# Resistor Detection Circuit

#### MisterBill2

Joined Jan 23, 2018
6,720
What I did not see is how accurately the setup needs to be, how close to 470K ohms does the resistance need to be for the LED to light. A circuit that might be more useful than a bridge would be a window-comparator using two sections of a LM339 comparator. as mentioned in post #13.

#### ConnaTC

Joined Jun 20, 2020
18
I’m going to try a couple of circuit including the window comparator to see what works best, In regards with tolerance i know the ohms can fluctuate depending on resistance from the wire connected to the socket i need to do more tests with a multimeter to determine this factor.

Does a window comparator need to be precise at 470knor can it fluctuate?
What I did not see is how accurately the setup needs to be, how close to 470K ohms does the resistance need to be for the LED to light. A circuit that might be more useful than a bridge would be a window-comparator using two sections of a LM339 comparator. as mentioned in post #13.

#### ericgibbs

Joined Jan 29, 2010
10,472
hi C,
A window comparator detects when a value falls within two set limits.
E

#### KMoffett

Joined Dec 19, 2007
2,899
In post #13 I included two trim pots to set the upper and lower acceptable limits. So any resistance between these is OK. Circuits are out there to do that.

#### MrChips

Joined Oct 2, 2009
21,651
I’m going to try a couple of circuit including the window comparator to see what works best, In regards with tolerance i know the ohms can fluctuate depending on resistance from the wire connected to the socket i need to do more tests with a multimeter to determine this factor.

Does a window comparator need to be precise at 470knor can it fluctuate?
The resistance you are attempting to detect is nominally 470kΩ.
If you allow for:
±10% variation, that would be ± 50kΩ
±5% variation, that would be ± 25kΩ
±1% variation, that would be ± 5kΩ
The resistance of connecting wires is very unlikely to exceed 10Ω.
Any variation you measure is going to reflect the tolerance of the resistor itself.

You can set the lower and upper limits of a window comparator to whatever you desire, for example, 440kΩ to 500kΩ.

Still easier to go buy a DMM.

#### MisterBill2

Joined Jan 23, 2018
6,720
A DMM req1uires an operator to have the skill to set the switch to the correct function, and to be able to read and understand the numbers and then evaluate that reading relative to some limits.
The reality is that there are a whole lot of people who can not do that on a consistent basis. That is why every production testing machine that I ever designed had a red light and a green light, for accept and reject. And that was a lot of machines for quite a few different companies over the years. Some of them also displayed numbers, but always RedLight/Greenlight.

#### MrChips

Joined Oct 2, 2009
21,651
That is why I suggested an LM3914 solution.
Use one LED of a different colour (green) in the mid-range output.

#### KMoffett

Joined Dec 19, 2007
2,899
ConnaTC,
Is this for production line testing, post installation verification, or something else on a large scale?

#### ConnaTC

Joined Jun 20, 2020
18
I do use a DMM on a daily on the field just wondering if a circuit could be made to give illuminated feedback as I’ve only used a handful of IC’s which i come on here for opinions and advice.

#### MrChips

Joined Oct 2, 2009
21,651
Here is a single LED solution using two TL431 voltage references.
Someone might be able to reduce in down to fewer resistors.

#### MisterBill2

Joined Jan 23, 2018
6,720
A system can be made that evaluates the signal from a digital meter and delivers an accept or reject output. I did one of those for checking that all of the fuel injectors were connected on an engine harness for a big auto company. That circuit was sort of complicated and I don't even thing the company that made the digital meter is around today.
BUT there are digital multimeters that interface with a computer, and it should be simple enough to have a simple program compare the measured value with the programmed limits. And it may be that the meter companies like Fluke or Beckman already have such a product that includes the Pass/fail limits system. Certainly this is not the only time such a function has been needed.

#### ericgibbs

Joined Jan 29, 2010
10,472
hi,
I would have thought a guy who uses a DMM as part of his field servicing anyway, should know how to read the resistance value from a DMM.?

E

#### ConnaTC

Joined Jun 20, 2020
18
Everyone makes mistakes bud sometimes expensive ones

#### KMoffett

Joined Dec 19, 2007
2,899
Minimum components in my opinion would be a Picaxe08M microcontroller, two resistors (470KΩ and 270Ω), an LED, a battery, a push-button switch, and a telephone plug. Well a cord and a case too. Simple to program with free programing software. A Picaxe would also require a USB-to-Serial cable.

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#### MisterBill2

Joined Jan 23, 2018
6,720
hi,
I would have thought a guy who uses a DMM as part of his field servicing anyway, should know how to read the resistance value from a DMM.?

E
I am presuming that the setup would be for use by somebody less skilled in the use of a meter. That is usually why semi-automated equipment is used.

#### Tonyr1084

Joined Sep 24, 2015
4,907
Something not mentioned (or asked) is: Assume three states (and for sake of discussion assume absolute perfection of values)

State 1: Open circuit.
State 2: 470KΩ.
State 3: zero ohms.

I point this out because I'm wondering exactly what it is you want to test for. Can you find different resistances in your test such as 330KΩ or 610KΩ (or other)? Wire lengths will affect resistance to some extent. Long runs will have higher resistances. And as already mentioned, resistors can have varied accuracy. Precision resistors are ±1% whereas common resistors can be ±20%.

If you're looking for any of those three states then the solution can be even easier.

#### KMoffett

Joined Dec 19, 2007
2,899
Microcontroller can handle all three ranges too. But at anything ranging outside of 470KΩ(+/-tolerance) needs intervention...I assume.
ConnTC, you still haven't been forth coming in what context this will be used and by whom.

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#### ConnaTC

Joined Jun 20, 2020
18
If i can detect all 3 ranges that be great As far I'm aware the resistors aren’t precision,

The circuit will be used by New field engineers whom don’t get supplied with DMM or other devices with Multimeter functions

#### Tonyr1084

Joined Sep 24, 2015
4,907
If i can detect all 3 ranges that be great As far I'm aware the resistors aren’t precision,

The circuit will be used by New field engineers whom don’t get supplied with DMM or other devices with Multimeter functions
So you're saying you need to detect a short, open or 470KΩ resistance?

Where is the 470KΩ resistance coming from? Is this something someone inserts into the circuit for testing?

#### KMoffett

Joined Dec 19, 2007
2,899
If i can detect all 3 ranges that be great As far I'm aware the resistors aren’t precision,

The circuit will be used by New field engineers whom don’t get supplied with DMM or other devices with Multimeter functions
Thanks! That helps.