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Designing a Pressure Sensor Without a MCU

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Mastermime:
Hello everyone,

I am trying to design a simple pressure sensor without the use of an microcontroller unit (Arduino, Axon, etc.) where once a force sensitive resistor reaches a certain threshhold pressure, send a signal to a Darlington transistor which will turn on a larger device.

The problem I am having is how to measure the pressure without a MCU.  I am not experienced at working with microcontroller chips such as the Atmega644 or anything like that.  Could I use a a microcontroller chip or is there an even simpler chip (something like a 555 timer) that could do the job?  Or would it be possible with no chip at all?

Any ideas are welcome

Thanks

MrWizard:
Whats wrong with a contact switch on a scale ?

waltr:
Convert the pressure sensor output to a Varying Voltage then use a Comparator IC (LM339 is an old one) to drive a transistor switch.

Really this is very easy and has been done long before micro-controllers were available.

Mastermime:

--- Quote ---Whats wrong with a contact switch on a scale ?
--- End quote ---
I'm a little confused on what you're saying here


--- Quote ---Convert the pressure sensor output to a Varying Voltage then use a Comparator IC (LM339 is an old one) to drive a transistor switch.

Really this is very easy and has been done long before micro-controllers were available.
--- End quote ---

Oh yes I forgot about a comparator.  I will design a schematic and post to see what you guys think


Edit:  I forgot to mention something. I'm measuring air pressure. Will an FSR be a viable device for this?

jwatte:
Only you will know whether a FSR will be sufficient for air pressure. If the pressure differential is large, I would expect it would be OK.
I would use an opamp rather than a comparator. I've found a comparators like the 339 to have a larger transition zone than an opamp set to 5000x amplification. The transition zone, in turn, is somewhere where your device may be half-triggered, which is bad.
You probably want to clean that up with a Schmitt trigger, too.

That being said: If you use a comparator, or opamp, you're already worrying about a DIP-8 socket and supply voltage. Why wouldn't you use something like an ATTiny85 in the same form factor, with an ADC, and some digital I/Os? The Tiny will run at 1 MHz or 8 MHz clock without any external components.

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