# Using force cells as a switch to detect presence of weight

Discussion in 'The Projects Forum' started by summersab, May 6, 2010.

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1. ### summersab Thread Starter Senior Member

Apr 8, 2010
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0
Without giving too much detail, I have an application that requires that a load cell be used to detect the presence of an applied force and work as a switch. When a force is applied above the basal force, the circuit goes high. When the force is removed, the circuit goes low.

The load cell is based off of a 1.5 kOhm strain gauge model, and I have it in a Wheatstone bridge of 1.5 kOhm resistors with 9VDC applied. When a force is applied, the change in voltage across the nodes is, as expected, a VERY low voltage (we're talking fractional volts). So, I've been looking at using an instrumentation amplifier to up the output voltage:

http://en.wikipedia.org/wiki/Instrumentation_amplifier

However, this doesn't solve the "switching" part of the circuit. I need the load cell to be connected to some sort of a circuit that will go high (5VDC) when a load of +-10 lbs is applied to the load cell (output of something like 0.05 VDC).

How do I do this? An analog-digital converter? I'm not familiar with how they work. My background is Mechanical Engineering (some electrical courses).

**Edit: Perhaps as a way of abstracting out the complications, is there a circuit that can detect a small change in voltage and throw a digital circuit high? If I used an ADC, something that could give an output of 5V for every input that is >0.05V. Make sense?

2. ### Bychon Member

Mar 12, 2010
469
41
Just set the amplification factor of the instrumentation amp so high that its output goes saturated. Then run the output through a resistor to a 5 volt zener diode to limit the output to 5 volts.

3. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
It would be easier to use an instrumentation amplifier to get a linear response, and then use a comparator with an adjustable threshold level w/hysteresis.

Don't try to build an instrumentation amplifier using opamps unless you are a glutton for punishment, and have lots of free time.

Have a read through this Intersil Application Note:
http://www.intersil.com/data/an/an1298.pdf

Have a look at the TI/Burr Brown INA128/INA129 inst. amps:

You might find excitation of the bridge using an AC signal an interesting topic:
http://www.analog.com/static/imported-files/application_notes/4924359049444287803AN_683_A.pdf

Rob Paisley's page on comparators is pretty good:
http://home.cogeco.ca/~rpaisley4/Comparators.html

4. ### summersab Thread Starter Senior Member

Apr 8, 2010
144
0
@SgtWookie, why is everyone against using op-amps? Sure, they're REALLY old technology, but that's what I used in undergrad 2 years ago. The reason I like them is that they're AVAILABLE - I can go to Radio Shack and pick one up for a buck along with a pack of resistors. If I get one of those ICs, I have to wait a week for shipping. Can I build a working instrumentation amplifier that works well enough with a LM324?

@Bychon, yeah, that's simple enough . . . thanks!

5. ### Bychon Member

Mar 12, 2010
469
41
With .05 volts of difference to amplify, you need at least a gain of 100 to get it to 5 volts. The 324 chip has an open loop gain of at least 15,000.

6. ### summersab Thread Starter Senior Member

Apr 8, 2010
144
0
That was Greek, sorry. Pretty much, "LM324 won't work - you can't go to Radio Shack for this one, buddy," right?

7. ### Bychon Member

Mar 12, 2010
469
41
Sorry you can't read Greek. I'll try it like this: You need at least a pound of gain from the chip. It has 150 pounds of gain. That means "plenty".

Last edited: May 7, 2010
8. ### retched AAC Fanatic!

Dec 5, 2009
5,201
316
He didn't say dont use an op amp, he said :
Op amps are also very current technology. The opamp you can get from Radio Shack IS OLD tech, but you can get very modern op amps , just not from RS.

9. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
I see this -
- and have to wonder if a sensitive basic microswitch would not work?

10. ### retched AAC Fanatic!

Dec 5, 2009
5,201
316
I thought that myself.

A spring return switch would be pretty easy.

11. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Go the micro-switch route if you really don't want to learn anything and just get the job done. For the moment. Until the switch fails, and then you have to deal with it again.

Opamps are great. Except for opamps like you used in undergrad; the 741's in particular. Please trust me on this; reading the material I've supplied links to will explain a LOT of things, and the instrumentation amps I've linked to will save you lots of gray hairs.

You're new. I usually have to explain things three times over to "n00bs" before they begin to realize that I'm giving them some straight scoop. I really don't have the time to do that multiple times for everyone...

You will not even approach the accuracy of the parts I've suggested using Radio Shack parts.

If you feel so inclined, beat your head against the wall trying to do so. You will fail, and I've posted documents explaining precisely why you will fail; but if you use RS parts, you will already be at a severe disadvantage.

Just trust me on this. Save yourself the heartache. Order a couple of the parts I've suggested, and get the job done.

Apr 20, 2004
15,815
293