Disassembly

To open up the phone, use a flat-head screwdriver to loosen the two screw in the base. The will not need to be fully removed, just unscrewed enough to release the shell.

Lift the shell off of the phone base.

Dial Pulse Switch

When a number is dialed on the rotary phone, the normally-closed pulse switch is opened and closed once per number dialed as the dial governor rotates back to the stop position. Dial a '1' and one pulse is sent. Dial a '7' for seven pulses. The '0' sends ten pulses.

The interval of these pulses in the US is a 100ms period, with 60ms closed and 40ms open. This can vary with phones from other countries.

Loosen the two screws holding the dial assembly to the mounting arms. Then, gently pull the dial free from the base, being careful not to pull on any wiring.

Use a multimeter to check the wiring -- one pair is connected to the speaker disconnect switch (which prevents the dialing pulse sounds from being heard in the earpiece), the other is connected to the pulse switch.

The blue pair on this dial is the pulse switch. Loosen the two screws on the network block and remove the blue wires -- you'll later connect them to the KB2040 to read the pulses.

Receiver Switch Hook

You'll use the receiver switch hook to enable or disable USB HID. This way, when the phone is hung up the virtual keyboard on a mobile device will be available, and when the phone receiver is picked up the dial will work as a number entry device.

Using a multimeter, determine which terminals of the network block are opened and closed by the switch hook. You'll run a wire from each of these terminal positions to the KB2040 later.

Optional RJ11-to-USB Cable Build

The simple thing to do is run a regular USB-C cable to the KB2040, snaking it through a gap in the phone shell. HOWEVER! You aren't here for simple, you're here for EXTRA. So to be totally extra, you're going to build a custom RJ11-to-USB cable, and then hijack the phone's RJ11 jack to run the wiring to the Kee Boar.

Cable Build

Cut one end off of your phone cable (make sure it is a four conductor RJ11 cable). A three foot length should work well (the stubby piece in the photo here is for illustration purposes only!)

Carefully remove the outer cable insulation, then strip and tin each of the four wires.

Solder the cable ends into the DIY USB-A plug in the order shown here.

USB Jack Wiring

In order to run the USB-to-RJ11 cable into the phone's RJ11 jack, you'll unscrew the three terminal screws connecting the yellow, green, and red wires from the network block. These will be connected to the KB2040, along with an added black wire, in case your phone's original jack didn't contain a fourth conductor. Different phones may have different numbers, colors, and positions of jack wires.

Wire colors and positions can vary, so it's important to use the continuity checker on your multimeter to connect the USB conductors to their associated pads on the KB2040.

These connections are:

  • Vcc to RAW
  • GND to GND
  • D- to D-
  • D+ to D+

KB2040 Wiring

Use terminal blocks (either a barrier block as shown here or Two Euro-style connector blocks) to run the phone wiring to the KB2040. Solder short lengths of wire to the KB2040 to make the connections.

The connections are shown on the Fritzing diagram above.

Euro-Style Terminal block with 4 pins
Connect this to that without soldering using these quick terminal blocks. They allow connecting of two sets of four wires together using just a flat-head screwdriver. These are...
$2.50
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Kee Boar Attachment

Use insulating Kapton tape or another material (I used adhesive felt) to protect the back of the Kee Boar from shorts.

Use a pair of zip ties to secure the Kee Boar to the base of the phone, making sure the terminal block and wiring is clear of any moving parts.

Re-Assembly

Place the dial assembly back into it's brackets and tighten the two screws.

Tuck any wires out of the way, then place the shell back on, making sure the jacks are secured against the housing.

Re-tighten the bottom screws and you're ready to go!

This guide was first published on Feb 08, 2022. It was last updated on 2022-02-08 16:03:53 -0500.

This page (Build the Rotary Phone Keypad) was last updated on May 24, 2022.

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