Telephone Ring Generator MK II

Do you own, collect or perhaps restore the older style of telephones which used a pair of gongs to annunciate an incoming call, AND long to hear the nostalgic peel of those bells ?

Presented here is a design for a "Telephone Ring Generator" which when connected to a suitable telephone will cause the bells to ring in the correct "cadence"- for what I might call the "British Post Office" type format. (in common use in many parts of the world)

From my research, the ringing sequence is such:
400mS ring, 200mS silence, 400mS ring followed by 2 second silence (3 second total - then cycle repeats) The ringing frequency of the bells is reckoned to be about 25Hz

The ringing sequence used in the USA is 4 secs ring, 8 secs silent, 30Hz ringing frequency, then cycle repeated. The design could in fact be "re-hashed" to deliver this format, a later update may follow.

From my own recollection of telephone exchanges c1960/70's the ringing (and also busy and disconnected ) signals were generated by micro switches actuated by sectored discs all rotating on the shaft of a constant speed motor !

Whilst there are designs and ideas on the internet that try to achieve the above; they range from feeding raw 110v mains AC into the phone (crude and dangerous!), oscillator into audio amplifier feeding the phone (may work but somewhat messy), to electronic devices using free running RC oscillators and a lot of associated components (high parts count) .

Often the need seems to be, in the Theatre where a telephone is required to ring during a stage performance.

The following design uses readily available components and utilizes the mains frequency as a reference for all needed frequencies and timings.
If we are to believe that AC mains frequency is held to very close tolerances then our ringing generator will also be very accurate !

Referring now to the schematic:

A plug pack or similar is used to obtain a low DC voltage (15v) to power the electronics - by conventional means; bridge rectifier, filter capacitor, 3 terminal regulator etc. Nothing new here.
Additionally low voltage AC from the transformer secondary is sampled through a diode to provide a unidirectional 50Hz signal which can be used as a "clock"
This positive going half-wave sinusoid is fed to a 4093 Schmitt trigger, its action being to produce a fast rise (and fall) square wave necessary to clock following logic IC's.
A clamping diode b/w the Schmitt input (pin 2) and the Vcc rail precludes the input from rising greater than one diode drop above Vcc, excess voltage being dropped across 1K resistor.
This cleaned-up 50Hz "clock" signal is now fed to a 7490 where divide by 2 gives 25Hz (for ringing frequency) and further divide by 5 give 5Hz or a 200mS interval suitable for producing the correct ring cadence.
The 200mS interval pulse train feeds a 4040 ripple counter which will advance its count for each falling edge of the incoming clock.

The sequential outputs of the 4040 are connected to the address lines of a 74150, being a 1 of 16 "multi-plexer" Each of 16 inputs is sequentially switched (in inverted form) thru to the output. The 16 inputs are hard wired to Vcc or Gnd as necessary to give the correct cadence (as stated above) for the ringing signal.
This "gating" signal is used to enable on and off the 25Hz ringing signal.
Gating is achieved in the 4093 NAND gate, its output then inverted directly feeds a VMOS FET gate which switches the unregulated 15v rail onto the transformer primary.
I preferred a VMOS FET over a bipolar transistor etc here because of their power handling capacity and very low "on" state resistance (milliohms) which allows most of the +15v DC rail be available to the transformer winding.
I recommend that the VMOS FET have some form of heat sink (even if small).
Fortunately these devices also have an internal reversed biased diode, b/w drain and source which protects them against failure from transient spikes (refer to manuf' data sheets).

The particular FET specified 2SK3296, I salvaged several from defunct P4 computer mother board (in power supply circuit). However suitable replacement types are easily available.
I have used a 5000 ohm to 4 ohm loud-speaker transformer (in reverse - so to speak) to step up the voltage to the level necessary the make the bell solenoid actuate the armature (1 : 35 ratio)
The specification for signal required to ring a telephone bell was a 25Hz , 75v swing (usually with a neg 50v DC bias).
The prototype I have managed to fit into a 4 x 3 x 2 inch diecast box, and I use wire-wrap sockets to mount all the components.

In summing up, I have constructed and tested the above design and find it works well, and the telephone ((((((( ring ))))))) is just as you might remember it !