For my 4th year project at university I had to select a group electronics project, this was a fairly large project to be completed with several others. I can't go in to details of the project because it still has potential as a commercial product but I wanted to talk about one aspect of it.

One part of the project involved creating multiple air pockets or bags, these each needed independent pressure control so they could be told to inflate or deflate on command. I worked on the majority of this section on my own and it gave me the most trouble.

The system I devised was fairly straight forward, there was an air bag or an inflatable football as I decided on, there were 2 air valves one to inflate and another to deflate and there was a pressure sensor and some electronics. Each of these things caused trouble.

Firstly the air bags were generally trouble free although they did have a tendency to pop when they were under pressure, the footballs were the only thing I could think of at the time.
The valves had a very particular requirements, they needed to be able to control pressures of between 0.5 bar and 2 bar, the problem with this was that all the commercial valves I could find only started working at 2 bar, some accepted a pilot switching supply but this was really to much work and expense for the project. We ended up settling for a solenoid purge valve from a car engine! This isn't really my area but this device is used to purge fuel vapour from the engine and operates at a low pressure and at 12V so was fairly good. In the long run they were very problematic, hard to obtain as most were normally open and I needed normally closed and they also drew a low of current, a big problem when you need almost 200 of them.
The final part was the pressure sensor, I ended up using a part from RS, this was a straightforward device that outputted a voltage proportional to pressure, the only problem was that it measure the pressure in the pipe network rather than the football so the valves would continually switch on and off as it filled or deflated, this could be overcome with filtering of the pressure reading or adding an air buffer before the sensor but I had other things to worry about.

Anyway, this project was some time ago and thankfully I don't need to work on this bit again, the reason I am writing this is because it dawned on me a few weeks ago that a blood pressure monitor that I have sitting in my draw implements most of this technology.

I dismantled my unit and inside were some of the parts I had so much trouble with a few years earlier.

Mounted on a single PCB was a motor driven compressor, a pressure sensor, a solenoid valve and what appears to be an emergency release pressure valve.
The whole unit runs from 3v and draws about 500mA when running, the compressor must be one way so when it stops compressing it doesn't let the air out, this eliminates the need for a second valve.
The pressure is a simple unit from Metrodyne an MPS-2100 series device.
The valve is a typical looking solenoid coil, the air pipe connects to one end and inside must be a plunger which gets moved into place to block the flow of air; so much simpler and neater than the purge valves.
Also in the air network is a small metal tube with no marking on, I am guessing that at a certain pressure it fails and safely releases the air.


Blood pressure meter internal electronics

The other interesting thing was the air bag being used, it was constructed from 2 pieces of plastic and heat sealed down the edges, this was something I had considered for my air bags but as I didn't have a heat sealer (they cost about £100) I opted for the simpler solution.

If anyone's interested the make and model of the unit above is, Yahorng - AK3000T V2.2. I think Yahorng is an OEM builder because I bought a re-branded one.