Phone ringer for stage use

Phone ringer for use on stage

Project (feasibility): I'm part of a theatre group and want to make a standard 'landline' phone ring on stage with an authentic-sounding ring. At present I'm assuming it will need to be remote controlled to avoid trailing wires and therefore should run from a battery but this may change.

I read that the UK ring signal is 40-70V AC at 25Hz and that the current needed is quite low - a few tens of milliamps. (Can anyone confirm these figures please?) The timing of a double ring followed by a longer gap is characteristic of the UK system and if it can't be made to sound realistic with relatively modest effort, then we'll carry on as now - just play a recording through speakers, which does the job but never sounds quite *right* to my ears.

My first task is deciding how to produce 40V AC at 25Hz from a battery. The first question is whether a transformer designed for 50Hz will work at half that frequency tolerably well, or not. If it will, I can produce the ring frequency at low voltage and then transform up to 40-70V but I feel rather doubtful. If not, I'll produce the DC voltage from some kind of inverter circuit then run a 25Hz oscillator to produce the warbly sound. Also, I don't know whether the output needs to be sine wave or will square wave have the same effect?

Other issues: the timing of the double ring with longer gaps will be by processor and assuming it needs RF to be remote controlled, I will probably use a ESP8266. It may make sense to generate the 25Hz as well as the double ring timing by processor but I'm open to suggestions.

For the current production, on stage in February, the phone rings a number of times and I want it to be triggered when required but to stop ringing when picked up (as it will of course). I'm not sure how to detect this to stop it ringing when the handset is replaced. Ideas?

This is not a definite, 'bespoke-PCB and smart enclosure with 3D-printed logo' type project, it'll be on stripboard using cheap bits; if it turns out to be difficult or expensive I may not do it and if I do, it'll be simple and not pretty!

Please first address my central concern: will a 50Hz transformer work on 25Hz. If not, how to produce 40V AC at 25Hz from a battery. Will a square wave input to a transformer be ok?

Thanks in advance for any help.

Nope a 50hz trans probably wouldnt like 25hz, it would saturate.
However if you half wave rectify it using a diode you'd end up with 25 hz pulses.
You can get power supply modules on ebay that can convert low voltage up to higher voltages:
**broken link removed**
You'd need an oscillator to generate the ring voltage.

So you could either generate the dc voltage with a power supply module, or use a transformer, if you can find a transformer with the correct voltages that is.

Bt say you can have a total Ren of 4 (I think), and a Ren 1 is 5.5mA, so 22ma is the max ring current.

The second project on this page is a line ringer, and you might be able to adapt the phone in use circuit to automatically stop the phone ringing.
The phone stops ringing when the receiver is lifted, and theres a socket where you can connect the studio desk into.
http://www.techlib.com/electronics/telephone.html

For a similar situation here in the US, I used a 120 V / 6.3 V "filament" transformer to step up a 20 Hz sinewave with no problems. The secondary was rated for 1 A and the actual current was way below that.

Processor > 50 Hz squarewave > filter/power amp (LM386?) > transformer

The sinewave does no have to be low distortion, so a simple single-pole filter should be adequate. Setting the filter cutoff freq lower than the fundamental removes more harmonic energy, but attenuates the fundamental. Fortunately, the LM386 has lotsa gain to make up the loss.

There are many threads and websites about using steps, coding, and other tricks to get a better pseudo-sine wave out of a PIC or other small processor.

EDIT - 25 Hz, not 50Hz

ak

If the half wave rectified solution above works then you may be able to use DC. A boost module to get around 50V (RMS equivalent of 70V) can be switched on and off at 25Hz by the Arduino. A bluetooth module could be paired with an Android phone and a simple terminal program to turn it on/off which means you don't need to write an App to control it. If you are comfy with HTML then an ESP can act as a web server and the phone controlled from a web page.

Mike.

Mechanical ringers in North American phones used 90V at 20Hz. The ringer mechanically resonated at 20Hz. I did not know that British ringers used a lower voltage and a higher frequency.

Look on ebay for a "BT Field Telephone" - a 704(a or b) or a 286(a or b).

Example:
**broken link removed**

(The newer 286 series) **broken link removed**

They take batteries and have a ring generator built in.

Connect two wires to the red & black line terminals and link to a master socket with the stage phone plugged in.

The ring switch will make it ring, though you have to emulate the cadence by hand.
And, you can talk to the other phone, making the conversation more realistic for the audience; talking to a dead phone always looks wrong...

How about a recording BUT instead of playing it through the PA, play it through a small speaker somewhere in the direct vicinity of the phone (perhaps on a table shelf underneath it concealed by a cloth) then the normal sound stage mikes will pick it up!

dr pepper said:

Nope a 50hz trans probably wouldnt like 25hz, it would saturate.
However if you half wave rectify it using a diode you'd end up with 25 hz pulses.
You can get power supply modules on ebay that can convert low voltage up to higher voltages:
**broken link removed**
You'd need an oscillator to generate the ring voltage.

So you could either generate the dc voltage with a power supply module, or use a transformer, if you can find a transformer with the correct voltages that is.

Bt say you can have a total Ren of 4 (I think), and a Ren 1 is 5.5mA, so 22ma is the max ring current.

The second project on this page is a line ringer, and you might be able to adapt the phone in use circuit to automatically stop the phone ringing.
The phone stops ringing when the receiver is lifted, and theres a socket where you can connect the studio desk into.
http://www.techlib.com/electronics/telephone.html

Click to expand...

I suspected it wouldn't work on 25Hz but not sure what would happen. Also AnalogKid below isn't so sure it won't but it seems unlikely to me.

I thought of DC-DC modules but a 10A one seems like overkill when I only need under 50mA.

Your linked phone ringer circuit looks a bit simpler than oneI found which is a ringer including an inverter and 25Hz oscillator for battery use but as I'd need a lot of parts I don't have, I'll look into modules first. I may return to these later.

Thanks for the confirmation on ring current.

AnalogKid said:

For a similar situation here in the US, I used a 120 V / 6.3 V "filament" transformer to step up a 20 Hz sinewave with no problems. The secondary was rated for 1 A and the actual current was way below that.

Processor > 50 Hz squarewave > filter/power amp (LM386?) > transformer

The sinewave does no have to be low distortion, so a simple single-pole filter should be adequate. Setting the filter cutoff freq lower than the fundamental removes more harmonic energy, but attenuates the fundamental. Fortunately, the LM386 has lotsa gain to make up the loss.

There are many threads and websites about using steps, coding, and other tricks to get a better pseudo-sine wave out of a PIC or other small processor.

ak

Click to expand...

You suggest the processor generates 50Hz square wave then filter it for the inverter? I can see that the wave shape is not critical so that's worth considering. Though if I need an inverter and still have to make 25Hz later I can probably use a voltage boost module.

I'll come back to other comments. Thanks all for your responses.

TYPO - I meant that the processor generates the 25 Hz squarewave directly, then filter than to an approximate sine, then run it through a small audio power amp chip to drive the secondary of a power transformer as if it were the primary. Then the original primary will be the new secondary with lotsa volts.

ak

I would avoid a transformer. Boost the voltage to around 30V and use a H bridge to produce a 60V peak to peak signal. Both of these modules (Boost and H bridge) can be had for less than $5 each on Ebay. Lot's of examples of connecting H bridge to Arduino around. **broken link removed**

Mike.

For a one off you could drive a trans with a square wave without a filter, its not 'nice' but its commonly done.
I'm surprised a 60hz trans ran off 20hz without trouble.
If your using a processor then you could safely run a 60hz trans on 20hz, by limiting the duty cycle to 8ms.

elfcurry said:

I suspected it wouldn't work on 25Hz but not sure what would happen. Also AnalogKid below isn't so sure it won't but it seems unlikely to me.

Click to expand...

I don't see why?, it's just a load of wire wound round a core, it will work (to some extent) over a fairly wide frequency range - and as the the amount of power required is tiny there shouldn't be any concerns over using a transformer at only half of it's rated frequency. Perhaps you aren't aware that mains transformers have often been used in home made valve amplifiers as the audio output transformer, where they perform 'acceptably'.

I've never used them in that way, although I have used frame output transformers from B&W TV's as mains transformers, I used one to energise the electromagnets on some old B&H PA speakers for many years. I've also used mains transformers for audio isolation in sound to light units in the past, again a common practice.

dr pepper said:

Nope a 50hz trans probably wouldnt like 25hz, it would saturate.
However if you half wave rectify it using a diode you'd end up with 25 hz pulses.
You can get power supply modules on ebay that can convert low voltage up to higher voltages:
**broken link removed**
You'd need an oscillator to generate the ring voltage.

So you could either generate the dc voltage with a power supply module, or use a transformer, if you can find a transformer with the correct voltages that is.

Bt say you can have a total Ren of 4 (I think), and a Ren 1 is 5.5mA, so 22ma is the max ring current.

The second project on this page is a line ringer, and you might be able to adapt the phone in use circuit to automatically stop the phone ringing.
The phone stops ringing when the receiver is lifted, and theres a socket where you can connect the studio desk into.
http://www.techlib.com/electronics/telephone.html

Click to expand...

I've just re-read this [bolded] and realised that each full cycle comprises a +ve and -ve halves, so if you remove all the -ve halves the frequency is unchanged. Also I think you've missed that this runs off a battery, so there's no 50Hz to start with! (I think if you full-wave rectify a sine wave you will effectively get a doubled frequency.) Though you corrected the frequency to 25Hz now.

Pommie, thanks for the HTML /Android phone idea, that may come in useful when I've cracked the 'authentic ringing' from battery problem.

rjenkinsgb I don't like this so much. I'd trust myself to get the ring timing more-or-less right but others not so much - and I'll be on stage!
Also I don't want to buy something like that only for it to sound less authentic (a waste of time and effort) and we don't need someone at the other end as actors in this group are reasonably plausible in faking half a phone conversation so it's not the aspect I'm trying to improve. Anyway this isn't high art, it's a farce but I'd like to make the phone sound right.

fourtytwo No, Im trying to get away from a recording and we don't have stage mics. It's the authenticicy of the sound that's my problem, not the location of the (backstage) sound effects speakers.

AnalogKid said:

TYPO - I meant that the processor generates the 25 Hz squarewave directly, then filter than to an approximate sine, then run it through a small audio power amp chip to drive the secondary of a power transformer as if it were the primary. Then the original primary will be the new secondary with lotsa volts.

ak

Click to expand...

Thanks, this makes more sense and if Nigel is right sounds like a good simple way forward.

I'll look through my junk for a suitable small transformer and then fake a rough sinewave at 25Hz with initially an Arduino and some sort of amp/drive. If I can get the basic ring right, then the ring timing pattern, remote control etc will be worth pursuing. If not, or it gets difficult then we'll carry on as now with a recording.

I wonder whether interrupting the signal at the primary causes any problems or if it should be uninterrupted at the primary and switched at the secondary?

Thanks for all your help.

audioguru said:

Mechanical ringers in North American phones used 90V at 20Hz. The ringer mechanically resonated at 20Hz. I did not know that British ringers used a lower voltage and a higher frequency.

Click to expand...

The figures I've seen on ring voltage vary but my recollection is something upwards of 40V and at the phone will depend on line length etc. Frequency is definitely different in US to here. I always heard phones on old US tv shows as sounding (musically) 'flat', like you should expect bad news! Ours sound (to my ears, anyway) more cheerful but with the characteristic ring-ring; pause ... repeat. Electro-mechanical bells in phones went out decades ago but the electrical standard remains (presumably) If I can't get it to sound right with modest effort and minimal cost then I won't waste time, mine or anyone else's.

dr pepper said:

Bt say you can have a total Ren of 4 (I think), and a Ren 1 is 5.5mA, so 22ma is the max ring current.

Click to expand...

I'm just thinking - if the 1 REN is only 5mA at the secondary, what current does the the 'primary' need?

Say if I find a 240V to 12V transformer and swap it round, the low voltage will require 20x the current at the high voltage side or 100mA.
Allow double that for inefficiencies including working at half its design frequency, I need to drive 200mA at 20Hz.

Yes that sounds about right, 40v at 5ma from a 20:1 transformer would be 100ma primary current or thereabouts, only the voltage will not be the primary rated voltage, so the actual power will be around 5ma x 40v approx 200mw.
I suspect that 40v is the minimum for here in the Uk at least, I'm think the exchange puts out 75v ring voltage.

A thought on trannys, the thing to bear in mind is volt seconds, or volts x seconds, if a trans is 240v : 12v at 50hz, so if you wanted to run the thing at half the frequency then halfing the voltages would put you back in the cores operating region - I think.

IMHO .. .. If all you need is an authentic ringing sound there's a mite too much thinking going on here .. .. .. ..

Depending on the style of the phone, the actual ringing sound is made by two fixed metal domes, repeatedly hit by an actuator driven by a pair of opposing solenoids.

I did something similar to this for a drama group several years ago by removing the solenoids and replacing with the mechanism out of a 9v 'underdome bell' ( Friedland, I think )
This was in the days when microprocessing was something you did to frozen peas so it was actuated by a bell push on a wire, but nowadays it would be quite a simple matter to use an Arduino or similar to copy the exact timings repeatedly. Lifting the receiver simply operates a microswitch would could be used to trip the reset. All from a 9v battery.

I'd use an IR remote to activate the ringing, there's often too much RF on stage as it is .. .. .. .

S

Musicmanager said:

IMHO .. .. If all you need is an authentic ringing sound there's a mite too much thinking going on here .. .. .. ..

Depending on the style of the phone, the actual ringing sound is made by two fixed metal domes, repeatedly hit by an actuator driven by a pair of opposing solenoids.

I did something similar to this for a drama group several years ago by removing the solenoids and replacing with the mechanism out of a 9v 'underdome bell' ( Friedland, I think )
This was in the days when microprocessing was something you did to frozen peas so it was actuated by a bell push on a wire, but nowadays it would be quite a simple matter to use an Arduino or similar to copy the exact timings repeatedly. Lifting the receiver simply operates a microswitch would could be used to trip the reset. All from a 9v battery.

I'd use an IR remote to activate the ringing, there's often too much RF on stage as it is .. .. .. .

S

Click to expand...

The phone for the current production doesn't have an electro-mechanical bell and I'm not going to mess about with its insides. My intention is to make something simple which any landline phone can be connected to, so it can be used for future productions too. I just need to provide the ring signal as though from the exchange, so the phone won't see any difference and the audience will hear a real phone ringing.

The IR remote is something I'd thought of but not because we're swamped with RF as this is quite a low-tech stage in a village hall.