I've made quite a bit of progress on the Selectric II project. Since the last post I managed to get a fair amount of the mechanisms necessary to actuate the keys figured out (at least for a first attempt). I made the mechanical additions using a bunch of Erector Set pieces.
For the time being I've used fishing line to tie the Erector Set levers to the solenoids. Initially I had it set up with fishing line going up over a bar above all the key press mechanisms (essentially pulling up on them) but this didn't work and wouldn't have survived the test of time (rubbing against a bar all the time...).
This past week I made my first go at the toner-transfer & ferric chloride etching process to make a Solenoid Driver board. I also did quite a few things in Eagle for the first time. The circuit for the driver board is below, it's a simple power FET (IRL2703) circuit with some blinkin' lights thrown in for good measure. Eagle files below, I used default part libraries and the Sparkfun Eagle Library.
I'm no expert on the toner transfer process, so just Google that if you're interested. I will document the equipment I used (some of which is not the greatest) here in case anyone is looking to buy this stuff for the first time:
- Laser Printer: Samsung ML-2525W
- Does an 'okay' job. Toner thickness could be better, and manual paper feed mechanism is wonky. Full sheets of photo paper recommended.
- Had a weird issue with linux (had to reboot printer after each print...) that I didn't take time to figure out completely.
- Laminator: GBC HeatSeal H220 Laminator
- The RadioShack 0.065 ish boards + 2xPhotopaper stalled a few times in this laminator, but it has a release button (kind of a clutch mechanism) that you can keep pressed down to help big pieces through.
- Next time I'm just going to get thinner boards from some web store.
- Ink Jet Glossy Photo Paper: HP 'Everyday' Glossy Photo Paper
- Not recommended. This doesn't soak up water well and doesn't peel off board easily. But it's Walmart's least expensive option...
- I also tried plain paper (v0.01 boards were made with plain paper transfer) it does a passable job on big traces.
- I also tried parchment paper (on it's own too flimsy to make it through printer).
- I'm going to try this paper again next time, but taped (with something other than scotch tape) to a normal piece of paper.
- Copper Clad Board: I used the 4"x6" double-sided sheets you can get at RadioShack because I was in a rush.
- Ferric Chloride
- Used the off shelf RadioShack stuff.
- It worked 'okay'. I know there are other cheaper chemical options, I might give those a try next.
- Please look up how to dispose of this stuff (especially after it's infused with copper particles)
- In a nut shell, take it to a local chemical disposal facility if your in a rush.
- If you want you can allow copper to settle to a sludge and poor off the reusable ferric chloride.
- To fully neutralise it you must slowly add Sodium Bicarbonate (Baking Soda) to it until the (obvious bubbly) reaction is stopped.
- I HIGHLY recommend researching this before attempting.
- Treat it like a chemistry assignment for school, write down everything ON PAPER that you need to do, from opening the bottle to how you're going to do disposal. That way you don't skip steps or start the process when you don't have everything you need to finish it!
- Understand the chemistry, maybe not the gory details, but know the chemicals, safety procedures and the gases this stuff puts off when reacting.
- Not required but proved to be very handy.
- I MacGyvered together an agitator using Tupperware, CDs, a servo, an Arduino, and Duct Tape...
- The Arduino commands the servo position with pseudo-random timing.
After etching and soldering on all the components I went to town testing it on the Selectric II. I only attempted to hook it into the seven levers needed for key presses though. I'll post again when it's further along, but I did make a build video:
Solenoid Driver (v0.2) Eagle Files: Solenoid Driver (v0.2).zip
- This is a board to drive 8 solenoids (at up to 30 V) at a time. It uses:
- IRL2703 Power FETs (with 5V compatible gates)
- [overkill] 6A/1000V fly-back diodes
- 1kΩ and 10kΩ resistors
- Sparkfun screw terminals (here)
- Standard 0.1" spaced headers