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Finger on the Pulse

A project log for The Phone Friend

Bring any old phone to life, no disassembly required!

stephSteph 04/14/2023 at 00:550 Comments

Detecting pulse dials is easy...

...but not fun.

Let's start with the part that *is* fun: detailed technical explanations. When your phone is on the hook, the Ring and Tip lines are (essentially) not connected. When you lift the handset, you close a switch that connects Ring to Tip, allowing current to flow through the phone so that you can talk and dial etc. 

Here's a phone circuit from 1905 that demonstrates the function of the hook switch:

The diagram shows how when the handset is lifted, the mic and speaker are connected through the battery. When it's hung up, the bells are ready to be rung with AC from the hand-cranked generator. It's essentially the same circuit used in every phone for the next 100 years.

With one exception...

...automated dialing. 

In the above circuit, a human operator was needed to connect each call to the appropriate line. 

The rotary dial allowed for calls to be connected without human intervention, and it did so by adding one switch connected to the dial. This new switch briefly breaks the Ring-Tip connection once for each number on the dial. 

 In this diagram, you'll also notice that the battery and generator have been moved out of the phone and into the Central Office. 

Detecting those Pulses

To get the number from a rotary dial, we not only have to count the pulses, we also have to know the difference between a short pulse and a regular ol' hang up, aka a really long pulse. Here, we demonstrate The Phone Friend's ability to detect the hook switch status as well as read the rotary dial pulses. In addition to the results printed to the screen, notice the LED on The Phone Friend light up to indicate the phone has been lifted off the hook, and blink off to indicate each pulse from the dial:

The code that runs that demo is here, and it's heavily commented for your reading pleasure. I'd paste it here, but the page formatting ruins my pretty comments. Basically, here's how it works:

That's it!

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