Testing for a resistor value without using a multimeter?

[Mattylad]

HI, I am trying to build a cable test set that an apprentice started for his college project, it's got a fair bit of money sunk into it already so it's a shame to just
stop and throw it away so I am finishing it off.

So anyway, one of the cable tests it does is to check for the presence of a 390r resistor between 2 pins, had I been able to do this I would have used an arduino but he did not have time to learn that (neither do I), so I am stuck with only the ability to add 1 LED hole into the enclosure (it has an expensive custom fascia).

So what has me stumped is how to measure a resistor value and illuminate a single LED.
I'm thinking that perhaps having a bicolour LED and below value or missing would be yellow, in the correct range to be green and above the range to be red.

Can you suggest how I may do this please?

 

[danadakk]

What V do you have to drive the LED and how much current you
want to drive it at ?Do you want the green on as well as the red
when over-range, or just one on for each range ?


Regards, Dana.

 

[betwixt]

Simplest method as you don't have a method to display the resistor value is to make the 390R half of a potential divider and pass a small current through it. Then use a comparator to see if the expected voltage is being dropped. You can use the comparator output as a 'good/bad' logic output to drive the LED.

Better still, use two comparators in a window configuration, then you can not only check the resistor is fitted but within reason, check it is also the correct value.

Example:
ground on of your two pins,
apply 5V (or something convenient) to the other pin through a 390R resistor,
use the comparator to see if the voltage is below or above half the voltage you applied.

If the resistor is missing, no voltage will be dropped and full voltage will appear across the pins. Anything less than full means some resistor is fitted. Half voltage +/- some tolerance will confirm your series resistor matches the value of the one fitted. The comparator output can directly drive the LED indicator(s).

Brian.

 

[BradtheRad]

Nowadays there's bicolor and tricolor led's in various terminal configurations.
You're not limited to the older type which lit red in one DC direction, green in the other DC direction, and yellow with AC.

The window comparator method (as in post #3) should make it feasible to detect all readings within 5% (whatever you choose) of your target value.

 

[D.A.(Tony)Stewart]

If you only had resistors and any NPN's I made this work with simulated testing.

http://tinyurl.com/2dnjmqnz

but will it interfere with the cable tester?? or does ever pin have resistors?

 

[Mattylad]

HI, many thanks for the suggestions.
What I have forgotten is to say that the rest of the tester is running at 24v DC, using 24v LED's.

There is 24v fed to one leg of the resistor as that goes into a connector, the other leg is wired back to another connector
so I can measure across both legs but will have to handle 24v on the input.

 

[D.A.(Tony)Stewart]

HI, many thanks for the suggestions.
What I have forgotten is to say that the rest of the tester is running at 24v DC, using 24v LED's.

There is 24v fed to one leg of the resistor as that goes into a connector, the other leg is wired back to another connector
so I can measure across both legs but will have to handle 24v on the input.

in which polarity?

You must specify the exact LEDs properly too and now draw a schematic.

 

[Mattylad]

Polarity for a resistor?

I just want to measure across 2 pins of a connector that has a 390r resistor in it to ensure that its the right value.
As much as that would be easy with a meter I cannot add a meter to the expensive fascia on the test set.
I'm in a bit of a pickle with this lol

 

[D.A.(Tony)Stewart]

You may have forgotten to say what kind of using 24v LED's are installed. I need specs.

Common Ground, 1A or 20mA or 5mm or stripled RED & GRN or what?

A photo or datasheet would be helpful.

 

[BradtheRad]

Principle of window comparator using 2 op amps. This shows how it responds inside a narrow window between two volt levels. One test point is 24V. Values and voltages are chosen so that the green led is now lit but goes dark if the Device Under Test (DUT) is a few percent higher or lower than 390 ohms.

You'll need to tailor values and voltages to suit your equipment. To add red and yellow led's, it's feasible to tap into each op amp output, as well as arrange volt levels and polarities, add logic gates, etc. It all depends on what kind of bi/tricolor led you wish to use.

 

[D.A.(Tony)Stewart]

390 to 24V will likely fry both 1/4 W resistor instantly as each draws 3/8th W

--- Updated Jan 10, 2024 ---

Any LM555 or equivalent , rated for 16V will work here.

3 White LEDs to drop 24 to <16V
Normal = flashing LED < 10 Hz
Shorted = LED ON steady
Open = LED Off

Test it here http://tinyurl.com/ysrqdene


Cheap solution may be soldered on 0.1" perf board with a socket for IC. Lookup LM555 datasheet for pin #'s.

 

[BradtheRad]

390 to 24V will likely fry both 1/4 W resistor instantly as each draws 3/8th W

Agreed. At first I thought the bottom resistor should serve as a 390 ohm 'reference' equal in value to the target resistance.

However the bottom resistor can be a higher value, thus getting by with lesser Amperes. Then all we need to do is apply new upper and lower volt levels to the comparator window.

 

[D.A.(Tony)Stewart]

Agreed. At first I thought the bottom resistor should serve as a 390 ohm 'reference' equal in value to the target resistance.

However the bottom resistor can be a higher value, thus getting by with lesser Amperes. Then all we need to do is apply new upper and lower volt levels to the comparator window.

The 555 does it all. The external resistors using the control R makes this solution easy. The cap makes the 3rd condition to capture all 3 states , matched , open short.

 

[KlausST]

Hi,

I´m with Tony. The thread misses basic informations.

The first one needs to decide is the accepted tolerance. A 390 Ohms resistor never is exactly 390 Ohms.
so maybe you allow a tolerance of 10% which is 39 Ohms.

Here a draft of how it could be done. (incomplete)

I tried to make it simple and rugged.
Important things are:
* you need a reference resistor R2. (the value you expect DUT to be nominally)
* you need two resistors to determine the tolerance (R3 = R_expected * tolerance. R4 usually is the same)
* and you need two equal resistors as current limiters. R5 = R6. The value may vary in a wide range. (1k), 10k, 100k .. what you have by hand.

It works in a wide range of power supply voltage V1.

Mind: comparators may need pull up resistors. Or they may need to reverse input polarity when they drive LEDs connected to VCC.
You need extra logic for output "DUT is wihtin both tolerance limits"

Klaus

 

[D.A.(Tony)Stewart]

The expert engineer will notice I missed something in describing the simulation response for the output LED and how it varies with external resistance.

Can you see it? How would you describe it's performance characteristics in 5 words or less. {Winner will get a bag of 200 Uber=bright LED's} in a self addressed envelope.)