auto_turret
New Member
Hello everyone, I posted a few weeks ago about making a digital switch box that tests resistances. I had an idea at first but then I posted here and got some suggestions from a few of you guys. Here was the post:
https://www.electro-tech-online.com...-16-to-1-switch-box-with-ics-any-help.113831/
One person suggested using an MCU with a 16 channel ADC. Another suggested a using reed relay matrix. So.. I put those 2 ideas together after scrapping my original and came up with something sweeeet!
It has:
An MCU with 6 ADC channels but only using 2,
32 5v W117SIP-6 reed relay's in SIP package,
4 74ls154 4-16 line demultiplexer
1 74ls193 counter
It does:
Cycles through connections, reads voltage drop, checks against calibration on a Pass/Fail basis.
Detects shorts/opens.
Self Calibrates interally to required resistances.
Accurate to +/-1mV voltage drop.
Tests up to 32 connections and calibration connections.
Test circuit protected from inductive kickback from relays
by shunt diodes.
The concept and beta circuit works beautifully.. all I'm waiting on are the rest of the deumux'es, the calibration resistors, and she's ready to go in her box.
I drew the schematic on 4 graph papers taped together so I can't fit it on the scanner. I'll have one posted along with source code for the uC eventually. For now here's a pic of the setup thus far in beta testing:
**broken link removed**
It'll all go inside a black project box, gonna use a DB-25 connection from the wire harness to the box. Before we connected them all individually.
I used the MCU to give clock and reset pulses to the counter, select the demultiplexers, and 2 ADC channels for 2 ranges for resistance testing. Channel 0 uses a 1k resistor on it's voltage divider, channel 1 uses an 1M.
Stability and hence accuracy ended up being an issue that took me days to resolve.. a combination of 4 things I tried keeps it stable within +/-1mV over a long period of time. At first I needed to recalibrate after cycling the switches 1 or 2 times it would start reading all fails.. now I can leave it on, come back an hour later with it staying accurate with 1 calibration. First I put the ADC power in series with 5v with a 15mh inductor. Then I put the voltage divider resisters in parallel with .01uf caps. Then put a .1uf cap between the ACD ref and GND. Finally, there's a 1uf between 5v and GND. Then finally my ADC stayed stable! For hours! Yay
!
So.. you power it up and it calibrates internally. Then you push a pushbutton to start the test. It tests all the connections it's programmed to test and gives you a pass or fail. Took me weeks upon weeks locked in a room to come up with this damn contraption. My GF thinks i've been ignoring her. To recalibrate, just turn it on then turn it back on. Guess I'll add a reset button.
It's all built on breadboard, so I hope it doesn't get dropped. I've never made a PCB before.
Unfortunately.. the circuit draws 100-125maA / hour. Means a 9v batter will last about 5 hrs.
When it's finished in it's entirety.. I'll post it up here.
Just wanted to let you all know again, that you've been extremely helpful and this project would have never got off the ground without me coming here for advice and ideas first.
Any comments/suggestions are totally welcome. Comments/suggestions are what got this project into production in the first place.
https://www.electro-tech-online.com...-16-to-1-switch-box-with-ics-any-help.113831/
One person suggested using an MCU with a 16 channel ADC. Another suggested a using reed relay matrix. So.. I put those 2 ideas together after scrapping my original and came up with something sweeeet!
It has:
An MCU with 6 ADC channels but only using 2,
32 5v W117SIP-6 reed relay's in SIP package,
4 74ls154 4-16 line demultiplexer
1 74ls193 counter
It does:
Cycles through connections, reads voltage drop, checks against calibration on a Pass/Fail basis.
Detects shorts/opens.
Self Calibrates interally to required resistances.
Accurate to +/-1mV voltage drop.
Tests up to 32 connections and calibration connections.
Test circuit protected from inductive kickback from relays
by shunt diodes.
The concept and beta circuit works beautifully.. all I'm waiting on are the rest of the deumux'es, the calibration resistors, and she's ready to go in her box.
I drew the schematic on 4 graph papers taped together so I can't fit it on the scanner. I'll have one posted along with source code for the uC eventually. For now here's a pic of the setup thus far in beta testing:
**broken link removed**
It'll all go inside a black project box, gonna use a DB-25 connection from the wire harness to the box. Before we connected them all individually.
I used the MCU to give clock and reset pulses to the counter, select the demultiplexers, and 2 ADC channels for 2 ranges for resistance testing. Channel 0 uses a 1k resistor on it's voltage divider, channel 1 uses an 1M.
Stability and hence accuracy ended up being an issue that took me days to resolve.. a combination of 4 things I tried keeps it stable within +/-1mV over a long period of time. At first I needed to recalibrate after cycling the switches 1 or 2 times it would start reading all fails.. now I can leave it on, come back an hour later with it staying accurate with 1 calibration. First I put the ADC power in series with 5v with a 15mh inductor. Then I put the voltage divider resisters in parallel with .01uf caps. Then put a .1uf cap between the ACD ref and GND. Finally, there's a 1uf between 5v and GND. Then finally my ADC stayed stable! For hours! Yay
!So.. you power it up and it calibrates internally. Then you push a pushbutton to start the test. It tests all the connections it's programmed to test and gives you a pass or fail. Took me weeks upon weeks locked in a room to come up with this damn contraption. My GF thinks i've been ignoring her. To recalibrate, just turn it on then turn it back on. Guess I'll add a reset button.
It's all built on breadboard, so I hope it doesn't get dropped. I've never made a PCB before.
Unfortunately.. the circuit draws 100-125maA / hour. Means a 9v batter will last about 5 hrs.
When it's finished in it's entirety.. I'll post it up here.
Just wanted to let you all know again, that you've been extremely helpful and this project would have never got off the ground without me coming here for advice and ideas first.
Any comments/suggestions are totally welcome. Comments/suggestions are what got this project into production in the first place.
Last edited:
