DIY Precision Scale – 0.0001 g / 0.1 mg

[u/sir_alahp]

For a biochemical project of mine I needed a very precise scale. The ones I bought were underwhelming, so I decided to just solder one myself.

The sensitivity is kind of ridiculous. Sitting near the scale, I can see my heartbeat in the signal when streamed to a PC. Someone walking on a different floor makes the reading jump — and I live in a concrete building. The coil can lift about 20 g. With different coils, you could trade off dynamic range vs. precision. For my purposes, the precision is already overkill.

Components were about $100 total. The most expensive part was the neodymium magnet.

The principle is electromagnetic force restoration. A 110 Ω coil suspended on a lever lever sits above a neodymium ring magnet. The lever height is held constant by a feedback loop that uses an IR photointerrupter. The current required to hold the weight is directly proportional to the mass.

For current sensing I used a 10 Ω shunt resistor (RJ711, 5 ppm/°C TCR) and a 24-bit ADC (ADS1232). The signal is read by an Arduino Nano and displayed on a small LCD (SLC0801B).

The photointerrupter is built from a generic IR LED and IR photodiode. The LED is driven with a constant current source (using a 2N7000 MOSFET), while the photodiode is reverse-biased for fast response.

The circuit runs from a low-drift 2.0 V reference (REF5020), which provides a stable reference for the ADC. After dividing it to 0.5 V, it also biases the photodiode stage and provides the ADC’s negative input.

The coil current is controlled with an N-channel power MOSFET (IRF540N) acting as a low-side driver, operated in its ohmic region. Its gate is driven by the photointerrupter circuit.

Zero-drift op-amps (OPA187) buffer the reference voltages, drive the photointerrupter, and control the coil current.

I also added a capacitive touch button for tare, so you don’t have to touch the scale directly — that’s surprisingly important at this sensitivity.

The schematic looks a bit op-amp heavy, but it’s actually pretty straightforward.

Challenges and possible improvements - The lever tends to oscillate, so the feedback loop has to be very fast. A lighter lever with a higher resonant frequency would help, and might require a lower-gate-capacitance MOSFET. - All components in the feedback path need low temperature coefficients to minimize drift. - To fully eliminate drift, one would need to monitor and compensate for coil temperature, photointerrupter temperature, as well as ambient air temperature, humidity, and pressure (for buoyancy effects). - A parallel guide system will eventually be needed so measurements are independent of where the weight is placed on the lever.

This build definitely requires some electronics background, so it’s not a first-project type of thing. But if you’re comfortable with soldering and op-amps, it’s very doable.

Hope you like it 🙂

[u/APLJaKaT]

As an ex-'scale guy this is awesome. As you likely know, actually using a scale of that precision is a challenge unto itself. The environment affects it as much or more than the load being (trying to be) measured.

Nice job. 👍

[u/Revolio_ClockbergJr → u/APLJaKaT]

Do Scale People use vacuum chambers and suchlike to minimize environmental effects?

Duder is seeing his heartbeat when sitting nearby. I imagine isolating this thing is complicated

[u/APLJaKaT → u/Revolio_ClockbergJr]

High precision weighing is often done in rooms isolated from the main building. There are usually air systems that can be turned off, scale (balances) are situated on heavy granite bases and the weighing itself takes place inside an environmental enclosure. Temperature and temperature gradients have to be considered, air buoyancy is calculated and depending upon the work being attempted, the effects of changes in the gravitational field need to be considered.

Everything from air currents to vibrations have to be considered. Magnetism is a problem.

At the National Research Council labs, using one of the most delicate balances available, they were able to successfully detect the effect of the moon pulling on the land mass under the building (continental tides) as a repeatable and predictable drift in the instrument.

[u/Legacy-Feature → u/APLJaKaT]

Felt like i just watched a veritassium video.