The Pulsynetic C69 programmable impulse timer. (Bell Ringer)

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One very useful accessory for Pulsynetic impulse clock systems was the adjustable programmer. Perhaps more commonly known as a "bell ringer".  Which was its usual task in schools.

This device could be used to ring electric bells and and sound factory sirens. It was commonly used in schools to mark the beginning and end of classes thanks to its reliability and accuracy. By removing the human element, classes, or other regular events, could be timed, almost to the second, almost indefinitely. Moreover the bells would ring simultaneously throughout the entire school without human intervention.

The Gents' Pulsynetic programmable contact maker.

The programmer relies on a beefed-up version of the Gent's slave dial mechanism. Through a simple system of gears the programmer's large ring is advanced at half minute intervals by the slave mechanism. On the impulse dial mechanism itself is the minute dial. Large slaves like this were sometimes used for larger public clock dials. Though usually the hands would be behind glass to avoid "weather resistance" to the limited torque available.

The very large ring at the rear has holes at 5 minute intervals where tapered pins can be inserted to actuate the electrical contacts. (on the extreme left in the top image) The disk on the left is the day of the week dial which allows the contacts to close. Or via a simple cam on its edge blocks them from closing. By this means the bells can be silenced during the weekend when the school is normally closed. A push button (at bottom left) allows the bells to be rung continuously as an alarm. Perhaps in the case of a fire drill or other emergency. The programmer was usually installed near the master clock in the headmaster's office or the school secretary. Security from tampering is thus assured.

The rocking mercury switch provided a fixed and limited period of bell ringing of 7-8 seconds. Without it the bells would ring continuously, once closed, until the pin finally cleared the contacts.

The Pulsynetic bell ringer. Or programmable contact maker.

Left click for much larger image. [300kB]

Full description:  The contact maker can be thought of as a clock, or time switch, with three dials which rotate against fixed pointers. These indicate the day of the week as well as showing the (24 hour) time and the minutes of the hour:

The electromagnet is activated by the master clock's brief impulses at 30 second intervals. This part of the mechanism is a modified, heavy duty, double locking, slave dial movement. The minute wheel and star wheel are rotated by the ratchet (or impulse) wheel as one unit.  All three turn once per hour driven by the electromagnet. The purpose of the 12 point star wheel is to continuously lift and reset the mercury switch and then release it again at 5 minute intervals.

The mercury switch fixes the duration of bell ringing to about 8 seconds. This is achieved by the mercury running slowly back, through narrow channels, away from its internal electrical contacts.

The day of the week wheel is really a simple cam driven by a pinion on the 24 hour wheel. The day of the week wheel has a normally-closed, bronze, contact finger rubbing along its lower edge. At the weekends an optional cutaway on the edge of this wheel allows these sprung contacts to open. Thus breaking the bell circuit. Which stops the bells ringing all weekend until the raised edge finally returns on Monday. Closing the contacts again.

Weekend silencing is a mechanical option. Gent's used to say that changing the programmer had to be attended to by their own engineers. Ringing is dependent on the weekend contacts being closed. This can cause some confusion in new owners/clock carers who cannot understand why the bells don't ring when a pin is set. A glance at the bottom of the day of the week dial should confirm whether weekend silencing is active.

Careful examination of the day of the week wheel in the image above shows that I have turned the removable back plate. The day of the week contacts remain continuously closed. Allowing ringing at weekends as well. The thin grey line of the back plate can be seen in the normally cutaway weekend sector. Removing the retaining pin and bush from the day of the week wheel will allow access to the backplate. Be very careful if you are tempted to try this. There is a large and heavy, bronze, toothed wheel (gearwheel) behind the day of the week wheel. As you slide the wheel free be ready for the unexpected weight. You will also need to reset the day of the week accurately when you replace it! (see setting instructions below)

The 24 hour pin wheel has holes around the periphery at 5 minute intervals. It is driven by a pinion from the minute wheel of the slave movement. When a stout, tapered, timing pin is placed in any of these holes it will eventually arrive at the bell contacts at the extreme left  of the mechanism. The pin closes the contacts, completing the bell circuit at the time shown on the pin wheel dial.Being a 24 hour dial the pins must be placed where they will cause the contacts to close only at a desired time. So pins are usually only required during the working hours of the school. So no pins are fitted during the evening or hours of the night. This simple arrangement allow night silencing without the extra complexity of driven wheels, cams and contacts.

Any single contact in the programmer's circuit cannot ring the bells. All must be closed at the same time. The bell contacts are closed by the timing pin for several minutes. The mercury switch is then released by the star wheel. Only if the weekend contacts are closed will an applied low voltage DC current pass through the entire bell circuit. Note that the bell circuit has no connection to the clock impulse circuit. Separate, labelled connection terminals are provided for both circuits (time and bells) at the front, inside the secure case. 

If a heavy current, or a higher voltage is required to be switched, then a suitable relay may be switched by the low voltage bell circuit. One should not try to switch mains electricity or higher voltages DC with the programmer's bell circuit. Higher voltages will cause arcing and should be strictly avoided. These higher voltages would make exposed metal, screws and wires live! Which could easily prove lethal if touched accidentally! A fire is also entirely possible due to overheating of components. 

Programmer time and day setting: Thanks to the clear markings on the various wheels the time and day of the week can be read off the programmer. Naturally these wheels must all be set accurately when the bell ringer is put into use. The day of the week wheel may be set quickly by removing the tapered pin and bush which retain it on its arbor. Do not lose these parts. The day of the week wheel may then be drawn gently forwards and out of engagement with its drive pinion. Then turned to the desired day before being very gently pushed backwards again to engage the large, toothed wheel with its driving pinion. The exact placement (rotation) will depend on the time already showing on the other two wheels.

It is easiest to set the day of the week dial at a particular time since it lacks any other sub-divisions than midnight between any two days. Midnight may be easiest to set accurately but is hardly a user-friendly time to be setting the mechanism. Using other times will require some guesswork. The time of day on the day of the week wheel is read from the point where the weekend contact finger rests underneath the wheel. So setting the day of the week wheel is best carried out at 6 or 12 o'clock. Being a quarter or halfway through the day shown against the bottom pointer. Midnight would of course be on the line between two days. Once set it will continue to rotate with the mechanism and show the correct days of the week. So setting only needs to be done once and can be checked for absolute accuracy if you should happen to be around at midnight. Check that the other dials agree that it really is midnight. Remember to replace the retaining drilled bush and tapered pin before you lose them. 

The minute wheel may be advanced by repeatedly pressing the armature against the electromagnet core and releasing it briefly. The slave movement which drives the entire mechanism has double locking. This makes it very difficult to advance quickly. (Unlike a normal slave which needs only the electromagnet armature to be pressed in to release the ratchet wheel)

There are certain midpoint positions of the locking and impulse levers which will allow the ratchet wheel to turn freely. But it requires some care and dexterity to avoid graunching the ratchet wheel with the levers if they should suddenly re-engage! Probably no damage will be done but it isn't a kind way to treat such a fine, old mechanism.

Freeing the minute wheel will also allow the 24 hour pin wheel to turn as they are geared together. The time on the 24 hour pin wheel is read at the bell contacts whether a pin is present or not. A pin may be inserted temporarily to ensure accuracy of reading the time showing on the 24 hour dial. There should be no disagreement between the time shown on the minute wheel and pin wheel unless the mechanism has been previously dismantled.

A black pointer is provided to show the time on the minute wheel of the slave movement. (Accurate only at the half minute time impulse)

Although the contact device is commonly known as a bell ringer it may be employed to switch low voltage currents for any purpose. You could even use it as an alarm clock with a suitable low voltage doorbell or buzzer. Or to switch lights on and off via a locking relay. 

Mercury


Great care should be taken with the glass, mercury switch. It contains liquid mercury within a thin glass envelope. It is held by strong spring clips but can slip free if touched.  Mercury vapour is highly toxic. Liquid mercury must never be allowed to remain if spilled indoors. The mercury switch is very shiny and the flowing metal very unusual in appearance. This may make the switch attractive to curious children. It is strongly suggested that the programmer case door be kept locked at all times and the key kept somewhere secure and completely inaccessible.

Mercury Spill Response -- Cleaning Up Small Spills


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