Making a Museum: Building a telephone switchboard


By Dana Sandoval

Have you ever seen a movie where someone picks up the phone and asks an operator to connect them to someone?

Before there were cell phones, two telephones had to be physically connected with wire for people to talk between them. It would be hard to have wires running from the phone in your house all the way to each of your friends’ houses, or worse, having a different phone for each person you want to call. And how would you call someone new? You’d have to get a new wire running to the pizza place to order pizza.

Telephone switchboards prevented this crazy wiring problem by connecting each phone to a central location. There, the wires from a phone making a call could be connected to the wires of the phone it was calling. Today computers make those connections when you call someone from a landline, but in the early days of telephones, telephone operators made the connections by hand.

The exhibits team at the MoSS has built a switchboard so visitors can see what it was like to be an operator.

Here are some of the steps we did to build this switchboard:

Research:

While we were pretty confident there was a way to make a switchboard, we didn’t know what it was. Could we use real phones or would we have to do something else, like have a switchboard for connecting tin-can phones? Fortunately, our research indicated that connecting phones is almost as easy as making a tin can phone. There are only two wires leading to a phone: electrical current flows in one wire and out the other, carrying the sound signal with it. If you connect one wire from each phone together, connect a battery (we used a 9-volt battery with a 300-ohm resistor attached to limit the current) and complete the circuit with the other wires, voila! You have a phone system just like that!

Testing:

At first we attached each element of the circuit with alligator clips so we could check that phones really can be connected this way. When this proof-of-concept test was successful, we needed to think about making the actual switchboard.
Prototyping: The first switchboard we made was a pink cardboard box. The practice model made it easier to figure out things like how big the board should be, where wires should go, and the best way to attach the battery.

3D Printing:

The battery and resistor are between the connectors that can be plugged into the jacks leading to each phone. We needed an effective way to hold everything in place on the switchboard, so we designed a battery holder to print on a 3D printer. We also printed covers for the plugs to make them easy to tell apart. If you look closely, you can see the lines between the layers of plastic that the printer added.

Building:

Once we knew how big to make the switchboard and where the wires would go, we drew everything up and used a laser cutter to cut it out of wood. We added rectangles to the edges so we could add sides that would fit like a puzzle. Now all it needed was all the wiring!

Soldering:

To make the connections between wires permanent, we soldered them together. Soldering uses a tool to melt soft metal, which is then used to glue two wires together. The metal is conductive, so the electricity can pass through it from one wire to the other. We soldered the two wires from the phone to a headphone jack, and both ends of the connectors to headphone plugs. When the connector is plugged into two phone jacks, a complete circuit is formed and the phones can communicate.

Buttons and lights:

In order for the operator to know which phones to connect, they need to know when someone is trying to make a call. We added another circuit to the battery which has LED lights and buttons so the caller and operator can signal each other when they need to connect.

The switchboard is made, now all it needs is an operator like you to come try it out!

MoSS’s communication exhibit will be at the Chandler Chuckwagon Festival at Tumbleweed Park on November 12th. Come visit us there!

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