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• 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
  

• Rev1 Display Bodge

  schuyler4 • 03/13/2024 at 00:45 • 0 comments

  To make the display work on revision 1, I bypassed the display driver and wired the pico IO pins strait to the segment LEDs. It is a little faint, as the display driver circuit worked completely differently, but it works to test the display.
  

• Rev1 Preliminary Accuracy and Precision Test

  schuyler4 • 03/10/2024 at 05:41 • 0 comments

  As the title says, this is a preliminary test of the first version of the multimeter. Before the test, I basically calibrated it using my Fluke 101 as a reference. The results of the test are as follows:

  2.5V
  Error: 0.569, Coeff of Var: 0.0102
  
  5V
  Error: 0.2342, Coeff of Var: 0.001
  
  7.5V
  Error: 0.2404, Coeff of Var: 0.0019
  
  10V
  Error: 0.0671, Coeff of Var: 0.0006
  
  10ohm
  Error: 48.5352, Coeff of Var: 0.0
  
  100ohm
  Error: 8.2285, Coeff of Var: 0.0
  
  1000ohm
  Error: 0.0443, Coeff of Var: 0.0
  
  10000ohm
  Error: 1.9391, Coeff of Var: 0.0
  
  40000ohm
  Error: 5.2247, Coeff of Var: 0.1246

  As can be observed, the error is fairly low for all the tested voltages. For resistance, the error is higher especially at the low and high ends. It basically isn't accurate at all at 10 ohms. I also did a test of capacitance and for 100nF it measured 579nF, so basically order of magnitude. For 10uF it measured 11.78uF, so close. I may be able to make capacitance better  (for 100nF) by increasing the sample rate.
  

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