Monday, 09-02-2004

I was in the middle of replying to an email today and i had a 'bing' moment... could a car tyre pressure gauge be used to measure the air pressure in a Lego Pneumatic system?

Only one way to find out... so off i went to Halfords... I found a little digital pressure gauge, price 3.99... so i bought 2 :)

Click on a piccy to open in new window.

The first unit, i took apart... i found that the tubing i'm using fits snugly over the pressure gauge nozzle, however the tube kept on popping off the gauge nozzle under higher pressures.

I decided to dismantle the unit so that i could file out the hole around the nozzle enough to clamp the tube using the body of the pressure gauge.

The battery fell out whilst i was dismantling the gauge... a bad thing to happen... the unit is factory sealed and i couldn't reset the gauge to read pressure again, perhaps the unit is calibrated after the battery is put in in the factory.


Ho hum... it doesn't work :( Air pressure sensor Sensor contacts (circled)


With one of the pressure gauges being unusable, i did the decent thing, and investigated its workings.

The middle picture shows the pressure sensor inside the gauge, inside is a pad that gets pressed against contacts with air pressure. The higher the pressure, the harder the pad is pressed against the contacts.

Circled in the right hand picture are 2 test points. With different air pressures, there is a difference in resistance across these 2 points. With high pressure, the resistance is around 40 ohms, with low pressure about 150 ohms and no pressure is an open circuit.

After discovering that the sensor inside the pressure gauge is little more than a variable resistor, can it be adapted to be wired up to an RCX?
I think so :)


With the second pressure gauge, I carefully trimmed the case and exposed the sensor nozzle. A small cable tie holds the tube onto the sensor nozzle. A large cable tie holds the body of the gauge together.

The pressure gauge is automatic, the unit activates when air pressure is applied. The air pressure must be released for the unit to zero before another reading can be taken.

The picture above shows the test circuit.

Switch 1, used to activate/zero the pressure gauge.
Switch 2, enables the pressure in the air muscle to be read.
Switch 3, activates the air muscle.
Switch 4, enables the stored system pressure to be read.


Max system pressure 37 psi 22 psi, pressure in the muscle after inflation 21 psi, pressure left in the tanks

I tried several times to obtain a max working pressure reading... each time, a hose came off between 34 & 36 psi, I managed to get a reading of 37 psi just before hose came of one of the tanks.

If i got my maths right, the air muscle uses about 40% of the stored air to inflate... gulp... J5's air storage needs to be huge!!!

In conclusion;
The maximum working air pressure for Lego Pneumatics is around 34 to 37 psi.
The type of air pressure guage used has definate possibilities in being adapted to provide continuous air pressure readings to a suitably programmed RCX.
J5 is going to need a large air storage capacity.


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This page is part of a diary of the design and build of my Lego Mindstorms robot J5 (based on Johnny Five from the film short Circuit.)
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