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
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.
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 (more to come, including video) ...
“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
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.)
LCD, blue, I2C serial, 16x2 char ... https://www.ebay.com/itm/1602-16x2-LCD-Character-Display-IIC-I2C-TWI-SPI-Serial-Interface-Board-Module-/262334510397?hash=item3d145af53d
Paxcoo Prototyping Boards ...https://www.amazon.com/gp/product/B01M7R5YIB/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
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
Solarbotics Power Regulator ... https://solarbotics.com/product/34020/