yerrs. Had this issue years ago with some PABX's that couldn't ring some analogue phones.
Simple lack of voltage.
Oddly enough you can use some audio transformers designed for very old audio amps from, way back that are still available as spares to pep up the output a bit. Or a small '100V line' transformer.
I suspected as much. Come to think of it an 80V p-p squarewave was probably what the GPO used, by reversing the 40V from their battery at
30Hz, to match the mechanical resonance of old bell clappers.En el artículo , Dave W escribió:
I thought the ringing frequency was 17.5Hz, but this is from memory, so could well be wrong. A quick google was inconclusive (some sites suggest 25Hz, which is conveniently half mains frequency, but I'm not certain this applies as the ringing current wouldn't have been mains- derived.)
I remember seeing the machine that produced the old 'brrrrr' dial tone in a Strowger exchange years ago. It was a fascinating piece of kit, about the size of a washing machine with a half-doughnut shaped clear plastic cover with what looked like hundreds of contacts and a rotating disc in the centre. Tried to find a pic online but failed.
Why do you need a PIC? I used a binary counter and a couple of AND gates to give the waveform: __ __ __ __ ________
which gated an AC ringing current. This was back in 1988 when I used it to supply a prop in Japanese lessons, but I've lost the original circuit during various house moves.
jgh
The only geniune Post Office exchange I went round had a long shaft with two motors (mains and battery) and 17Hz (actually it was c/s then) generator. So, indirectly mains derived. the battery motor was in case of mains failure - there was also a big flywheel.
Yes I think you're right about 17c/s originally. This website says now
75V rms at 25Hz.:
"call"
Which may go through in less than a second or take several or even fail completely.
If actor has had to wait for a couple of seconds for the phone to start ringing they ain't going to let it ring... Then you hit the problem that the phone has been "answered" but the delay from pressing "hang up" to the remote phone stopping ringing means it's ringing after being answered.
I speak from experience of doing various forms of what the OP is doing for TV and radio. Mobiles are just not reliable or consistent enough to be involved in live action, not unless there is a plan to edit around the inevitable pauses. You can't edit a live stage play...
One overseas am-dram group I am familiar with was lucky enough to have a techy who was able to make this device work. Stage Manager pressed the button, the phone rang with the right tone (ring-ring-pause). Picking up the phone reset the system and the ringing stopped.
This is the only reliable way to do it.
There used to be a feed of Post Office 17Hz (actually 16 2/3Hz) on the bays at TvC.
I have one such device in my "shed". I built it in the early 1970s. It uses relays to get the "ring pause" bit and a thyratron to generate the ring tone.
I only went inside a Strowger exchange once (that would probably get my friend sacked these days) can still remember the noises.
And when the it breaks, you get this
That looks familiar. At GEC Trafford Park, they had their own phone exchange, and in the window was a device very like that.
They had a private link to another Manchester site, for which you dialled a prefix. The phone booklet showed a similar code for the link in the reverse direction.
Curiosity whetted, I wondered what would happen if I dialled out, then back again, and added my own number. It was no surprise that it was engaged, so I hung up. At this point it all went wrong - the bell trembled, and when picked up again the line was dead. I had to request manual intervention. :-(
Chris
En el artículo , Graham. escribió:
Thanks Graham. Not the one I saw though, I think that on in the video must be a much earlier one. The one I saw was cylindrical and painted pale yellow/GPO beige.
While I like designing stuff in discrete form, the PIC only has 8 pins compared to 28/32 for a couple of 74/CMOS chips. And a built in oscillator which does both mark/space and the pukka ring frequency.
Maplin used to produce a kit to make a phone ringer. In the days when they were interested in electronics.
Problem is being to get the required voltage and current to ring an old bell phone.
Funnily enough, my circuit running off 20v PSU and throwing out 40v P-P rang various old bell phones just fine, it was the modern warblers that didn't respond. I was actually very surprised when it worked originally, expecting that proper coils and clappers would be too much for it. It was slightly quieter and a little less crisp perhaps but very close and I was more than happy ith it (and I'm quite finickity about such things!)
Or even Light Straw.
Not really. audio amp and 100v line transformer
Or even a small mains transformer driven back to front.
Not sure what power you need butt class A for occasional use is probaly OK.
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