Monday, 16 February 2015

Raspberry Pi Ultra Sonic Press Up (Push Up) Counting Machine using Scratch

Looking at back at recent posts I've noticed:
  • I've not done any electronics related projects recently.
  • I've not done much programming with my kids recently.
When doing some press ups* recently I lost count and couldn't log them on my Jerks exercise logging system.  Hence this gave me the idea to build a machine to count my press ups.

(*Push ups for you readers in North America)

I saw you can buy one from Amazon and there are some available as smart phone apps but this isn't the Geek Dad way; I wanted to build one with a Raspberry Pi, program it in Scratch and use some electronics.  Ideas:
  • Mount a light dependent resistor - as I go down it blocks out light from the LDR, as I go up it allows light again and counts a press up.
  • Use a camera and a QR code.  As I go down it reads the QR code and counts a press up.
  • Use an ultra sonic sensor to measure distance as I go up and down.
I decided to use an ultra sonic sensor - purely because I'd hadn't used one since a University project about 20 years ago.  Here's the one I bought, an HC-SR04 module which has both a transmitter and a receiver.  They're widely available on the interweb.

To integrate the sensor to my Raspberry Pi I used the utterly amazing, epic and awesome Cymplecy Scratch GPIO system (did I mention that this site is brilliant).  This chap has developed a series of Scratch releases that allow you to integrate with many and various third party boards and bits of electronic kit that you can connect to the GPIO.  There's a specific page on Ultra Sonic module integration; it's available here and the solution only uses one GPIO input/output which is ace.

The resistors shown on the breadboard diagram on Cymplecy's site are essential to stop you blowing up your Raspberry Pi; they create a potential divider to reduce the +5V output from the HC-SR04 to a GPIO friendly 3.3V.  The resistors are:
  • Brown-Black-Red = 1kOhms
  • Red-Red-Red = 2.2kOhms
I won`t do the maths here, but see this site for an explanation of potential dividers.

The system works brilliantly and with this code block you can get Scratch to show the measurement from the ultra sonic sensor and move the Scratch Cat around.

So this worked beautifully, next I needed a robust housing for the press up counting machine to avoid me breaking it.  Enter Lego, the tool of tinkerers everywhere.  Here's a series of before, during and after shots:

So the breadboard is mounted perpendicular in the Lego housing, meaning the sensor points straight up.  Hence as I'm doing press ups I'm moving up and down vertically over the sensor.  The Raspberry Pi sits separate to it.

Here's a quick video:

The basic algorithm for the press up code is as follows:

boolean = false
  Take Measurement
  if Measurement < Low AND boolean = false
    boolean = true
  if Measurement > High and boolean = true
    increment press up count
    boolean = false

So this uses a simple boolean to log when my body is low down (say < 10cm from the sensor).  The spots the bottom part of the press up.  Then when I move up, if my body is high up (say > 30cm) AND I've previously been low down then a press up is counted (and the boolean is reset).

The basic Scratch code is shown on the screen shot below (put your email address in a comment below and I'll send you the file).  This does a good job of counting press ups (although you have to go quite slow as the sampling period is only one second) and getting the Scratch Cat to say them.

What I wanted to do with my daughters is jazz it up a bit.  In preparation for this I recorded a number of different audio messages for Scratch to play out.  I used this site which takes a text string input and provides a downloadable MP3 file.  These files can then be put in the /usr/share/scratch/Media/Sounds directory on your Raspberry Pi.  Armed with this I spent an enjoyable afternoon with my daughters making the press up counter more interesting.

  • Changed the background to make it look better.
  • Added a sound at the start to tell you that "It's press up time!".
  • Added If statements to detect each Press Up (up to 10) and play an audio message with the number ("1" for the first press up, "5" for the fifth etc).
  • Added some motivational (semi-abusive) messages to keep you going.
  • Added a "Well Done" message at the end.

Here's a video of it in action!

Here's a screenshot of the Scratch code (top part). A but hard to see so, again, put your email address in comments below and I'll send you the Scratch file.

...and bottom part:


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