Tutorial: Op-amp circuit for force sensor signal conditioning

This is a description of a simple opamp circuit that will translate the variable resistance of a Flexi-Force sensor into an analog voltage and maintain linearity across the sensor's measurement range.

http://www.tekscan.com/flexiforce.html

http://www.national.com/ds/LM/LM124.pdf

Your feedback and topic suggestions for future tutorials are welcome.

Category:

Science & Technology

Tags:

• lm324
• opamp
• op amp
• turorial
• flexiforce
• flexi-force
• flexi
• force
• analog
• amplifier
• conductance

[AgentDexter47]

doesn't the 8 volt regulator need at least 1.5 or even 2 volts more than in 8 volts it gives out?

and also could this schematic be adjusted to match the curve of an NTC resistor for precise temperature measurement

[bkraz333]

@AgentDexter47 Yes, a 1.5V drop is typical for the 78xx series. The "9V" unregulated adapter that I am using will output about 11V when loaded at 50 to 100 mA, which is more than this circuit will ever draw. So, we can be sure to always have +8V coming out of the regulator. You could use a similar circuit for a thermistor, but I think the curve is not exactly 1/R, so additional calibration would be necessary (I think).

[xXdenhartXx]

Awesome video, off-topic question what do you use the M3 copper foil/tape thingy to? I have a bunch of those rolls, but haven't found a good use :P

[bkraz333]

@xXdenhartXx It's very high-quality shielding tape. If you are building something that must contain RF, or will be used in an environment that is very noisy, the shielding tape will help prevent your circuits from coupling unwanted RF noise.

[ntjbjhq]

Keep 'em coming, more like this!

[SuperDisco91]

You live near Jameco! Lucky!

[electroniceeltube]

Nice videos you have!

You could reduce the different number of voltage regulators of your design if you use the internal voltage reference of the atmega. It is 1.1V or 2.5V depending on the exact model and it is usually more accurate than the 5V of an standard linear regulator. Just put an 100nF to ground on the Aref.

With the scope on the highest sensitivity it looks like your speech is picked up by the sensor and can be seen on the screen.

[clickfix]

Cool! Analog signal conditioning is one of my favorite topics.

My first thought when i started watching this was that you where going to use an antilog amplifier (a circuit that uses the logarithmic voltage response of a semiconductor) but this is perfect when you just have a variable resistance, same technique is used for tailoring potentiometers, never saw that using an opamp before though.

I'd like to experiment with this my self, maybe you could eliminate the inverter using an antilog amp...

[120ohm]

I love watching your videos because your projects are always different and you're so good at explaining things! You should come teach at Purdue!!!

[squiggyreck]

Awsome..enjoyed it.

[GeorgeGraves]

Really liked the video! Very helpful. It does make me wonder what kind of results you'd get by scaling the signal in software.

[leoclub626]

I disliked the video by mistake. I was trying to push like button and later I did. I don't know if it matters. Sorry.

[chatch15117]

i like lm358 but ebay suppliers keep sending the lm358m which is tiny and hard to solder. can you look at a circuit for me? im tapping rssi voltage output on a fm receiver. atmega88pa based.

[xXdenhartXx]

@bkraz333 Thanks! :)

[DracoXul]

I really enjoy your videos .. love the tutorials .. I would also like to know what you are doing with all of the pressure sensors .. :)