hacking the BDM-100 blood pressure meter

Introduction

Because I like demolishing things and to a lesser extent because I am interested in ways to implement a working R/C submarine "on the cheap", I bought and demolished a BDM-100 blood pressure cuff.

This device is capable to pressurize a bladder, to deflate it controlled and to measure pressure. The mentioned R/C submarine would need a swimming bladder to control it's depth, which is measured by pressure. So, it might be usable... This page just documents what I am doing, to enable you to learn from my mistakes.

Originally these things are strapped to your wrist, they inflate a cuff and they detect your heart's pulse at varying pressures. Thus it can at least shut off the blood supply to your fingers. the manual of the BDM-100 indicates it can deliver pressure up to 280 mmHg

How to get it and what you get

I bought the BDM-100 at "kijkshop", but I also bought a cheaper one (18 euro's) in the Lidl supermarkets. The BDM-100 is fairly well documented on the Internet. It works with 2 AAA type batteries, so at 3 volts. it was also reasonably expensive, as in about 35 euro's.

What did I get out of the device ?

• a plastic bladder with two connection points
• one plastic type of valve, probably a safety relief valve, with o-ring
• one compressor with a 3 volt motor
• one electrically operated valve. Without current, the thing is open, with current applied it shuts off the airflow.
• a few tubes and suchlike to connect it all
• a circuit, containing a Metrodyne mps 2000 pressure sensor chip and a microcontroller
• an LCD screen
• some more o-rings

How to disassemble the blood pressure cuff

first, open the battery compartment. then, remove the screws on the backside of the device. these screws can be covered by the bladder, so be gentle when bending the bladder. Now the case front and back can be split open. beware of very short wires and tubes. what holds the halves together is the connection between LCD and circuit board. I cut the wires (battery, compressor and valve) to be able to remove the compressor and valve, and to open the case further.

With hindsight, try to locate two round holes in the inside of the front cover, under the display panel. if pressure is applied there, the glass of the display will detach. after that you can wiggle the LCD through a slit in the front case and the front can then be thrown away. a picture is worth a thousand words

remove all screws found around the circuit board. this causes the tubing to detach from the circuit board. also you can now remove the circuit board by pushing on the plastic clips holding it in place.

Remove the screw found in a recess on the outside of the case. this is the top side when using the display as an orientation. of the case. Then, locate a clear plastic pin under the circuit board and close to the battery compartment. when this is pushed the right way, the bladder will detach. Remove the velvet cover of the bladder carefully.