Monday, March 4, 2019

VR Pneumatic Recoil Rifle Stock: DIY Part 5: Recoil Device and Driver

Recoil:  the big one. -----------------------------------
Soon a product called force tube *Go support their kickstarter* will be releasing an electric recoil.  Stated to provide 60N force over what looks like a 1cm throw of an inserted slot in the butt pad.  It bumps the shoulder.   But my too powerful for fields G36 airsoft GBBR kicks fairly nicely and I wanted something that kicks at least as hard as that.   Electronics likely isn't the way to go for a solid kick unless you are using some sequential fired coilgun like solenoid to get the momentum transfer (or impulse).   At least not without a huge amount of power and likely a pretty heavy block of copper wire for the solenoids and some powerful ND54+ magnets that need to be sufficiently impact protected. 

So looking into it I came up with a design using the following for a


So I took my Stock from this.

To this.

Granted I'm now cleaning it up to look nicer, but hey it works beautifully!

And here is how.

The basic parts needed are as follows.

1. Double acting pneumatic cylinder  OD  1.125" to 1.5",   Stroke  2-5"   Cost  20-60USD 
Longer throw is better but depending on where you mount it you may not be able to have that much length without interfering with a controller or getting recoil at a strange angle.  Ideally it impacts the but plate to transfer momentum directly to the shoulder as transferring it just to the rifle stock will likely overpower any magnets you have holding your controllers to the stock.   I could see modifications to MAMUT and PROTUBE stocks to allow a kick in one direction to bottom out a stop on the magnet blocks while minimally influencing their effectiveness for rapid release and reattachment, but for now its easier just to put the weight right up against the shoulder. 

2. Directional Control Electric Solenoid Air Valve 4 Way 2 Position 1/4 Inch 12V DC   20-40usd
gives about 20ms response which is good.

3. 100ft 1/4" DOT Air Line Bags Air Ride Suspension 100 FT 1/4 inch 100'  30-40usd to connect the compressor to the recoil device. Run it down the cable for the HMD, then along the sling to the stock. A Quick fitting is recommended to detach the sling from the HMD.

4.  5 PCS 1/4" OD x 1/4" NPT Swiveling Elbow Metal Push In to Connect Tube Fitting   10-20usd  for nice easy connect/disconnect keeping a fairly low profile.


and a bunch of lesser parts.

555 timer
12V - 5V regulator  POLOLU 5V  600mA --accept no substitute, this thing is cool and efficient.
Ho31 A57 Nfet
2x  1kohm POT
1x 10kOhm POT
2x 1n4148 diodes
100nf cap--or so
some resistors 1kohm to 10kohm.
NTE3036 phototransistor
prototype board
0.8mm coax wire  1/2ft
1  4mm or 5mm superbright white LED  3-3.4V operation 20mA max
and other random minor circuit components
some zip ties for initial setup
1-1/4" to 1-1/2" OD aluminum tube (or larger) to protect you from the bounce weight.
1 to 1-1/8" steel rod at least 2" long for a bounce weight 8oz with threads to mate to cylinder.
Ability to drill hole in steel rod and cut and drill aluminum tube as protective sleeve
Some creativity and sensibility in how you mount everything.
1/8"OD steel or brass tube 2-3cm long for trigger pivot rod.
1x  4.7mm OD x 3-6mm long ND magnet
1x  1/8" OD  3-5mm long ND magnet

PC7 epoxy
JB weld
liquid electrical tape
hot glue
5min epoxy
a torch never hurts 

Total cost can be as little as 200 dollars, or as much as 400 depending on what you already have and if you buy a spare controller for the hack.--recommended just in case...

------Fire control circuitry:
This is pretty simple.
You hack the haptic controller but stop when you get to the haptic linear oscillator as outlined in part 4.

For the Oculus touch you will find it shorts at 11mA and opens at 3V, so just enough to drive a superbright LED, white  with a 3V threshold and 20mA limit.  You may need to play around with this but if you get red the breakdown is lower and you need a resistor so you don't over source the controller.  If your threshold is too high it just wont light up.   Also if you get too low of a threshold diode the diode will light up on haptic pulses for mic presses,  slide release, mag release etc, so try to get an LED close to the 3v limit, but still too bright to look at and draws 10mA at 3V (use 2 AA batteries and a DMM to test this)  This way the LED filters undesired haptics.   Test it in game before closing up the controller for good. 

Next remove the haptic and run a 0.8mm coax, or 2 thin silicone wires up through the controller and out to the magnet mount where you will drill a strategic hole and install the LED.    now drill a corresponding hole and mount the sensor inline to pick up the signal.  I housed the photodiode with a small brass tube for robustness and to reflect extra light.  I also installed a tinny 1/16" magnet beside the brass tube to attract the metal can of the photodiode so I can easily remove it for testing and let it snap back in place.

The below circuit takes it from the photodide and converts short and long haptic pulses into a sequence of pulses as long as you tune it.  So you have to tune it for the weapon you are using.  I find this easily done with one of the two POT's  RV1 or RV2.  These control the up and down pulse width thereby controlling pulse width and frequency.  You typically want your up pulse to be no longer than 20-30ms and your down pulse 30-100ms depending on the weapon your simulating.  Since we will put a physical dampener on the return stroke (down pulse) you want it longer so If you are short stroking try turning the down pulse longer and the up pulse shorter.

EDIT: Note do not use LiIon batteries. The stock kicks too hard and after 10k rounds or so compacts the foil in the batteries.  Use a tethered 12V supply parallel to the airline or NiCd  NiMH or other batteries proven to take a beating with radio controlled vehicles.  (dont use LiPos without great dampening)
Also lesson learned, You need 12V to drive the solenoid so I used 3 lithium batteries  6000mAh.  However!  if you use a cheap step-down converter it will likely be inefficient, overheat and fry.  I used a Pololu 5V  600mA and it stays nice n cool.   other Chinese crap tend to burn out with a drop from 12V to 5V at a few hundred mA.  You may need to play with the cap and resistor values but these should be pretty close to what you need for 600-900RPM fire rates.

You may also put an interrupt switch in line with the signal from the 3036 photodiode to the 555 timer pin 4 which is only "on" when the front magnet mount is engaged.  This helps further with the white LED to prevent misfires and is quite effective.    While I look forward to hacking a Forcetube control board after their kickstarter, you almost don't need to.  But the tuning per weapon may get old after a while.  It takes 10 seconds typically but may take 2 minutes if your a little slow or didn't design enough range in the circuit PWM with RV1 and RV2 (variable resistors or POTs).

Operational current on standby at 12V is about 15mA  and up to about 150mA peak per shot fired.   3 lithium batteries at 6000maH will last a while.

You are using 60+Psi to accelerate an 8+ oz mass toward your arm.  Make darned sure you have built a sleeve and frame of sufficient robustness so you CANNOT GET HURT.  Test fire it on the floor first.  A lot!   Also make sure the mass impact the frame right against your shoulder, or hits a movable part that hits your shoulder directly to reduce shock waves travelling through the frame and making the accelerometers in the controllers jump.

Operating it like the picture below without a mount and without stops is dangerous. Don't do it.  Plus without a dampener at the end of each stroke your likely to destroy the cylinder.  This particular model had integrated air dampeners.  Most dont.

Part 1: Intro and disclaimer
Part 2: Trigger and grip mod:
Part 3: Fore grip Mod:
Part 4: Modifying the Haptic for a signal
Part 5: Pneumatic Recoil parts and Fire Control Circuit 
Part 6: Scent Module


  1. Please... Help me contact you... I need your help with doing this... You're a genius Sir!

  2. You're a genius Sir
    Please send me your email