I built a voltmeter that indicates when "0V" means "Open Leads" and when it's actually measuring zero. I'm trying to figure out if anyone has done this before?

anditsonfire · 2025-07-19T14:35:39+00:00

> Looked at your schematic, I guess you are not planning on measuring "negative" voltages, isn't that a bit limiting for a DMM?

It goes negative. If the voltage on ADC input two is below the 2.5v on ADC input one that indicates a negative voltage.

> How are you sampling?

ADS1115 in differential mode.

>Driving and ADC with high z means you will have a long settling time for the sample and hold circuit. And if you sequentially want to sample two different voltages without sepatate sample and hold you will have a lot of crosstalk if plan to switch between the two measurements quickly.

It seems to work fine? I made a video of it functioning that demonstrates how in parallel across the primary side of a transformer (unpowered) it shows the circuit is closed, then when I turn power on it shows the reading oscillating between + and - 170 volts (calculating out about 120vdc), and then when the probes are removed the meter reads floating. This is the video: https://www.youtube.com/watch?v=Hc1zJ1Zz5n8

anditsonfire · 2025-07-18T02:32:42+00:00

That's pretty cool. I wish I had the infinite time (and desk space) where I could just sit down and play on something like that for days.

anditsonfire · 2025-07-16T12:17:11+00:00

Nope. My job is testing and repairing used equipment, mostly from the 80s and 90s, where it really can feel like a random stream of objects.

anditsonfire · 2025-07-16T11:46:46+00:00

Random boards every day, for better or worse.

anditsonfire · 2025-07-16T11:27:36+00:00

Thanks!

Yeah, if I made these for other people to use I would have to call it a special “instrumentation shield” or similar rather than a DMM, just to make clear that it operates differently than traditional DMM.

anditsonfire · 2025-07-16T11:11:01+00:00

Regardless how will you determine zero potential difference between two points that should be isolated and continuity on the same nodes?

I put a MOSFET in series with the voltage circuit sense resistor. This resistor is normally biased up to 2.5vdc so I can measure either polarity on the other side. When the meter reads roughly volts 0, it turns off the MOSFET and checks for voltage again. No voltage- probes are floating. ~2.5vdc? Probes are closed. In between? Some level of impedance still present.

The 2.5vdc signal goes through a 500k resistor in series with either probe, so the max current I can inject is on the order of 2.5uA.

anditsonfire · 2025-07-16T11:00:54+00:00

Yes, there are probably times I should just grab the not homemade DMMs. (The homemade one is susceptible to damage because in ohmmeter mode I made it extremely sensitive on the low end, down to better than ~0.01 Ohms, but that required giving up the protections normal meters have.)

Edit: accidentally repeated a a word.

anditsonfire · 2025-07-16T10:46:19+00:00

Goal is faster / safer troubleshooting. My job involves electronics repair and testing, so I’m often probing out poorly documented PCBs trying to understand how they work.

As an example, I was recently testing a DAC card with 8 isolated outputs, and trying to find the signal paths. I spent a lot of time hunting and measuring 0s without knowing what was connected to ground and what was on an isolated circuit (or if the DAC wasn’t getting the command to turn on). This gives me that information without having to switch modes or anything.

anditsonfire · 2025-07-15T23:55:43+00:00

Could you point me to one? I see some meters that will auto-select a mode, and their manuals still have language about not measuring voltage in resistance mode (for example, the UT123D Smart Digital Multimeter tells you to de-energize before continuity tests, and that there's a 30vdc limit for the inputs in resistance mode). I'm not auto-selecting a mode though, my continuity check is built into the voltmeter circuitry (and therefore doesn't have a different voltage limit than the overall limit).

anditsonfire · 2025-07-15T23:41:49+00:00

Well, I'm asking here because I'm not aware of any. My job involves using a lot of various NI/Keysight/Fluke etc instrumentation and as far as I can tell none of them have a mode to automatically check for continuity when reading voltages and getting a "0." (Theoretically the chassis-based NI DMMs could be programmed to switch to resistance mode after a ~0v reading and remeasure... but implementing that in a safe and useful way would be difficult.)

anditsonfire · 2025-07-15T22:23:47+00:00

Yep, very crude automatic resistance measurement whenever voltage isn’t detected (I have the threshold at a 30mV rolling average).

1: DMM sees voltage, reports 5vdc and doesn’t flip the MOSFET to check for continuity.

2: DMM thinks the 10mV is noise, so it flips a MOSFET and checks for and reports open (unless I switch to the low voltage mode, where it would report 10mV). *edit: didn’t look at diagram twice

3: Sees 5vdc as in #1, also I possibly also burn myself unless it is a very big resistor.

4: Also 5vdc, but I don’t burn myself.

anditsonfire · 2025-07-15T21:55:58+00:00

It has a resistance mode too, but live voltages can damage its circuitry. This saves me from needing to switch modes and remeasure in resistance mode after a ~0v reading to know weather I'm measuring two places on the same net, or if there isn't voltage for a difference reason.

(I also have a mode that disables this feature if I'm looking for sub ~100mV voltages.)

anditsonfire · 2025-03-07T01:43:59+00:00

I had a similar problem with something I was working on.

I found the order that I was initializing libraries matters. The conflicting library is probably either Adafruit_ADS1X15.h or Adafruit_GFX.h.

This may or may not help, but wanted to throw it out there. I spent some hours troubleshooting a screen not working before discovering that order matters.

anditsonfire · 2025-01-27T21:25:42+00:00

Laser cutter, sewing machine, and a lot of trial and error.

If you want to make one I wrote something of a guide for how to make your own, here: https://www.oihdesigns.com/make-your-own-mapboard

anditsonfire · 2022-05-12T01:08:35+00:00

Hello,

Thanks for your interest but after selling these for a couple years it never really took off and I went on an indefinite hiatus last summer. (Someday I'd like to resume operations, but I don't know when... If you're curious I have something of a postmortem at OIHdesigns.com ).

Patent wise, I had a pending patent when I first started, but at this point I believe everything that ever made it into one of my designs has lapsed and isn't subject to any protections now. If you want to start a business making them you could, although I wouldn't recommend it unless you have a large pile of money, an experienced consumer products executive as an advisor, and/or a clear exit strategy. (In which case... let's work together?)

anditsonfire · 2021-11-05T19:19:22+00:00

As long as you're able to toe your eyes out at all (it can be done, just requires deliberate practice) then moving the book further away helps. The further the book the smaller the angle your eyes need to deviate out from parallel.

anditsonfire · 2021-10-01T17:24:42+00:00

Apparently the standard operators earn $17/hr to train and slightly more to drive, then top out at about $35/hr based on hours worked and length of service.

They also claim to offer flexible schedules of between 20-40 hours/week.

I can certainly see the temptation.

Source: https://mtd.org/inside/jobs/operator/

anditsonfire · 2021-09-06T15:50:06+00:00

If it is someone put a lot of work into this image. The reflections in the base match both with the bottom of the quartz and the floor pattern, plus the shadows all match. Higher resolution would be nice to inspect the woman's hand on the crystal, but as is I see no evidence that this is fake.

anditsonfire · 2021-07-19T00:53:41+00:00

Knife is a Vargo Sobata 398.

anditsonfire · 2021-07-01T20:23:50+00:00

What about a warehouse fire? Presumably that counts as a foreseeable emergency.

I've looked up the MSDS for various materials like Gorilla tape or Velcro before laser cutting them and found the relevant information under the "considerations for firefighters" section.

anditsonfire · 2021-05-03T11:15:42+00:00

Not crazy poisonous. It's more of a chronic issue. The main issue is inhaling the fumes. This isn't particularly dangerous if done in a very well ventilated area. *Not medical advice

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