Arduino Project: HLT volume display using bathroom scales

[Jim]

Well an update: hooked everything up and....


....it doesn't work I get a steady reading from the amplifier regardless of any load on the scales and if I tweak the gain resistor then also, nothing happens.

Debugging analogue electronics isn't my strong point! Perhaps the amplifier chip is knackered, I'll recheck all wiring (again!).

Do you have a 'scope or high impedance meter to check levels at the input and output?

[Andy]

I have a digital multimeter, that's it!

[Jim]

I have a digital multimeter, that's it!

Well that should do. Have you checked the levels with it?

[Andy]

Yup, all seem OK.

[Jim]

So the output level from the amp changes when you change the load on the sensors?

[Andy]

The multimeter reads 0v from the amp output, the arduino shows a voltage from the same pin, but very small. The small voltage doesn't change when weight is applied to the scales.

Also, tweaking the variable resistor which changes the ampilfier gain make no difference to the output, which is odd. I'd expect that to do something even if the input signal was rubbish.

[Jim]

What's the most sensitive scale on your meter?

Do you know what kind of voltage to expect from the sensor PCB?

Edit: reading back in the thread you have the sensors connected directly to the amp, so if there's no change from there the problem might well be in the way you've configured the sensor wiring and supply.

[Andy]

Meter should read down to 0.01v (that's the display resolution anyway). The load scale voltage will be tiny, hence the need for the amplifier chip.

[Jim]

Yeah it could well be too low for your meter to read.

[Fil]

have you tried with a single load cell rather than all wired up together, im way out of my depth here, but if you have 2 cross wired they may cancel each other out when under load? which may explain the unchanging reading..??

[wmk]

Well an update: hooked everything up and....


....it doesn't work I get a steady reading from the amplifier regardless of any load on the scales and if I tweak the gain resistor then also, nothing happens.

Debugging analogue electronics isn't my strong point! Perhaps the amplifier chip is knackered, I'll recheck all wiring (again!).

Why not post a pic of the setup? It's hard to tell what's going on otherwise, it kind of sounds like you're using the 3 wire load cells like 4 wire cells? Two of them should be used in tandem with a couple of resistors to make a full wheatstone bridge.

[JabbA]

You might have the same issue I did Andy? I originally had the load cells sit flat between two pieces of MDF with the full HLT on top of that which resulted in no change in reading when the load was removed. After a bit of a think I simply threw some money at the problem

I supported the edges of the load cells on pound coins so the middle could flex and provide a change in resistance, I also placed a penny on top of each load cell to give a more solid point of contact. A couple of pics to illustrate:




Cheers,
Jamie

[Andy]

Nice!

In my case I'm using the load cells in-situ in the digital bathroom scales so they should work without any mods.

I'll take a pic of the setup and post up, but to round it all off last night, my home PC died. Just instantly powered off and now no signs of life, suspect the PSU has blown.


Grrrrr!


PS. Are you *the* Jamie on STW ?

[bobsbeer]

Did you manage to get this working?

[Andy]

Hi, no - never pursued the investigations into the static reading from the load cells. So it's "on hold" pending a bout of enthusiasm again