This sensor is based on the Lucas NovaSensor NPC-410 Series pressure sensor. The circuit below contains the usual powered sensor interface, but I used an LM358 dual opamp in place of the usual LM324. The 78L05 regulates the voltage from the RCX down to 5V. This is used both for the pressure sensor bridge and to invert the signal so that the RCX will read 0 for 0 pressure and 100 for 30psi. NPC-410-030D-3L is a 30psi max differential pressure sensor available from Digikey as #11003-ND for $17. You could use a gauge type pressure sensor, but the differential model allows you to measure vacuum as well as pressure.
Pictured below is my finished pressure sensor circuit board. I used the corner of a Radio Shack PC board to construct the project. As you can see it is pretty compact. The lower tube in the photo is used for pressure and the upper tube is not connected. If you want to measure vacuum, you connect the upper tube and leave the lower one disconnected.
In operation the RCX is set to Light sensor type. The charts below show how the RCX reading relates to actual pressure and to the number of times a large LEGO pneumatic pump is compressed in a system that has the small air tank like the Search Sub model. I find that the LEGO pneumatic parts start to leak at about 40psi so the operational range of the sensor looks good.
I haven't committed the project to a decent enclosure yet. For now I'm using a box made from beams with plates for the top and bottom.
To check the calibration of the pressure sensor I purchased a Monkey Grip brand digital tire pressure gauge from KMart for $8.00 and disassembled it. The original product is pictured below along with the setup you need to use to measure LEGO pressure with it. Unfortunately, the tire pressure gauge was not designed to measure pressure continuously. It expects you to press it up to a tire stem, remove it and then look at the numbers. A LEGO pneumatic switch provides the needed "press and remove" action. To make a reading, you toggle the switch down and then back up. The LEGO pneumatic T is just there to act as a plug to prevent the incoming air supply (blue tube) from rushing out when the switch is in the down position. The gauge takes a single reading of the pressure and will hold the value for several seconds. Only a tiny amount of air is lost for each reading.
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