# Bodgers – RPM & KPH

Last Saturday we had a look at how we might figure out how far and how fast a bike is going using a Raspberry Pi. We used a very basic set up with just a micro-switch attached to a toy trike with a little nut taped to the front wheel, each time the wheel rotates the nut would “click” the micro-switch. We would need to use a reed switch or a hall effect sensor and a magnet attached to the wheel if we were to use this on a real bike.

We started of our coding by looking at the time.time() function. This function returns the number of seconds, in decimal form, since 01 January 1970. If we want to time an event all we have to do is use time.time() to get the start time and use it again to get the end time and then subtract the the start time from the end time.

We used this to get the the amount time it takes to do one rotation. Now we want to find out how many rotations we have per minute or RPM (revolutions per minute). As our result is in seconds the easiest thing to do is calculate revs per second so we divide 1 by the time it takes to do 1 rotation and then multiply the answer by 60 to get RPM.

Now we wanted to get KPH(kilometres per hour) so first we measured the circumference of the wheel and found it was 50cm or 0.5 Metres. We then calculated metres per minute by multiplying our RPM by 0.5 and we then multiplied this by 1000 to get KPH.

Here’s a picture of what our results could look like when displayed using Pygame. We will look at Pygame later on as it’s an excellent way of displaying information. See you all after the break.

Declan, Dave and Alaidh