ElMo Cannon Creation

A long time ago, I posted about adding a solenoid to ElMo, that we could use to kick the ball as part of the skittle challenge.  Well, I found that Tris10 Robotics sell something called the kickball kit - a whole heap of capacitors that can be charged up and then discharged rapidly into the solenoid.  The force could apparently puncture a football - which appealed to the hooligan in me :)  We sent them some cash and a parcel arrived from Australia.  This weekend it was time to put it together.

The kickball comes as a kit of parts

that need to be soldered to the circuit board

 The finished circuit - comprising a few headers, 3 capacitors, a relay switch, a few resistors, a diode and a transistor

I was sceptical as to whether my soldering skills were up to the job - but it works!  For now we have only tested it using the supplied arduino code - I thought I'd rather fry the arduino than the pi if things went wrong!

The solenoid is attached to the kickball controller using 2 pins.  There are two more pins for Vin and ground - I used a 9V rechargeable battery first of all.  Another pin is used as the trigger (set the pin high for 70ms to trigger the 'kick' - you then need to wait ~10s for the capacitors to recharge) and a final optional pin can be used to query the charge of the capacitors (I didn't connect this - I don't think we need this for PiWars where we only need one kick).

9V was certainly enough to move the test ball - but it wasn't enough to kick the ball from a standing start and knock any skittles over.  The diddyborg we are using for ElMo has a 12V battery pack and when we connected that we did manage to knock over one skittle - yay!  I finally tried wiring up 2 9V batteries to give 18V and that felt pretty punchy - we managed to knock over 4 skittles with that.  So we might power ElMo with 18V (for that challenge at least).  It may be that with the robot pushing the ball, the 12V kick will be enough - once we have a moving robot (hopefully this weekend!) we can think about how to attach the cannon and do some more tests (and post some video)