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Old 31st Oct 2017, 10:02 pm   #1
Sparks
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Smile Telephone cradle switch(es)

Please indulge me for these questions. On any given corded telephone, what exactly do the switches connect and/or disconnect when the handset is lifted and replaced ?
How many switches are there ? Do they operate simultaneously ? Are they simple on/off switches or is there a changeover function ?

Thankyou !
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Old 31st Oct 2017, 10:41 pm   #2
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Default Re: Telephone cradle switch(es)

On 'traditional' dial telephones, there are two switches that operate when the handset is lifted. One set of contacts makes the loop by providing a low-impedance path between A and B lines by putting the induction coil, microphone and dial-pulse contacts (normally closed) in circuit. A relay at the exchange ('A'-relay?) detects the change-of-state and dialling tone is sent to the telephone.

The second set of contacts makes a spark-quench circuit by shunting the dial-pulse contacts (via a further dial contact called a 'dial off-normal' contact) with a CR circuit. This gives a clean dial pulse to signal to the exchange.

Both switches are lift-to-make on older (332) telephones, and are not changeover contacts. They operate simultaneously. More modern 700 series phones use a C/O contact on the spark-quench cct, but one set of contacts is strapped together. GPO instruments were made to be flexible and so this contact may have another use (of which I can't think of right now) dependent on its configuration.

I can't speak for anything more modern than an 8782 DTMF push-button instrument with bell, other than to say there's at least one switch for making the loop and seizing the line.
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Old 1st Nov 2017, 6:19 am   #3
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Default Re: Telephone cradle switch(es)

Most '3 wire' telephones -- those with the separate bell shunt wire to pin 3 of the BT socket -- have 2 contacts on the cradle/hook/gravity switch. Both make (close) when the handset is lifted. One connects the telephone to the line, the other connects the bell shunt wire to the 'a' line wire (via a zener network sometimes) to for the anti-tinkle function. In a lot of cases the second contact is designed to make before the first when the hanset is lifted and break after the first when you hang up.
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Old 1st Nov 2017, 8:43 am   #4
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Default Re: Telephone cradle switch(es)

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Originally Posted by russell_w_b View Post

The second set of contacts makes a spark-quench circuit by shunting the dial-pulse contacts (via a further dial contact called a 'dial off-normal' contact) with a CR circuit. This gives a clean dial pulse to signal to the exchange.
This set of contacts also prevents the series bell capacitor being by-passed (shunted) and ensures the a.c. ringing signal is not routed to the bell partly via the receiver and the microphone, which it would do if there were only one set of contacts breaking the loop.
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Old 1st Nov 2017, 3:50 pm   #5
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Default Re: Telephone cradle switch(es)

Thankyou Russell and Tony.
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Old 1st Nov 2017, 4:19 pm   #6
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Default Re: Telephone cradle switch(es)

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Originally Posted by Sparks View Post
Thank you Russell and Tony.
You're welcome. Your question got me thinking and I looked into a tele 706 with the regulator strapped out as was done on long lines.

The equivalent d.c. resistance shunted across the capacitor via the second gravity switch, the mic, the receiver, the bridge resistor and the ASTIC is typically 830 ohms, although this may go higher or lower depending on the state and position of the carbon mic. As this is in series with the bell-bobbins there would be a d.c. resistance across the line of 1830 Ohms, which would sink 27mA and probably not seize the line. The continuous passage of this current through the carbon mic would eventually 'fry' it. So this second switch contact is also an indirect break to keep d.c. out of the speech circuit.

If the mic resistance dropped to, say, 230 Ohms, then it might just seize the line.

I'm sure I've seen some continental instruments where the gravity switches are more simply arranged though.
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Old 1st Nov 2017, 7:12 pm   #7
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Default Re: Telephone cradle switch(es)

If you look at telephones made for non UK system with only 2 wires, it is almost the same. The splitting from 2 to 3 wires done in the UK master socket i just moved in to the telephones used in many other countries. The minimum you need if you have to improvise is one single contact breaking the connection when you go on hook, most telephones has more due to the need of not having any loud clicks in the receiver when you press the hook-switch, and pretty often on European telephones something disconnecting the ringer when going off hook is pretty common.
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Old 1st Nov 2017, 10:17 pm   #8
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Default Re: Telephone cradle switch(es)

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Originally Posted by dagskarlsen View Post
'...and pretty often on European telephones something disconnecting the ringer when going off hook is pretty common.'
Thanks, Dags. This is what I was on about, and I'm pretty sure there's been a circuit on here of some continental telephones where the bell is routed via the gravity switch.

The other thing I neglected to mention is that d.c. should not flow through the receiver or it will eventually become depolarised. On the 706 instrument hitherto described, this would amount to a quiescent d.c. of about 23mA if the capacitor was shunted by the speech network.
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Old 2nd Nov 2017, 5:46 pm   #9
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Default Re: Telephone cradle switch(es)

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Originally Posted by dagskarlsen View Post
'...and pretty often on European telephones something disconnecting the ringer when going off hook is pretty common.'
I've just dug out the circuit-diagram for my 1968 Ericsson Dialog (model No: 115.139) and the gravity switch is indeed configured like that.

There are two sets of contacts: one N/C and one C/O set. The bell capacitor sits before the changeover contact, and when the handset is on rest, the line A is routed via the N/C contact of the C/O set to the bell and back to the line B.

When the handset is lifted, the N/O contact of the C/O set closes, switching the bell capacitor into series with a resistor (spark-quencher) across the dial pulse contacts, and the N/C contact opens, disconnecting the bell from the line.

The other, N/C set of contacts is connected directly to the A line and switches in the mic and pulse contacts.
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Old 2nd Nov 2017, 9:05 pm   #10
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Default Re: Telephone cradle switch(es)

Here is one of the more complex cradle switches:http://www.classicrotaryphones.com/f...h=111769;image

A more typical one used in Norway:
http://www.classicrotaryphones.com/f...ch=74833;image

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