The Circuit

The circuit above is the breadboarded version of the circuit you'll build on a FeatherWing Doubler. You may want to test it on a breadboard before moving to soldering it together.

The sections of the circuit are:

  • Feather RP2040 A1-M0 connected to keypad matrix
  • Feather RP2040 TX pin connect to audio RC filter circuit to reduce noise, then to A+ input of the amp breakout
  • Amp output connect to phone handset speaker
  • Phone switch hook connected to Feather En and GND
  • USB cable breakout power and GND to Feather USB and GND

FeatherWing Proto Area Circuit

Transfer the circuit from breadboard to the proto area of the FeatherWing Doubler.

The seven position header will be soldered to the underside of the board so it can be plugged into the back of the matrix keypad later (see below).

Solder two black lead wires with ferrule crimped ends to the Doubler GND.

Solder another wire to En (this will enable/disable the board, essentially powering it off when the handset is in the cradle).

Solder a red wire with ferrule end to the USB pin on the Doubler. This will be connected to USB power for battery charging.

Fit the amp and Feather onto the Doubler.

Keypad Preparation

The keypad assembly is composed of two assemblies -- the DTMF circuit board, and the keypad matrix itself. We don't need the DTMF board, so we'll desolder the ribbon cable and then add header pins to connect to the FeatherWing.

Use a solder sucker and/or desoldering wick to remove the ribbon cable cleanly.

Here you can see how the keypad works -- the plastic buttons press down on the elastomer pads, causing the conductive rubber pills to short the associated row/column traces.

Note, the pcb is single-sided with the exception of the row two and row three bridges routed as wires through the back of the board!

Keypad Header Pins

Solder in the header pins -- the matrix pcb has eight pads, but we only need to connect to the first seven, based on how the matrix is designed, using three columns and four rows.

Attach the Boards

Plug in a LiPo battery to the Feather, then fit it behind the keypad.

Press the Doubler socket in to the first seven pins of the keypad.

Next you'll integrate the Feather board with the rest of the phone.

This guide was first published on Feb 25, 2022. It was last updated on 2022-02-25 11:11:35 -0500.

This page (Build the Dial-a-Song Circuit) was last updated on May 24, 2022.

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