logic-gatespiezotransducer
I am taking apart this non-invasive blood pressure monitor to see how it works. The engineer decided to use (what looks like a piezo-electric element to measure the relative pressure from the cuff, only I cannot decipher the circuit and how it works. It appears to be using a quad NOR gate to amplify the signal before it is sent to the MCU. The question I have is how is it able to measure the exact pressure from the cuff when NOR gates output digital signals?
Attached are photos and schematics.
Yes, to both of your questions. The symbols used for "+5V" or "Vcc" or "ground" are simply notational conveniences which mean "all the stuff with this symbol is connected together". We can also assume, if it isn't explicitly drawn, that there is a 5V power supply connected across "+5V" and "ground".
It doesn't mean anything beyond that. It's a common beginner mistake to think that "ground" means something special. It doesn't.
An exactly equivalent schematic can be drawn without those symbols:
simulate this circuit – Schematic created using CircuitLab
...but since power supply connections are so common, that would mean some very long lines to follow over the schematic. Not a big deal for a simple schematic as you have, but for something non-trivial with a schematic spanning many pages, you can imagine...
I believe this is the simplest way to achieve what you are looking for:
simulate this circuit – Schematic created using CircuitLab
Edit: As KingDuken points out in the comments, this is known as an imply gate.
Best Answer
The diaphragm is a variable capacitor. The resistor feedback (three sections of NOR, U3 output to U1 input) acts to bias the gates as a linear amplifier cascade. I think you are looking at a capacitive barometer sensor, possibly with oscillation frequency dependent on pressure. It might also be a nonoscillating amplifier, which would be useful as a stethoscope (blood pressure is usually measured with a pressure gage and a stethoscope).