February 13, 2018

Test Equipment You Can Build
Whatever there is to test, we'll be detailing the design of an existing (or original) instrument to measure it.
The emphasis will be on low cost and build-ability. 


We are introducing our theme for this new year: "Test Equipment You Can Build" ... Whatever there is to test, we'll be detailing the design of an existing (or original) instrument to measure it.  It may be a kit.  It may be a design concept that we flesh out for ourselves to build.  It may be an inexpensive gadget that someone else already offers. 

The emphasis will be on low cost and buildability.  When parts are needed, we'll be sourcing small runs for those interested in building along with us.  Our episodes will go through the design, build and usage of our "Test Gadget of the Month".  We'll introduce a CWTD Test Gadget in each episode and spotlight the guys who have built one since the previous episode.

We'll have to hurry and maybe even stick in a extra episode or two, but by the end of the year each one of us on CWTD will have a bench full of inexpensive-yet-essential Test Gadgets for measuring and checking out your projects on the bench and in the field!

Oh yes, we'll also be wrapping up our project from last time (the CWTD GPS-DO), and we'll have an occasional topic totally hosted by a guest speaker ... and you'll love the first topic!

73, George N2APB & Joe N2CX

Listen to the Podcast  (will be posted shortly after the live episode)


CWTD Episode #85:  Test Equipment You Can Build


Game Plan for 2018




Nano Digital Voltmeter

A digital voltmeter can be the basis of many test equipment functions

Nano ... An easy-to-implement voltmeter

                Internal 10-bit ADC

                Inexpensive LCD display to provide readout

ADC range is 0-to-5V in 1023 steps for good resolution

Inputs can be scaled to extend range

Usual ranges are 1-2-5 pattern

                Suggested ranges are 5V, 10, 20V, 50V

Chip considerations:

For ADC accuracy input impedance needs to be 10K or less

To protect inputs have to limit applied voltage to no more than Vcc + 0.5V

Resistive dividers can provide scaling

Schottky diode from input pin to Vcc prevents zapping chip

Limitations of level zero scheme

                Need to switch dividers to set range

                                Auto switching is future possibility

                Input resistance of voltmeter is only 10k

                                Good enough for many applications

Other possibility for future more versatile voltmeter is external DMM chip




The DC Voltmeter Test Gadget

“Manual DVM” --  Uses a manual switch to select appropriately-sized resistive dividers to handle max readings.  Diodes on the uC analog port to protect input.  uC always reads and displays voltage.  If voltage at 3FF (max for 10 bit adc), “OVERFLOW” displayed and user rotates “Range” switch to one lower … until a voltage is displayed.




1) Turning the pot is a way to present a  variable 0-12V DC input to the Arduino input A0 for measurement.

2) Clamping diodes D1 and D2 ensure that input A0 signal stays within pin limits of ~5V.

3) R4 prevents input pin from floating when nothing applied.

4) R5 is current protection for the input pin.



1) Turn rotary encoder to set desired scale: 5V, 10V, 20V, 50V.  (We only have 12V max)

2) Turn potentiometer on Gadget plug-in board to vary input voltage.

3) See result displayed on LCD.



1) LCD, blue, I2C serial, 16x2 char:

1) https://www.ebay.com/itm/1602-16x2-LCD-Character-Display-IIC-I2C-TWI-SPI-Serial-Interface-Board-Module-/262334510397?hash=item3d145af53d

2) http://www.mpja.com/mobile/Serial-2-X-16-LCD-Display-for-Arduino/productinfo/31164+MP/

2) Paxcoo Prototyping Boards ...https://www.amazon.com/gp/product/B01M7R5YIB/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

3) Arduino Nano Mini ... https://www.ebay.com/itm/Arduino-board-Micro-controller-V3-0-ATmega328P-CH340G-5V-16M-Gift-USB-Nano-MINI/331853764136?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D1%26asc%3D20160908110712%26meid%3Dcab8be711bfb4b628ebb2c5ee08f606c%26pid%3D100677%26rk%3D1%26rkt%3D28%26sd%3D253099138551%26itm%3D331853764136&_trksid=p2385738.c100677.m4598

4) Solarbotics Power Regulator ... https://solarbotics.com/product/34020/ 





Download Sketch

Serial LCD Library ... Add THIS zip library (NewLiquidCrystal_1.3.5.zip) to your system.  After downloading, use the Arduino IDE menu to go to (SKETCH / INCLUDE LIBRARY / ADD .ZIP LIBRARY) and point to the zip file.


WATCH VIDEO ... <click here>





Prototyping Boards ... A Standard for the CWTD Test Gadget Station





Arduino Nano ... Our Controller for the CWTD Test Gadget Station ...




Convenient Power Regulator ...


First Implementation (See video) ...






Next Steps


  1. “Auto-ranging DVM” --  Use an electronically-switched set of voltage dividers with open collector driver (like old SN7407) engaging selected driver.



                           i.       uC has min resistive divider “on” at startup.

                           ii.      uC reads port

                           iii.      if reading an “MAX” then engage next lower divider and goto ii

                           iv.      display reading


  1. Interface a “DMM-on-a-chip” MAX133 – For longer-term design implementation in this “test station” that we’re building up could use a special-purpose chip that’s used in cheap DMMs: MAX133 (https://datasheets.maximintegrated.com/en/ds/MAX133-MAX134.pdf), $23 in DIP40 pkg, yielding a most easily achieved solution that really is workable on the homebrewer’s bench, *and* is uC controlled for data logging purposes downstream.    (PS:  End-game implementation, for each one of our “test adapters” in this series, can be small plug-in pcb for each subsystem.)