chalky - street chalk printer

Description

openchalk is a (multiples of) 5 bit wide box that fits 5 chalksprays. solenoids actuate the cans according to current speed. you can input a text message and openchalk will write it to the ground if you tow it by hand or by bike.

but why? chalk is considered non-permanent in many eu countries and thus you will not violate the law by writing messages on the street.

and it's easily built in a fablab or hackerspace near you. all plans on github.

Details

this would not work without chalk spray, which is available from ebay for around 2-5€. i bought three different brands, all use the same can, so the spray container will fit most. it also doesn't matter if the spraycan sprays to the top or to the side, just flip the printer accordingly.

the spray can container is made from lasercut wood, with a few acrylic parts thrown in because of mechanical stresses. the actuators for the spraycan are obviously solenoids (readily available from ebay all over the world). the control of each solenoid happens thru a non-exotic mosfet, an very commonly available optocoupler and an arduino.

the only part you may have to design yourself is the wheel encoder. in my case it's 3d printed and will probably only fit my dog trailer. its made from a turn encoder (like those used in reprap ultipanels) and a ball bearing (had 8/4 laying around). use your imagination, even a button with a long lever positioned near the spokes may suffice.

this project can be routed, laser cut or hand build.

Components

5 × irfz44n power moset

5 × ltv817 Optocouplers and Optoisolators

5 × 1N4001 Diodes and Rectifiers

5 × 100 ohms resistor

5 × 680 ohms resistor

Project Logs

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first test done
mnt • 06/03/2015 at 22:02 • 0 comments

very first test done, works, but needs finetuning
things to fix:
- the wheel encoder needs to be 0.5-1mm closer to the wheel, it's slipping sometimes (thus the space in the photo)
- fine tune nut settings, each can seems to deliver a different amount of chalk
tomorrow starts gulasch programmier nacht gpn15. maybe i should go and finetune there.
testmount below trailer
mnt • 06/03/2015 at 21:55 • 0 comments

should probably be higher (for bigger dots) but lets test that first
money shot
mnt • 06/03/2015 at 21:54 • 0 comments

forgot front view
the encoder joke
mnt • 06/03/2015 at 21:53 • 0 comments

basically only an rotation encoder (like used in ultipanel), ball bearing, m4 screw and cutted foam. looks unfinished, but works. pulls pin 2 of the arduino to ground if bike moves (internal pullup of pin 2 is on, so no external pullup needed).
left 1st version, right final (3rd) version
mounted to my dog trailer
one of our cats likes the dog trailer too
building can box
mnt • 06/03/2015 at 21:38 • 0 comments

mechanical and electronics done, no software yet.
also: old lipo=dead. old lead accu=dead. bought a new 12V 4800Ah 2.6A current lead accu. works.
the solenoids:
spraycans mounted
test firing made a mess on my bench
the (simple) electronics: mosfet, opto coupler, 2 resistors, 1 protection diode per solenoid. see fritzing sketch for compontent list.