# Resistor Detection Circuit

#### Tonyr1084

Joined Sep 24, 2015
4,895
I'm sure a nine stage comparator could be built. Just a little beyond my skill level. That's all.

#### KMoffett

Joined Dec 19, 2007
2,899
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#### MisterBill2

Joined Jan 23, 2018
6,714
Like I said, I can't figure out how to get three states (three LED's). I can (and have) draw a two state tester. In the circuit below Open or Short you get a red light. Only when the resistance is between the upper and lower limits will you get a green light. And yes, you can use two 10KΩ pots for the limit controls.

I used a 9V battery and calculated for a current of 10 mA through each LED. Resistor values shown are exact requirements. Use of very close standard resistance will yield good results. Instead of the 700Ω resistor, a 680Ω will produce nearly the same results.
View attachment 210488
You can tie the two comparator outputs to each other as a wired-or, because they are NPN open collector outputs. Then when both outputs are in the off state the green LED can light, but when either output is low the red LED will be brighter and the green LED will be off.
The reason to not have one valid state be all LEDs off is because it is very hard to tell the difference between zero and nothing.

#### ci139

Joined Jul 11, 2016
1,671
part 1 http://tinyurl.com/ybb3e9zu
part 2 . . . not really into this . . . but you should detect the U/D HV --e.g.-- +42/-30V , set a current detect resistor in series with the 470kΩ . . . so :
• $$I_R=\frac{V_{HV\ Hi}-V_{HV\ Lo}}{470kΩ}≈150µA$$
• requires voltmeter-circuit + ammeter-circuit + voltages' pre-scaler + analog multiplier divider + ...
• ... something that does something with the output --e.g.-- with the derived resistance value in the multiple of volts (... say 470mV if the R=470kΩ)
will be quite a complex circuit - - just one option

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#### MisterBill2

Joined Jan 23, 2018
6,714
OK, making it a bit simpler electrically, one button for the short test, which uses just an LED and a resistor sized to allow about 20mA if the there is a very low resistance, less than 20 ohms. So press the short button and the "zero ohms" LED comes on. Next the 470K test, that uses an MPSA darlington with the green LED in series with a limiting resistor in series from the emitter to battery negative. The drain goes to V+, The gate has a 470K to negative and the tested circuit from gate to V+. And iif neither test lights an LED then the connection is open. (Or the battery has failed.)So a third pushbutton is for a battery check by simulating a short.

#### ci139

Joined Jul 11, 2016
1,671
the meter (an overkill) without some final aritmetics (the RED graph)

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+ a log amp v. (not complete . . . but the "functional" part)
if the V(Y,M) falls below the "CYAN horizontal" (1.3649V in this particular case) the Rx will be above the 470kΩ and "the way around" (whatever that suppose to be then)

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+ more detail (there are too many ways to implement the CC-sink - so i bared that for not to over-trash the circuit)
D2=ON < 470k , 470k - 5% < D3=ON < 470k + 5% , 470k < D1=ON

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#### ConnaTC

Joined Jun 20, 2020
18
Again thanks for everyone’s input,
I'm going to try a window comparator and see how i get on with it if that fails due to the shitfall of telecom circuits il try another

#### MrChips

Joined Oct 2, 2009
21,648
No one likes my solution in post #43.
Works like a charm validating the resistor to ±2% resolution.

#### ConnaTC

Joined Jun 20, 2020
18
Just revisited your post I completely missed it I apologise looks like a simple enough circuit to use il look into it and test it out

#### MrChips

Joined Oct 2, 2009
21,648
If R1, 1MΩ variable resistor is too sensitive to adjust you can replace it with 500kΩ variable.
Or you can use a 330kΩ fixed resistor with 100kΩ variable. The proper value for R1 is around 355kΩ.

#### ci139

Joined Jul 11, 2016
1,671
like a charm
suppose ,
but it lacks a calibration button/-resistor of 470kΩ
and it (both LED-s ON) does rely on the ? internal hysterresis ? of the CMOS "inverter" ? (TTL has such ... tiny ... but i'm not sure about CMOS)