Last year I started planning a physics experiment that would require me to measure tiny forces. My plans for that included building a "microbalance" out of a salvaged analog needle movement.
I chose several items with needle dials from my dad's junk collection, and ended up extracting the movement from an old tachometer. It consists of an electromagnet, two delicate coil springs, and the needle itself, which is attached to an axle that permits it to rotate over the electromagnet. Running a small current through the electromagnet changes the angle of the needle; otherwise, the springs damp its motion and keep it in a fixed position. There's also a metal tab that you can rotate to adjust the neutral position of the needle (this turned out to be very handy). Once I had the movement out of the tachometer, I designed and 3d-printed a new housing that would hold it in the right spatial relationship to a t-slot optical interrupter.
An optical interrupter contains some kind of light-emitting device in one pillar, and some kind of photosensitive device in the other. The idea is to attach a tiny shutter to the needle and suspend it in the slot of the interrupter. Even a slight weight on top of the needle will push it down and block the light with the shutter, changing the output signal of the photosensitive device. You can use that altered output to drive more current into the electromagnet and pull the needle back up. The larger the load on the needle, the more current you have to put into the electromagnet to get the shutter back out of the light, and the more voltage you need to drive it. So the voltage across the electromagnet serves to measure the weight on the needle. Neat, huh?
I had two previous projects, both showcased on YouTube, to use as inspiration. I leaned more heavily on the second one (Applied Science), since that circuit is more detailed and easier to adjust - look at all the potentiometers! The op-amp on the right and its resistors are merely an added layer of amplification on the measurement voltage, to make it easier to display, so ignore them for now. The op-amp on the left corresponds to the one in the TI video.
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| Screenshot from "Weigh an Eyelash--Build a Microgram Scale" by Texas Instruments. https://www.youtube.com/watch?v=n90whRO-ypE |
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| Screenshot from "Measure the mass of an eyelash with a DIY microbalance" by Applied Science. https://www.youtube.com/watch?v=ta7nlkI5K5g&t=256s |
Now ... dear readers with electronics experience, do you see anything *odd* about both these op-amp circuits? They're configured like non-inverting amplifiers, but the resistor that would normally connect the '-' input to ground is missing. All that's in that path is the photosensitive device of the optical interrupter. Maybe this works out if the device is a photodiode, as shown in both circuit diagrams; there might be some amount of voltage drop across the diode even when the light is shining on it and it is ON. But my Adafruit optical interrupter has a phototransistor instead. When this thing turns ON, its resistance becomes (approximately) zero. That gives me an amplifier with "infinite" gain. Whoops! The op-amp is physically limited in the amount of voltage it can produce, so its output gets as close to the positive supply voltage as it can. My needle was always stuck in the "up as high as possible" position.
So it turned out I couldn't just imitate the circuit from Applied Science's project - not with the parts I had. Since the transistor is (again, ideally) a binary on-off switch, I realized I didn't need proportional gain feedback at all. Toggling between two different voltages on the electromagnet would be more appropriate. This makes the needle oscillate, but so long as the oscillations stay around some stable equilibrium point, that's okay.
I reconfigured the op-amp circuit to be a summing amplifier. In one leg of the sum was my bias voltage, which I could adjust to set the needle's neutral position at the point where the shutter just began to break the light beam. In the other leg, I put a pull-up resistor connected to what I'll call the "recovery voltage," then connected the phototransistor between that and ground. With the transistor OFF, the output of the op-amp is (bias voltage) + (recovery voltage), which is enough to lift the needle. With the transistor ON, the output of the op-amp is only (bias voltage), which lets the needle sag under its own weight and the weight of whatever's on it. This produces a cycle: the needle drops, the transistor turns OFF, the voltage increases, the needle rises, the transistor turns ON, the voltage decreases, repeat. When you add weight to the needle, it takes more time to rise and less time to fall, so the *average* output voltage increases because the circuit spends longer in the "transistor OFF" phase. You can treat it like a pulse-width modulated signal.
And this actually worked!! For a minute or two. Then my fancy chopper-stabilized op-amp mysteriously died. My best guess is that current into one of the pins exceeded its unusually low "operating" limit of 100 uA. (When not "operating," it has a 10 mA limit like a more normal op-amp, and that's the level of protection I was providing with my resistors.) And my spare op-amp was killed by electrostatic discharge - this is the first time I've seen that ruin a part in real life, but take it from me, you DO have to worry about it! Especially if you live in a nasty dry climate (grumble).
I thought I was stuck until I could order more parts ... but then I realized that since I wasn't doing proportional control anymore, I didn't really need an op-amp at all. I adjusted the mechanical bias on the needle movement until it would hang in a good neutral position when unpowered. Then I set up the simplest feedback mechanism possible: transistor OFF powers the electromagnet, transistor ON cuts the power (via a pullup resistor). With no delicate op-amps to suddenly give up, this version worked long enough for me to get a demo video.
For the future, I should find a way to reintroduce a bias voltage, so I don't have to depend on moving the mechanical bias point to calibrate the scale. I should also add some kind of averaging or filtering circuit on the output to smooth the measurement voltage, and amplify it to something bigger than a few mV. But I've got the basic idea in hand! As seen in the video, it can detect a bit of polyester carpet fuzz which is so light that it electrostatically sticks to the needle.
Until the next cycle,
Jenny
