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Testing for a resistor value without using a multimeter?

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Mattylad

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


1704635991547.png

1704636078177.png

1704636132130.png


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