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Michael Drolet -- 2014

Switchboard under rigorous testing
In our recent production of Lanford Wilson's "Hot l Baltimore", much of the action took place around the aging hotel's decrepit telephone switchboard. 

The Props Dept was called upon to build a working representation of a switchboard, based on the image to the left.

Rather than use a prerecorded sound effect to signal inside and outside calls arriving at the switchboard, we decided to use an electromechanical bell and buzzer to signal incoming calls.
A buzzer consists of an electromagnet which operates a switch connected to an armature. 

Applying 16 VAC power to the electromagnet pulls on the armature, opening the electrical contacts, and de-energizing the electromagnet. 

Spring action causes the con
tacts to close, re-energizing the magnet; and the cycle repeats itself.

The armature moves back a
nd forth making a "buzzing " noise as long as power is applied.

We used this sound for
calls coming from rooms within the hotel.
Adding a clapper and gong to the buzzer turns it into a bell.

If we operate it with a 2 second ON, 4 sec OFF cadence, it sounds like a typical North American telephone ringer.

If we operate it with a different cadence (0.2 second ON, 0.4 sec OFF, 0.2 second ON, and 4 sec OFF), it sounds like a typical UK telephone ringer.

We used this bell sound for calls originating outside the hotel.

This unit, the "Quasimodo 5000" bell ringer, contains a transformer to convert 120 Volt AC (house current) to 16 Volt AC and switches to operate a bell or buzzer onstage, as necessary.

During rehearsals, the unit is operated manually by the Stage Manager.

In performance, it can be operated from the Sound Booth, and is fitted with 3-pin XLR connectors so that we can use the theatre's existing cabling to connect it to bells and buzzers onstage.

We decided to automate the ringing and buzzing under control of Show Cue Systems (SCS) software; which we used to run all the other sound cues in the show.
As well as playing sound, SCS can send control messages to external hardware to perform different tasks.

This unit, the "Highly Liquid MD24" can interpret these messages, in Musical Instrument Digital Interface (MIDI) format to drive small relays inside the box.

The relays mimic the physical switches on the
"Quasimodo 5000" bell ringer, causing the bell or buzzer to sound.

This circuit board, mounted inside the MD24 holds four small relays.

A relay is like an electromechanical lever.  It enables a small control
signal (from SCS) to operate a device (bell or buzzer) requiring a much larger current or voltage.

Each relay operates on 5 volts DC from SCS and can control the 16 VAC current to operate one of four bells or buzzers.

The relay does this by using an electromagnet to close a switch contact, just like in the buzzer.
relay board
SCS MIDI A "CONTROL SEND CUE" from SCS, in MIDI format.  "Note #" 60 with a "Velocity" of 127 causes relay #1 in the MD24 to operate.  This causes the Quasimodo 5000  to ring the bell.

A similarly formatted message, with a different value (0) for "Velocity", will cause the same relay to release, stopping the ringing.
A few lines from the SCS Cue list:

Cue 470 <1> causes the bell to ring for 2 seconds, then Cue 470 <2> shuts it off for4 seconds.

Cue 475 is a loop which jumps back to CUE 470, repeating the process. When the Sound Board Op sees someone pickup the phone, CUE 480 is manually activated, stopping the ringing and breaking the loop.

Cues 455,460 and 465 act similarly  for the buzzer.

A glitch in our programming prevents the ringing or buzzing to be stopped on the first ring (or buzz).  To be investigated.
The many incandescent lamps on the switchboard could consume several amps of current at 3 VDC and 12 VDC.  An ATX  computer power supply was rewired with a terminal strip to provide easy access to 3 VDC @ 8 amps and 12 VDC @ 7 amps.

Some lamps were lit up by the actor's action of plugging in one of the patch cords.  Others (red) were lit up by operating the row  of switches on the bottom panel.

Lights indicating incoming calls were lit up by an operator with a switchbox backstage.  It was connected to the switchboard by a CAT6 -- 8 conductor cable.
backstage switches