Author Topic: Designing a Pressure Sensor Without a MCU  (Read 698 times)

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Offline MastermimeTopic starter

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Designing a Pressure Sensor Without a MCU
« on: March 24, 2013, 11:40:03 AM »
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

Offline MrWizard

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Re: Designing a Pressure Sensor Without a MCU
« Reply #1 on: March 24, 2013, 12:54:30 PM »
Whats wrong with a contact switch on a scale ?

Offline waltr

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Re: Designing a Pressure Sensor Without a MCU
« Reply #2 on: March 24, 2013, 01:34:14 PM »
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.

Offline MastermimeTopic starter

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Re: Designing a Pressure Sensor Without a MCU
« Reply #3 on: March 24, 2013, 08:27:02 PM »
Quote
Whats wrong with a contact switch on a scale ?
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.

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?
« Last Edit: March 25, 2013, 07:22:14 AM by Mastermime »

Offline jwatte

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Re: Designing a Pressure Sensor Without a MCU
« Reply #4 on: March 25, 2013, 12:21:52 PM »
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.


Offline MastermimeTopic starter

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Re: Designing a Pressure Sensor Without a MCU
« Reply #5 on: March 25, 2013, 06:27:20 PM »
Quote
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.
Ok good idea with the op amp and Schmitt trigger. Attached is a schematic with an op amp and Schmitt trigger.  I hope I understood what I read correctly.  Tell me what you think.

Quote
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.
I don't really want to go this route b/c its a completely different design and I almost have figured the one above out.  It just seems more simple.

Offline jwatte

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Re: Designing a Pressure Sensor Without a MCU
« Reply #6 on: March 25, 2013, 08:50:07 PM »
Seems like it should work to me. I'm used to VCC-to-GND opamps, where GND is the most-negative, but if you have a symmetric supply (-12V available) then that looks legit. Probably want a resistor from the opamp to the transistor base, and a large resistor between base and ground, for good measure. Also, if you're driving a large load, you probably want that transistor to be a MOSFET instead of a BJT, but if that's just something like a 60 mA relay coil, probably makes no difference.

Offline MastermimeTopic starter

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Re: Designing a Pressure Sensor Without a MCU
« Reply #7 on: March 25, 2013, 11:28:50 PM »
Quote
Seems like it should work to me. I'm used to VCC-to-GND opamps, where GND is the most-negative, but if you have a symmetric supply (-12V available) then that looks legit.
That was a mistake.  I am using a car battery.  I guess I misunderstood the tutorial I was watching.

I updated my circuit with the new additions to the transistors as well as to the op amp (1 resistor).  I am not completely sure of the low trigger point equation.  So would I just set it to a minimal voltage level such as 1v?

Let me make sure I'm getting everything straight.  When the pressure increases, the voltage output of the voltage divider will increase above the upper trigger point and in order for it to stop, it must drop below the low trigger point, correct?

Btw thank you for the help.  Much appreciated
« Last Edit: March 26, 2013, 11:56:30 AM by Mastermime »

Offline jwatte

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Re: Designing a Pressure Sensor Without a MCU
« Reply #8 on: March 26, 2013, 07:36:07 PM »
I wonder if there will always be current from +12V to the base of that transistor through those resistors? The opamp may not be able to source/sink enough current to counteract that. But you reached my limitation in analog electronics, so I'd suggest googling some more for helpful circuits, or perhaps Soren is online and will have a better idea?

Offline MastermimeTopic starter

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Re: Designing a Pressure Sensor Without a MCU
« Reply #9 on: March 26, 2013, 09:40:41 PM »
Ok thank you for your help. I'd never thought there was a limit to your knowledge lol.   

 


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