Retrofit an Old CPAP Machine Into a 'ventilator' Like Device

Retrofit an old CPAP machine into a ‘ventilator’ like device

If you have an older model CPAP machine in your closet that you don’t use and have hobbyist, or better, soldering skills, you can convert that old CPAP into a ventilator-like device. This instructable will guide you on how to do this for the CPAP that you have and uses very common components that you may already have in your electronic parts ‘junk box’. First some background on CPAP devices:

CPAP machines are designed to help people with Obstructive Sleep Apnea. These people stop breathing intermittently through the night. Sleep Apnea is a very serious condition and can shorten a person’s life if not treated. A CPAP machine keeps their airway open (not obstructed) all night so they can breathe much better.

CPAP stands for Continuous Positive Airway Pressure. It provides positive pressure only, continuously. When we breathe, our diaphragm creates both positive and negative pressure as we inhale and exhale. A ‘real’ ventilator will do exactly what our diaphragm does, it will create both positive and negative pressure. So, a CPAP device is NOT a substitute for a ventilator.

There are more advanced CPAP machines called BiPAP that (from what I have read) doctors are using as an alternative to ‘hard to find’ ventilators. These devices produce two Positive Pressures, a high and a low. The high pressure is for inhaling and the low pressure is for exhaling. A doctor will prescribe this type of machine to a sleep apnea patient with more serious respiratory problems since the machine makes it easier to exhale.

So, this instructable will retrofit an older model (that is, dumber) CPAP machine to work like a BiPAP device with the exhale pressure at zero. The inhale pressure will be what the machine is set to provide.

Please note that if you construct this device, YOU MUST USE IT UNDER A DOCTOR”S CARE. Just because you can solder, doesn’t make you a medical professional trained to set up the operation and use of this device on a sick person.

OK, so how does this work? The older CPAP machines had to be manually set up for the pressure level they deliver, this was usually done in a doctor’s office. The machine would NOT automatically turn itself on when you wore the mask and started breathing (as the more modern ones do). You had to press a button to start it. And you had to press a button to stop it.

This feature (or lack of it) is how this instructable creates the two pressure levels. A relay switches the machine on, then switches the machine off. It does this repeatedly at a rate controlled by a knob. This, ‘respiration rate’ is another control that needs to be set by a doctor. The average rest respiration rate is about 15 breaths per minute for a man of average build. But, obviously, this will be different for different people, only a doctor can set this correctly.

Please see the picture of the assembled project. I used a Respironics REMstar plus CPAP machine that I had sitting in a box, gathering dust. Please find a close up picture of the respiration controller. A schematic of this design is also shown. The potentiometer will produce respiration rates from 8 to 93 breaths per minute. Note this is not on a linear scale. The mid-range setting is about 15 breaths per minute. 10 breaths per minute is at the 20% point and 20 breaths per minute is at 65% point.

The LED will blink with the relay. Use your ‘stop-watch’ feature on your smartphone to set the respiration rate with the potentiometer. The relay will click on and off, twice per respiration cycle (inhale phase and exhale phase). The LED will do the same.

The use of a relay makes this project possible on many different CPAP machines. The actual relay used here is a double form-C type. Form-C relays have Common, Normally Open and Normally Closed connections and so can be wired into most any power switch circuit.

Download the complete PDF as there are alternative circuit designs shown besides this one.

Good luck, and remember: CONSULT WITH A DOCTOR BEFORE USING THIS MACHINE ON ANYONE. Let the doctor read through the CPAP machine’s manual to set up the proper pressure level and let him adjust the respiration rate on the project board. You may wish to mark the respiration rate knob and it’s PCB with a permanent marker (or nail polish) ‘hash mark’ so this adjustment point is recorded.