Under the Hood
This section digs deeper into the structure and workings of the system. Some details are for “DIY makers and phone geeks”. If you aren’t, you may enjoy just browsing the pictures.
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Layout of the components
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The image below outlines the layout of the main system components. It supports two simultaneous callers. This means that 4 of the 8 phones could be connected (2 + 2) at the same time. Why allow for two callers for such a small system? It was more fun to watch and listen with two callers dialing at once! The four devices at the base in the picture below are rotary stepper switches. These will be explained later. See the Motivation page for more on relays and their operation.
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Components layout, front view. (3’ 6” tall)
Rear view of the system
Closeup of some control relays with dust covers
Phone System basic operation
The signage below (greatly simplified, showing 4 of 8 phones) was originally used at the Science Fair demo. It shows the basic idea for connecting two phones. Only one Line Finder and one Selector is used to make one call. The second pair of steppers are reserved for a second concurrent call, if needed.
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When a caller goes off-hook, a Line Finder (LF) auto steps to find this caller’s phone line. So, if caller #4 goes off-hook then the LF would step to position 4 and the caller would receive dial tone.
A Selector (it should have been called a Connector) responds to the caller’s dialed digit. So, if #8 was dialed, the Selector would step to position 8. If phone #8 was busy the caller received a busy tone. If idle, the ring cycle started. If the called phone answers, a talk path would be established between #4 and #8, using a quite talk battery. It takes many relay operations to control the actions of the steppers.
Four rotary stepping switches -- the heart of the system
Located on the rear base of the system were three 24V NiCad rechargeable batteries. With 20/20 hindsight, using several independent supplies was not wise. Fewer supplies should have been a design goal, it was not.
As a self-taught novice, my documentation skills were a joke. The docs that remain after 50 years are incomplete and often ambiguous. As they say, “It made sense at the time”. This makes restoration a challenge.
Original circuit diagram for Line and Cutoff relays
There were many design challenges. For example, using a common two-wire dial phone, I could not find a simple way to ring the called phone’s bell and simultaneously detect the called phone going “off hook” -- answering. It’s a good problem!
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In the circuit above there are three wires (see far left) going to a phone. The R wire is dedicated for ringing the bell. The T and S wires are the talking path (loop). So, each phone connects to the system with 3 wires not 2. No one knew or cared but me.
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Only years later did I discover how to detect off-hook while ringing a 2-wire phone. First, superimpose the AC ring voltage with 48 VDC on the talk path. Second, use a slow acting relay (the AC ring voltage does not engage it) to detect off hook. Live and learn.
The Soundproof Box
Here is another construction challenge. It turns out that most phones need a 20 Hz AC signal to ring the internal bell. Using the 60 Hz wall socket AC will not work -- the bell just “hums”.
So, I built a 20 Hz relay-based oscillator that provided a square wave ring voltage. Works like a charm. However, the oscillating relays are very noisy. My grandfather built a soundproof box (2 images below) to contain the oscillator and other relays providing timing intervals for the busy and ring signals.
The box rested on two railings in the middle rear of the system and had a 16-wire umbilical cord that connected it to the other relays.
Understanding the Rotary
Stepping Switch
(with two videos)
Soundproof box internals with five “noisy” relays inside
San Francisco Bay Area Science Fair Signage
