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I have a 0-1kg load cell with 4 wires: Excitation + and -, and Signal + and -. I haven't had any problems wiring them up correctly without amplification, and my voltmeter shows me a 1 mV difference between no load and full load (1 kg).

The signal has to feed into an ATmega microcontroller, so ideally I would like to be able to use the full 10-bit analogue scale when reading the values (voltages from 0 to 5 V). In any case, as it stands now, I only get 1 / 1024 as a reading under full load.

I have a prototype deadline for a product soon, and silly as I am I don't have any instrumentation amps left. I tried wiring the signal wires to a general-purpose NPN transistor, but I'm not getting any amplification. This is the transistor configuration that I tried: ignore the double cell symbol for voltage, I couldn't find anything better.

Transistor amp circuit

I also tried adding resistors between drain and ground, but that didn't help.

Is there any way I could obtain a usable reading using components I have right now? Ordering a proper In-amp would take too long, and I don't really know what kind of op-amp would be suited for this kind of measurement (although that might be an option).

I don't have a big selection of stuff available, mainly different kinds of transistors. And of course diodes, capacitors, resistors, and the like.

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    \$\begingroup\$ There are transistor amplifier circuits. (But that is not one of them.) How about an opamp? (got any of those?) I assume the load cell has a low impedance so any type opamp should be better than what you have. \$\endgroup\$ – George Herold Aug 19 '14 at 15:47
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You will never be able to get accurate amplification of a 1mV full scale DC signal with a simple circuit using discrete components (I do know a way, but it's not simple). Even a crappy LM324 will do better (because the transistors are matched and on one chip).

Tell us what you have and we can, perhaps, help. An instrumentation amplifier can be made with three op-amps, and as I said, even a crappy 10-cent LM324 will be better than trying to use discretes.

enter image description here

In general, an LM324 will probably drift about 5-10uV/K so you can expect (at best) errors commensurate with that- about 5g ~ 10g/°C. A better amplifier would reduce those errors.

By comparison an B-E junction (as in your voltage follower) changes -2mV/K so you'd see 2kg/°C change- worse than useless.

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  • \$\begingroup\$ Thanks. I might be able to get access to op-amps, and price isn't a big issue (I'm not paying for it myself). So would any op-amp work, or is there a category/part better suited for my needs than others? \$\endgroup\$ – LS97 Aug 19 '14 at 16:42
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    \$\begingroup\$ Precision op-amp, low offset voltage and low offset voltage drift. A low-noise 'zero drift' op-amp would be ideal. \$\endgroup\$ – Spehro Pefhany Aug 19 '14 at 16:52
  • \$\begingroup\$ You can request a sample of an instrumentation amp on TI's website and generally receive it within a day or so. It might be an idea to do that in parallel with trying to get your op-amp circuit working. Spehro is absolutely correct about trying to use discretes vs. even the worst op-amp. It can be done (Philbrick!) but it will take you way longer to get it right than it would to just get the right components. \$\endgroup\$ – John D Aug 19 '14 at 18:01

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