station I worked at used a Uniselector system to search round a number of resistance thermometers monitoring temperatures on a boiler.

As I recall it gave more trouble than it was worth and I replaced it with a simpler relay based system.

Not much help to the OP but it does prove that it could be done. I think it was manufactured by Cambridge Instruments or EIL but I can't be certain after all these years.