Strain gauge beep - help please!


Thread Starter


How do I hook a strain gauge up to a beeping device, so that when a certain load is applied, a beeping noise occurs?

Imagine holding a Jeopardy clicker - with your thumb hovering a button. Is there a way I can put a strain gauge load cell on the button, so that it measures the force applied by the thumb? Also - I want it to beep when a certain load is applied.

What is the best way to solve this?


kalpak dabir

You can buy an off the shelf strain gauge amplifier module that will generate an output of 0-10V or 4-20mA. Alternately look at the FlexiForce ( piezoresistive sensors, cheaper and easier to interface and mount.
You should be able to purchase a display meter with alarm relay outputs.. Their are hundreds of vendors for these display models... Look one up on Newark, or Tension measurement inc....

Just purchase a load cell and display meter thats within your tension range.. Then calibrate your display meter to your load cell... Program your alarm limits into the display meter to close a set of dry contact relays. The dry contact relays will feed the power to your alarm device..

After that your off and running...


Curt Wuollet

The easy, but expensive, way would be to get a strain gauge amplifier or signal conditioner that has a setpoint or alarm output. To do this yourself you will need a dual precision op amp and misc. passive components. You don't need strictly need linearization, but it's usually best to put the strain gauge in a wheatstone bridge configuration because a difference amplifier will reject common mode noise. Use any of the stock circuits from the IC manufacturers (Google strain gauge amplifier) and use the second op amp as a comparator on the output. Apply the desired load and tweak the comparator threshold so that the output just switches. I'll leave the beep as an exercise for the reader. With good op amps and decent layout this should require only infrequent calibration.
If you don't know what you're doing, you should really go with the commercial product. Alternatives are the easier to use silicon force transducers or a microswitch and a spring.



Bruce Thompson

In our age of computers, PLCs, etc. it is of great enjoyment to devise programmable methods of implementing solutions while retaining the ability to change them quickly.

But must we completely forget analog circuits? A very simple solution to this problem is to setup the load cell as a wheatstone bridge with a constant excitation voltage. As the differential voltage on the bridge exceeds a limit (adjustable by resistors) this triggers a Darlington transistor (remember transistors) to turn on the piezo beeper. When the load is reduced, the differential voltage drops below a lower limit that turns off the transistor.

All passive. All simple. Changeable limits and it can be produced in your garage for about $4 in parts.

Curt Wuollet

Almost, but with a full scale reading of 30mV/V it would be difficult to get a sharp turn on with just a transistor. But with a low cost instrumentation amp and a low cost opamp open loop and your darlington as a driver it would be relatively easy to get an accurate and reliable beep at a given weight. But unless you do this sort of thing, a load cell signal conditioner with an alarm function would be a lot easier, if more costly.

cww who does these sorts of things.
How about using a wireless load cell, you can use a handheld device to check the reading everywhile. Moreover the sensor could be programmed to send a reading on a regular time basis.

I do not know if you need this wireless link from the irst place, but this could help.