Easy DC?

Well, six months after the fact, I believe I may have located the source of the pretty disastrous issues I had at the last Pi Wars. I actually managed to complete just three of the seven events so it was all a bit heartbreaking tbh.

I had severe issues using a PS3 controller and I fully believed that to be the issue and therefore have looked at moving away from this to another controller type for next time round. The Rock Candy PS3 controllers come highly recommended by Brian Corteil (aka @CannonFodder) who has used them to great effect, in fact winning – twice!

So, you may have seen FRED-209, my new Pi powered Nerf Tank. This uses same ZeroBorg motor controller and I had it linked to an original PS3 controller. I’ve tested extensively in the week and then this Saturday I took it to Cotswold Jam and bam… the controller fails. Niet… Zip… Diddly. Can’t get it to connect, then if it does it drops after a few seconds.

This is *identical* to the behaviour X-Bot exhibited at Pi Wars. So much discussion ensues as to the cause. I know of several other folks who also had issues with these controllers last time round. However, Brian C. lent me a Rock Candy one to test and this also showed exact same behaviour. The RC ones are 2.4ghz and the PS3 ones are Bluetooth so still wasn’t sure if it was radio interference of some kind. Another good guess was noisy DC motors causing problems.

But… I think we have a winner. This DC-DC converter board is a damn liar…


I recently recommended this type of thing having seen similar at Pi Wars last time. I now conclusively retract that recommendation! I used a similar board last time , without a read out, and I believe this suffers from the same issue.

Note the difference in alleged and actual voltage….


This difference is not enough to shut the Pi down, but it is enough to mean power hungry peripherals, say a bluetooth dongle for example, are just enough to make the power dip enough that the USB experiences issues and that kills the controller.

If this were the only problem then simply setting it higher to start off with would solve the problem. We did crank it up to 5.18v on the multimeter which may be getting a little hot for the Pi, but it then ran flawlessly for the rest of the day at Cotswold Jam with loads of people giving FRED-209 a good hammering.

I do though believe there is good reason to suspect that the setting on the board drifts over time. Whether this is vibration from the pot moving the micro screw that you use to set the output voltage or something else, it does seem to change. This is regardless of the state of the batteries being used.

So, I am now hunting for a DC-DC board that sticks out a guaranteed 5v that can be relied one. Core Robotics have one in the pipeline but it’s not available yet. I’d be interested to hear from other teams who used these or similar boards or PS3 controllers last time and also experienced issues.


Until next time….

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Say hello to FRED-209… (You have 20 seconds to comply.)


This one’s been a while in the making, I’ve had a whole summer of fun putting it together. Tbh I can’t remember what first inspired the idea for this. I’ve been wondering about Nerf guns for a while (another amazing thing that wasn’t around when I was a youngster) My main thought was ‘can you use a servo to trigger it?’

Turns out you can :)

First experiments….

Flushed with initial success, things got out of hand remarkably quickly….


Whilst a fun idea, it obviously wasn’t going to fly that way so I decided I’d see if I could design a ‘Nerf Tank’ from the ground up. I’ve use the ZeroBorg controller again from Pi Borg and a PS3 controller (despite one dying on me at PiWars!) I chose this as the base as I have working controller code and I wanted to focus on the actual 3D design and build this time so I could complete in time for the next Cotswold Raspberry Jam.

The Nerf in the pic above is the Elite Stryfe, about £15 from various places. Or – your local car booty for a whole lot less! ..


I pretty rapidly disassembled it to get a good understanding of the innards. Turns out it’s two flywheels that actual push the dart out and a fairly simple push mechanism to give it a shove. Hmmm… two motors – and a servo. This could work!


I then got OpenSCAD out again, still my favourite 3D design tool, I love its overall simplicity and the fact you can do fairly complex shapes in just a few lines of code (without having to learn what a zillion buttons do) I drew out a basic design for the chassis and I made larger versions of the same motor mounts I made for the PiWars bot.

This is the original chassis I designed, combined with the gorgeous chunky wheels I found on eBay. Sadly the motors turned out not to have enough grunt to move the thing so a rapid and somewhat disappointing switch was made to the Rover 5 chassis you can see in the final version (which was also an eBay find) Also in the below picture is the design for the tilt mechanism which would eventually be controlled by two of the shoulder buttons on the PS3 controller.


Here’s the completed barrel and mechanism box, you can see the servo mounted underneath which attaches to a nice simple pusher. Printing the lid was fairly tricky as it and the box are the largest single pieces I’ve printed at 24cm long. The lid also required a lot of support material which was a bit of a bugger to remove cleanly as it covered such a large surface area.


Servo pusher…..


And flinger bit….


I had to tweak a couple of dimensions on the box to make sure everything lined up nicely but it went together really quite quickly in the end.

Testing time!!

You’ll notice this version is black. And really quite poorly printed. After two crappy results I finally worked out something pretty important. Cheap PLA isn’t worth the saving. I spent many hours trying to work out why the results were so poor when previously the printer had been excellent. Turns out it wasn’t calibration, or heat settings, or bed temperature – just naff filament. It works ok on smaller items but the moment you try and print something larger you’ll run into all sorts of problems with warping, lifting and delamination.

I therefore binned the black, and splashed out on some shiny orange (flouro red was the as described colour but hmm…) I also had to redesign the bottom plate to fit onto the Rover 5 chassis. It doesn’t look as good as it did on the chunky wheels but hey, it works.


I’m planing to take this to Cotswold Jam so thought it may be a good idea to give the kids something to aim at (apart from each other!) So – evil alien hordes it is. I’ve also rigged up a Pimoroni Blinkt, the lovely little 8 LED board. This works as a visual countdown, starting with all green LEDs and then one going from green to red every 15 seconds. The plan is to mark out an arena and then the player has two minutes to knock down as many targets as possible and then return to base before time runs out.


Doing a little target practice with the finished build…

(Apologies for rough quality of video – it was downsampled during upload to youtube for some reason.)

I’ll put the STL files for the chassis and the Nerf cannon on Thingiverse when I get some time later this week, I’ll also upload the controller code to GitHub.

I’ve got some future plans for this chap too. Adding a camera and web interface controller seems like the logical next step – and guaranteed to cause remote mayhem! Further to that, I’ve started experimenting recently with OpenCV so am very interested in adding a level of autonomy / target detection. Hopefully FRED-209 will be slightly better behaved than his namesake!

Thanks for reading – pop along to Cotswold Jam on 23rd September if you want to have a play – there’ll be prizes for the highest scores!

Until next time…..

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