Dial-A-Song: Part 2

For part two of the Dial-A-Song project, I’m going to go through the hookup of the phone’s keypad so that it can act as an input to the Raspberry Pi.

The first thing to do was remove the short ribbon cable that came on the keypad and replace it with something a little longer so I could more easily hook it up to the Pi. Here’s the old ribbon cable:

Dial Pad and Tone Generator

And now with the new, more colorful, cable:

Keypad Wiring

On the other end, I attached a 2×5 IDC female header (I didn’t have a 2×4, so there’s a couple wasted pins). This is what will eventually connect to the Pi interface board I’ll create, but for now I mocked up a breakout board (the green perf-board) so I could connect it to a breadboard along with Adafruit’s great “Cobbler” kit.

The keypad’s internal wiring is setup as shown below (which the red wire being pin 1 and increasing to the left, from the image above):

  5 6 7

Most matrix keypads are very similar so, the keypad pins should be connected to the Raspberry Pi GPIO as follows:

Pad | Pi
1   - 25
2   - 24
3   - 23
4   - 18
5   - 22
6   - 17
7   - 4

Now for the software. Originally, I was going to use a library designed for specifically this purpose. It did exactly what it said it would with the exception that it uses polling to detect the keypads. This works, but required a significant amount of the CPU resources to get any decent response time. Often around 80%. This was not acceptable.

So, I decided to try converting the library to use the interrupt capability of the RPi.GPIO library that comes pre-installed on Raspbian. It took a little fiddling because the Pi GPIO pins are really susceptible to noise making button debouncing a real issue. But in the end, it works quite well:

import RPi.GPIO as GPIO
import time

class keypad():
    def __init__(self, callback):
        self._count = 0
        self._inInterrupt = False
        self._callback = callback

        # CONSTANTS 
        self.KEYPAD = [

        #hook the rows (1,4,7,*) to these GPIO pins
        self.ROW         = [18,23,24,25]
        #hook the columns (1,2,3) to these GPIO pins
        self.COLUMN      = [4,17,22]


    def __colInt(self, channel):
        time.sleep(0.05) #give it a moment to settle
        if GPIO.input(channel) > 0:

        #remove interrupts temporarily
        for c in range(len(self.COLUMN)):

        #get column number
        colVal = -1
        for c in range(len(self.COLUMN)):
            if channel == self.COLUMN:
                colVal = c

        #continue if valid column (it should always be)
        if colVal >=0 and colVal < len(self.COLUMN):

            #set rows as intputs
            for r in range(len(self.ROW)):
                GPIO.setup(self.ROW[r], GPIO.IN, pull_up_down=GPIO.PUD_UP)

            #set triggered column as low output
            GPIO.setup(channel, GPIO.OUT, initial=GPIO.LOW)

            # Scan rows for pushed key/button
            rowVal = -1
            for r in range(len(self.ROW)):
                tmpRead = GPIO.input(self.ROW[r])
                if tmpRead == 0:
                    rowVal = r

            #continue if row is valid (possible that it might not be if the key was very quickly released)
            if rowVal >= 0 and rowVal < len(self.ROW):
                #send key info right away
                #This avoids nasty boucning errors when the key is released
                #By waiting for the rising edge before re-enabling interrupts it 
                #avoids interrupts fired due to bouncing on key release and 
                #any repeated interrupts that would otherwise fire.
                    GPIO.wait_for_edge(self.ROW[rowVal], GPIO.RISING)
                except RuntimeError:

                print "Invalid Row!"
            print "Invalid Col!"

        #re-enable interrupts

    def __changeWrapper(self, channel):
        #if there is already another interrupt going on (multiple key press or something)
        #return right away to avoid collisions
        if self._inInterrupt:

        self._inInterrupt = True
        self.__colInt(channel) #handle the actual interrupt
        self._inInterrupt = False

    def __setInterruptMode(self):
        #set the first row as output low
        #only first one needed as it will ground to all columns
        for r in range(len(self.ROW)):
            GPIO.setup(self.ROW[r], GPIO.OUT, initial=GPIO.LOW)

        #set columns as inputs and attach interrupt handlers on rising edge
        for c in range(len(self.COLUMN)):
            GPIO.setup(self.COLUMN[c][/c], GPIO.IN, pull_up_down=GPIO.PUD_UP)
            GPIO.add_event_detect(self.COLUMN[c][/c], GPIO.FALLING, bouncetime=250, callback=self.__changeWrapper)

    def cleanup(self):
        print "Cleanup done!"

It's super simple to use as you can see from the code below. Instantiate the class with a callback function and that function gets called when there's a keypress on the pad. Just note that the callback will be running on the context of a different thread from the main thread. While it uses interrupts, RPi.GPIO cheats a little bit and the interrupts are actually running on background threads and so will the callback. Not usually a big problem but something to be aware of.

import time     
if __name__ == '__main__':
    def keypadCallback(value):
        print "Keypad: " + value

    key = keypad(keypadCallback)

        while True:

    except KeyboardInterrupt:

You can grab the full library and find any updates to it on GitHub.
That's all for now. Check back soon for more progress on Dial-A-Song!


  1. Hi there,
    Great write up, thank your for documenting this! I’m working on a project that would benefit greatly from your code. However, I get the following error when I try and run it:

    File “keypad.py”, line 116, in
    key = keypad(keypadCallback)
    File “keypad.py”, line 26, in __init__
    File “keypad.py”, line 103, in __setInterruptMode
    GPIO.setup(self.COLUMN, GPIO.IN, pull_up_down=GPIO.PUD_UP)

    Any help would be greatly appreciated!

  2. Rats! I accidentally didn’t paste the full error, sorry about that.

    Traceback (most recent call last):
    File “keypad.py”, line 116, in
    key = keypad(keypadCallback)
    File “keypad.py”, line 26, in __init__
    File “keypad.py”, line 103, in __setInterruptMode
    GPIO.setup(self.COLUMN, GPIO.IN, pull_up_down=GPIO.PUD_UP)
    TypeError: an integer is required

    1. Sorry about that. Not sure how that ever worked for me… I’ve fixed the library and committed the changed to GitHub. Grab the latest and you should be fine.

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