Telephone ringer project

[Thomato]

Dear all,

I am in many amateur dramatics societies and in one of them we would like to make two telephones ring onstage. This involves sending a signal down a line to the phones. I was hoping to find an old telephone exchange system, but no luck yet.

I have found a few ideas for circuits, but would ideally like to make a ringer output that is realistic. This would be about 70-80v at 25Hz. I cannot find any way to make a basic analogue frequency divider, so wonder whether my best bet is as follows:

I use a 3:1 transformer to take mains UK voltage from 240v to 80v.
I then full-wave rectify that to make 80v DC.
I use an oscillator circuit to create 25Hz and put it through a relay (power transistor?) to pulse the 80v into a square-wave AC signal.

Would this work, or can anybody think of a better/simpler way of doing it, please?
Also, as it's a long time since I did any real electronics (GCSEs in 1997!) could somebody please remind me of the basics of creating the 25Hz please?

Many many thanks!
Thomas

 

[Gary B]

We used to use “subcycle” transformers to generate 30 hz from the 60hz power line. It is basically an LC tuned winding of a transformer. Personally, I think it is easier to just use the power line frequency. Frequency selective ringers for use on party lines (don’t have many of them anymore) operate from 20 hz to 60 hz. No matter which frequency is used with the old two-bell ringers, the tone is going to be the same and I cannot tell the difference between a bell tapping at 60 hz from one operating at 20 hz. Note that the ringer is strictly AC and is blocked from the 48 volt DC talk battery by 0.1 μfd 200 volt capacitor in the telephone’s network.

 

[Thomato]

Thanks Gary!

Basically, then, I should be able to just use a 3:1 transformer and connect the 50Hz signal to the ringers? That'll be fantastic! Wiring, here I come.

 

[canadaelk]

Check out theHV440, app. note AN-H37 at Supertex.com. It works quite well. E

 

[ericgibbs]

Thanks Gary!

Basically, then, I should be able to just use a 3:1 transformer and connect the 50Hz signal to the ringers? That'll be fantastic! Wiring, here I come.

Why not a MP3 player loaded with ringer sounds.???
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[Thomato]

A proper phone ring doesn't cut off in the same way as an MP3 file. If someone picks up an old bell phone there's a distinctive "ding" from the handset as it finishes ringing, an echo. If you hear them both you'll realise the MP3 doesn't sound quite authentic because of this. By the time you've set up the system to work the echo onto the end of the file then also make sure it's ringing in stereo and sounds as though it's coming from the stage it makes sense to just make the things ring.
Thanks though!

 

[Gary B]

FYI: In the US the ringing cycle is 2 seconds on and 4 seconds off. Ringback tone (what the calling party hears) is 1 second on and 3 seconds off. If burst ringing is enabled the first ring can be as short as one second to as long as 3 seconds. The Central Office puts one second of ringing on the line before switching to the ringing machine, so the timing of the first quiet period could be anything from 0 to 4 seconds.

If you really want to sound authentic, the timing or cadence of the ringing and silent periods that people recognize without even realizing that they are doing it. Use a stop watch to check the cadence of your local phones and duplicate that as closely as possible. Since most countries adhere to TIA/EIA 464D standard, the 2 on/ 4 off is probably what your audience will expect. If you are trying to simulate a system that was in use before 464A was adopted (late 1950’s and earlier) the cadence could be almost anything depending upon who built the CO.

 

[BobW]

I built a ringing circuit several years ago in order to use some regular telephones as an intercom system. It's a simple multi-vibrator using a couple of NPN power transistors driving the centre tapped secondary of a small power transformer. The output is taken from the primary. I could post a schematic, but it will take a couple of days to dig it out, since I'm out of town at the moment.

Edit: Here is the circuit:
**broken link removed**
The transformer a small power transformer 120 primary, 12 centre tapped secondary, wired as step up (ie., reversed windings).
I no longer have the exact values of the resistors, but they are in the 100 to 270 ohm range and should be 5 watts. R3 is in the 10 to 50 ohm range and can possibly be eliminated depending on the input voltage. The frequency is dependent on the transformer and the R1, R2 resistor values. So, you will need to adjust the resistor values to suit the transformer. The circuit is admittedly crude, but I found it to be very reliable.