Adding Pen

I’m interested in adding a pen to my 3pi.

I guess the options are either to make something short enough to fit under the PCB (c. 9mm?) or to put the nib through the central hole where the expansion header goes. If I choose the latter, to what diameter can I safely enlarge the hole?

Where by ‘safely’ I mean with no loss of function other than the ability to solder a header into that hole. Can I make the hole larger safely if I also carry out some ‘green-wire’ repairs of damaged traces?


How important is it to you that the pen be at the center of the robot? A safer place to drill out a larger hole would be in the prototyping area at the front of the bot (and in this location the pen could basically also act like a front ball caster). However, note that there are some traces that pass through this prototyping space along the underside of the PCB, so you would need to be careful not to damage these!

- Ben

It makes the maths easier if it’s in the exact center.

I think a 3mm diameter hole, or thereabouts would be big enough, so if it’s safe to put one there, I’d prefer to have the hole in the center.

(edited for clarity)

Assuming you’re willing to give up the expansion header connections and make minor repairs, it should be ok. The main problem is that the reset line runs right through the middle of the expansion header, and I think you will cut the trace if you drill there. This picture shows the top side of the PCB with the reset trace highlighted (the gap in the trace between the two vias is connected on the bottom side of the PCB):

If you drill where I put the blue circle, you will probably cut the highlighted trace, which prevents the reset pin of the ISP header from reaching the reset pin on the microcontroller (i.e. you will no longer be able to program the board). One way to fix this would be to connect the ISP header reset pin to the right reset button pin with a wire (the lower dashed yellow line), or you could PC6/RST pin in the expansion header to the right reset button pin with a wire (the upper dashed yellow line).

Also, be wary of shorts when you drill out the hole. I suggest you clean it out so that stray bits of metal aren’t shorting things together that aren’t meant to be shorted.

Please let me know if you have any questions, and please let us know how it turns out. I haven’t heard of anyone adding a pen to their 3pi before, so I’m very interested to hear how it goes.

- Ben

This is also what I want to do. A pen at the center is the easiest way to draw, say a square. Any other place will make the program very much more complicated, if possible at all.
Of course we can draw more interesting things like recursive patterns, or re-draw a maze after having explored it…
Do you have any solution for this problem?

The 3pi was not intended for this kind of application, so the best solution I can offer is the one posted above. I am curious to know if tgdavies ever got this working, though!

- Ben