Adjustable Air Pressure Switch

Written by: admin@makezilla

I have a cheap-o electric air pump that puts out 120 psi (so the manual says), but I need a steady 20 psi for a project that uses a small pneumatic cylinder.

Oh, what to do, what to do.

Have you priced air pressure switches or regulators? Sheesh! Expensive!!!

So here's my home made air pressure switch. Nothing fancy, but it works fine for my simple needs.

I use a propane tank as the pressure reservoir, with an accurate air pressure gauge attached. The propane fitting is converted to standard pipe thread with an adapter.

Make sure all tanks, fittings, and air lines you use are rated for the working pressure that you expect. Safety first.

3D model created (by MEEEE!!!) with AutoCAD 2008.

Step 1: PVC Parts

PVC fittings for this project:
3/4" MPT plug
1-1/4" to 3/4" FPT adapter
1-1/4" coupler
1-1/4" pipe nipple
1-1/4" FPT cap

A 1-1/4" pipe plug can be used instead of the 1-1/4" adapter and 3/4" MPT plug (save some $). I'll be mounting my air inlet fitting thru the 3/4" plug, so I have options if I want to change the fitting later.

If you have never used PVC cement, it would be a good idea to consult with someone who has. It's not difficult, but there is a definite technique required.

Step 2: The Diaphragm

The diaphragm will bulge upward, pushing the actuator shaft, whenever there is pressure at the air inlet. 

1. The rubber washer is 2 in diameter and 1/16 in thick. Material is neoprene (I think). It can be found at the hardware store waaay in the back where they keep the hardware bins. It must be carefully marked and cut to fit the ID of the 1-1/4" coupler. The hardware is #6-32. Use a screw long enough to leave several threads exposed.

2.When you are putting the diaphragm together, schmear RTV sealant on all the parts. Tighten the screw only enough to make sure the RTV is making a good seal. Too much pressure will distort the rubber. Wipe off the excess sealant.

3. Cut a piece of 1-1/4" pipe 2" long. Set aside.
We need to cut two spacers from the leftover pipe. The spacers are 3/16" thick. They must be square and uniform in thickness.
I used a back saw and clamped the pipe securely in a miter fixture. I'm sure there is an easier way to do it. (I have no lathe. Sigh)
Use a saw to cut a gap in each spacer. This will make assembly easier.

4. The white piece on the left is used as a backstop for the diaphragm. Without it the spring would push the diaphragm down (and ruin it) when there is no air pressure applied.
Material is 1/16" ABS. Any thin, stiff material will work. The diameter is the same as the ID of the coupler.

Step 3: The Diaphragm Assembly

1. Use PVC cement to install the backstop and the first spacer. Apply just enough cement to avoid air pockets. (Use a Q-Tip)
Apply RTV sealant to the top of the first spacer and the outer part of the diaphragm.
Use enough sealant to ensure an air-tight seal without getting it all over the works. (Have more Q-Tips ready)

2. Place the diaphragm on top of the first spacer, apply more RTV, and cement the second spacer against the diaphragm.

3. Cement the 2" long pipe nipple into the coupler on top of the second spacer.

4. Cement the 1-1/4" adapter into the coupler.
Use Teflon thread tape or pipe dope to install the 3/4" MPT plug. This part of the plenum will be under pressure.
I will be drilling thru the 3/4" plug to install the air inlet fitting. Probably a small hose barb.

Step 4: Pressure Adjuster Screw

The adjuster screw is moved up or down and applies a force on the spring. The higher the down-force, the higher the pressure setting.

The threaded ferrule and the hollow screw are parts from a broken (and very ugly) table lamp. These parts attach the harp, where the lampshade hangs. The lamp power cord goes thru the hollow screw. A home center will carry these in the lighting department.

Thread diameter is 3/8". The adjusting screw is 1-1/2" long.

Drill a 1/2" hole in the center of the cap, and epoxy the ferrule into place.

Step 5: Actuator Shaft

The shaft will move up and down with the diaphragm to operate the limit switch.

1. The shaft is made from 1/4" brass tube 4-3/8" long. Solder a 1/4" brass washer 3/4" from the bottom of the shaft to hold the spring. Your dimensions will probably be different.(Ponder this step until the shaft length and spring position can be determined.)
The bottom of the shaft (on the left) fits over the diaphragm screw to keep the shaft centered.
The top of the tube must be closed so that it will actuate the switch. Solder a small piece of brass sheet to the end, then file it down to the shaft diameter.
The spring is from the local hardware store. It is 3/8"D and 1-3/4" long. The wire diameter is .055".
A second 1/4" washer is placed above the spring to make contact with the adjuster screw.
This configuration gives me a max pressure setting of 35 psi. A heavier spring will give a higher range. You will be able to replace the spring and modify the shaft later if needed.

2. Install the shaft assembly. Make sure it slips over the diaphragm screw. Don't use sealant on this joint, there is no pressure in this part of the plenum. We want the cap to be removable, so finger tight is good.

Step 6: Mount the Switch

1. I'm using a MicroSwitch style limit switch from a dead microwave oven. It was a door safety interlock. Current rating is 16 amps.
The MicroSwitch open and close points are (theoretically) at the same actuator position. Too large a displacement will cause the pressure to drop considerably before the switch is re-energized. You can fine tune the switch position by screwing the PVC cap up or down.
Any momentary pushbutton switch will work if it has a consistent open/close point. It must be a normally closed configuration. When the desired pressure is reached, the actuator is depressed, and the switch opens.
Note: An option is to use a normally open switch, and drive a relay connected to the air pump using the relay's normally closed output.
If you use a switch with a low current rating (such as a tactile pushbutton), you will need to drive a relay to supply power to the air pump.
The switch is wired in series with the air pump's power input. An inline fuse holder can be added to the power cord.

2. To mount the switch I used 3/4" x 1/16" aluminum strap. I'm sure you'll find something in your scrap bin that will work.
Self-tapping panel screws attach the brackets and switch.
There must be clear access to the adjuster screw. This will be turned in or out using your fingers to set the pressure point.
The shaft height stays the same no matter where the adjuster screw is. That means that the shaft and diaphragm will move less than 1/8" (with proper switch positioning) at any pressure setting. Very little stress on the diaphragm.


Final Thoughts

With a heavier spring and higher pressure settings, flats may have to be filed on the adjuster screw so that a small wrench can be used. (Or add a knurled knob to the adjuster screw? Let's be creative.)

Higher pressure settings will create more force on the threaded ferrule. I would consider finding a more substantial way to secure the threaded ferrule into the cap.

Looking at the pictures, you might think that this thing is a piece of junk. But, it turned out to be a precise pressure switch to rival them expensive store-bought ones.


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