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• Nonlinear Response Fix

  schuyler4 • 07/10/2026 at 22:25 • 0 comments

  So here is a simple solution to the nonlinear response caused by adding the protection resistor.

  The op amp has input protection diodes that clamp to the rail, so the zener isn't actually needed. And when it is removed it isn't cracking open at all, so no nonlinear response.

• REV3 Inadequately Protected Against Stupidity

  schuyler4 • 07/08/2026 at 18:33 • 0 comments

  That is to say, if you have the meter in ohm mode but you are actually measuring a somewhat high DC voltage, it is going to stop being a multimeter and start being a smoke machine. I learned this the hard way.

  Looking at the schematic it is clear why this is the case. 

  In ohm mode the switch will be thrown so R37 will be bypassed. That means unfortunate U3 is just going to eat that voltage (which could be almost 60V out of its operating range) right in the face and D5 ain't going to do shit to protect it. D5 is also going to eat it. 

  There is a rather simple bodge that solves this problem: 

  Adding that 10k resistor will allow D5 to clamp the voltage U3 sees to 3.3V and adds a layer of protection. 

  And here is the bodge bodged on:
  

  As always it isn't that simple though. U3 is going to squirt or sip some bias current which is going to generate an 8uV offset. In voltage mode this will correspond to a 0.4mV offset in the actual reading. This can be calibrated out though.

  The bigger issue is D5 is a 3.3V zener, but it does start to crack open a bit when measuring higher voltages. This generates a non-linearity in the multimeters voltage response. This is a big problem. More on this to come.  
  

• Improved Steady Display

  schuyler4 • 12/23/2025 at 04:47 • 0 comments

  REV3 is coming along well. I have fixed most of the problems, and it is nearly ready for a final test after the new enclosure is completed. 

  
  One of the most annoying things about REV2 was flicker in the display. Most of the time it was just a few counts of the least significant digit, so not consequential for reading accuracy, but it was annoying and looked amateurish. 
  

  It turns out it was mostly a firmware/algorithm improvement that fixes this. Hardware filtering before the ADC definitely matters, but what really made the display look good was using an update customized IIR filter instead of just a windowed running average, and adding some hysteresis between real measurement and what was displayed. I considered adding hysteresis a while back, but I ended up not doing it because I couldn't find any evidence that it is used in commercial meters. After further study, I think it is likely that it is implemented to some extent, but just not documented. It is obviously common practice with single bit  analog to digital (comparators), and I just don't see how commercial meters can keep as steady as they do without it. I only added a little over a count, so a little over 1mV of hysteresis in voltage mode. 

• REV3 Prototype

  schuyler4 • 09/30/2025 at 02:50 • 0 comments

  The prototype for REV3 has been constructed.

  Unfortunately, there seem to small memory effects with the voltage reading after increasing the input impedance to 5.21M ohm in this revision. I'm going to attempt to create an auto zero circuit to compensate for this. In addition, there are a routing issues, and additional IO is really needed, as in this revision the LED pin on the pico board is used.

• REV3 Design

  schuyler4 • 08/20/2025 at 02:38 • 0 comments

  After using REV2 as basically my daily driver for over a year, I decided it was time to make another revision to take care of some of the problems that are present in the REV2 design. REV3 has not yet been fabricated yet, but here is an overview of the changes. 

  Better Resistance Measurement Circuit

  REV2 basically just utilized a voltage divider for resistance measurement. This approach is not ideal because the ADC codes per ohm is not constant. Also there was only two resistance ranges. The combination of these factors meant that the dynamic range of resistance measurement was not very high - only about 30Ω to 500kΩ, and the accuracy could be as bad as 10% at resistances near the limit of a range. To fix these problems I have implemented a constant current source to measure the resistance in REV3.

  Additionally there are 4 current ranges: 1mA, 100uA, 10uA, and 1uA. 
  
  Diode Mode
  I have realized that diode mode is useful enough to warrant inclusion on REV3. This will be implemented using one of the current ranges reading out the voltage instead of calculating the resistance in firmware.
  
  Analog Continuity Circuit
  Previously continuity mode was implemented in software as part of resistance measurement utilizing a threshold. In REV3 I have added an actual amplifier and comparator that will be used with the current source to check for continuity. This way, hopefully the continuity threshold can be very low. I am aiming for 0.5Ω. No More Capacitance Mode
  I decided that capacitance mode implemented with an RC step response did not work well enough, and I don't use capacitance mode on a multimeter very often anyway. So REV3 will not have capacitance mode. 
  
  USB B Power Plug
  Using the micro USB on the Pi Pico was not ideal. There is now a USB B connector: 
  
  
  

• cal2

  schuyler4 • 06/02/2025 at 02:42 • 0 comments

  I have done a second calibration on both the full featured multimeter version, and the voltmeter version after ~1 year. No adjustments were needed. I have backed off the voltage spec to 0.5% relative accuracy. Both versions showed slightly higher error when measuring 1V compared to the first time I calibrated them. This leads to a large decrease in accuracy. 1V is low in the voltage range, so small differences lead to large errors.

• Rev2 Final Calibration

  schuyler4 • 04/14/2024 at 03:52 • 0 comments

  I have completed the final calibration for the second revision. The data is as follows:

  Voltage:
  Meter, Fluke 101
  -60.28, -60.4
  -54.70, -54.81
  -49.86, -49.90
  -44.88, -44.97
  -39.86, -39.93
  -34.93, -35.00
  -29.89, -29.94
  -24.90, -24.94
  -19.92, -19.95
  -14.94, -14.96
  -9.956, -9.97
  -4.972, -4.978
  -0.987, -0.989
  0.001, 0.000
  0.989, 0.989 
  4.975, 4.984
  9.960, 9.97
  14.94, 14.97
  19.92, 19.96
  24.90, 24.95
  29.89, 29.95
  34.81, 34.87
  40.00, 40.08
  44.80, 44.88
  49.90, 50.03
  54.97, 55.11
  61.04, 61.1
  
  Resistance:
  Meter, Range, Fluke 101
  29.57, 1, 31.7, 
  98.33, 1, 103.3
  332.3, 1, 332.8
  1081, 1, 1044
  10730, 2, 10520 
  21930, 2, 21410 
  100500, 2, 97300
  339800, 2, 315200
  530000, 2, 484000
  
  Capacitance:
  Meter, Range, Fluke 101
  50.88n, 2, 49.2n
  96.73n, 2, 96.6n
  1.006u, 2, 1.009u
  10.03u, 1, 9.92u
  45.57u, 1, 47.32u
  

  This works out to 0.3% relative accuracy for voltage, 10% for resistance, and 5% for capacitance.
  

• Rev2 PCB Assembly and Bring Up

  schuyler4 • 03/22/2024 at 06:34 • 0 comments

  I have assembled and brought up the revision 2 PCB to the point where it reads voltage. Additional firmware is needed for mode switching and calibration is also needed.

• Rev1 Enclosure

  schuyler4 • 03/20/2024 at 03:57 • 0 comments

  I have mounted the first revision into an aluminum sheet metal enclosure with some holes cut in it. The first revision has been repurposed into just a voltmeter because there are some problems with the switching circuit.
  

• Rev1 Voltage Test

  schuyler4 • 03/14/2024 at 03:44 • 0 comments

  I did another test with the rev1 prototype, this time only measuring voltage, and bringing it all the way to the maximum voltage of 60V. Again I am doing calibration and comparison to my Fluke 101 Multimeter. This has a problem as the Fluke 101 does not have more digits than this multimeter, however, I am ignoring this for now. I was able to stay under 1% error for all measurements, and I think this could be improved with a better calibration procedure. Here are the results:

  Voltage(V), Error(%)
  0.989, 0.0
  4.982, 0.71
  9.97, 0.76
  14.96, 0.81
  19.95, 0.86
  24.94, 0.85
  29.94, 0.88
  34.99, 0.86
  40.03, 0.83
  44.9, 0.81
  49.89, 0.85
  54.84, 0.9
  59.8, 0.71
  

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