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Old 03-30-2009, 12:42 AM   #1 (permalink)
Spider!
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Join Date: Dec 2008
Location: Houston, TX

SMAC mag - Spider!s Minimal Analog Control

This thread is for my EP automag project using a common LM556 chip for control of my pneumag ram. I will try to post all the problems and developments, but for now it works well and I will be wringing out the whole thing for a while.



I've always had trouble with ball collisions in my pneumag, since one break pretty much renders the accuracy from my revolver barrel kit useless. I wanted a way to prevent the trigger from keeping my automag from recharging properly. I went from a classic valve with an RT on/off to an x-valve with a ULT and still wasn't satisfied. I thought about a UTB, but I wanted less. I wanted something that ran semi-only, no modes or switches. I also wanted something cheap enough to put on a classic valve, where the board didn't cost more than a good stainless valve that would do what I want.

In the way back, when FORTRAN roamed the earth in large heaps and KP2 rifles were just starting to shoot semi-auto, paintball players talked about electric control of markers. Not electronic, electric, with capacitors, resistors, and such. Back then, the actuators were large and needed a lot of power. Most frames were small. Over the years people have discussed analog controls off and on, but they are generally as much work as a new micro-controller board without the features.

During that time I collected (some times inadvertently) various parts for an analog control circuit that I wanted to build. I noticed the new solenoids on Air Soldier Products and was interested by Hilltop Customs' EP mag on AO. While reading about the mess in Lukes garage (lukescustoms), someone asked why he worked that way. I know a few machinists, and I knew that probably that is the way he likes to work, with parts in hand. That reminded me that I have always wanted to do this, and some things have changed over the years that would make it work.

Things that have come together over the years are:

Power: LM556 timer chips are capable of 200 mA output, micro solenoids are down to 0.5W. This lets the old cheapie timer circuits drive the solenoid directly.

Precision parts: 1% resistors and 5% capacitors are easily available and much smaller.

Temperature stability: Metal film resistors and capacitors have very good temperature stability. The timer chips have always had good temperature tolerance.

Timing: with all of the info on other markers with solenoid dwell between 7 ms and 16 ms, as well as regulator recharge times, picking a dwell of 11 ms and a total hold out of about 49 ms was easy. A classic valve could use a little more, but I'm willing to live within those windows.

With all the discussion of tickler performance, I decided to take an old air gauge and drill and tap it for a 10-32 barb and checked it's output. Since I put my pneumag together, I had scratched a vernier on the side of my tickler to keep track of the setting. It turns out that it was usually running at about 40 psi, and occasionally about 60 psi when I had sear problems.





I gutted my old pneumag frame and setup a breadboard with my circuit and a test timer. The circuit is a delayed re-trigger LM556 with a couple of changes that I will get to. I will be posting the exact circuit I used and the web site that was most useful (there's a bunch).



I checked the timing with an old surplus oscilloscope. The time delays were just about perfect as calculated. That's when I realized this might really work.



I decided to throw out a few components...the on/off switch, the indicator light, and most significantly, the PC board. Since the chip is the largest electronic part, I decided to wire directly to the chip and it would be my board.


Specs:
Infinite trigger sampling rate (no MHz here)
Audio/visual indicator (marker goes "bang")
Battery saver switch (remove battery after play, you should have about 50 hours of run time on an alky battery).
Firing rate about 20 bps
3 shot burst (fan trigger with three fingers)
6 shot burst (fan trigger twice)
mechanical backup mode (get the other mag out)
eye mode - level 7 or level 10

The best part is that you can wire the chip in any form you want and stuff it in other places. I may try a venomous designs ule rail. The actual parts cost is about $10, so it's a good entertainment value.

Here's how the guts fit finally.


The switch is an 80g switch that I bought a bunch of to replace timmy board switches (that never needed it). Due to my frame's previous uses, the switch mount didn't come out clean, but the idea was to drill a hole of a diameter that would just fit the switch and pin the switch through, with the hole on the edge of the frame so that a slot would get cut for the wires to come through. I couldn't get a bit to run straight in my press on my carved up frame, so I drilled it by hand.




Not only does this give my classic valve a home, I now have an x-valve to put in the next project.
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Last edited by Spider!; 04-08-2009 at 12:16 PM.
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