Greetings everyone!

I am a long time lurker but a first time poster in need of guidance on something that I've been conceptualizing. (Mods feel free to move this thread if I am posting this in the wrong area)

I am looking for a low-tech method to detect the difference between a 1K resistor, a 2K resistor, and a 2.2K resistor without using a meter. I was thinking of building some sort of box with 3 LED's where each light would correspond to the amount of resistance detected.

So if the resistance is 999 ohms or less, none of the lights should illuminate. If the resistance is greater than 999 ohms but less than 2000 ohms, the first light should illuminate. Greater than 2000 ohms but less than 2199 ohms, the second light should illuminate and so on.

Just to give you some background on what I'm doing, I install alarm systems and sometimes the resistors are out 'in the field' next to the door contact sensors. Rather than pulling the sensors out to read the color bands, I'd like to build a tool like this one to determine if it is one of the common resistor sizes that is used on most alarm systems.

Ideally I'd like to make the led illumination isolated from all of the other lights, however, I wouldn't be opposed to the lights illuminating in succession.

Thanks!

 

Where would you connect this "LED box"? The reason I ask is because, depending on where you make your connection, you may be dealing with the wire resistance as well. The wire resistance could throw off your measurements especially since not all the wire runs will be of the same length.

 

This device will be used at the location of the alarm control box.

Basically the box (which I imagine would be the size of a pack of cigarettes) containing the LED's would have a couple of wire leads with alligator clips on the end of them. To test for the resistance in each circuit, I would disconnect the loop from the alarm control panel and clip my alligator clips to it.

Sometimes I go into environments where the alarm control panel is already removed from the closet, garage, or wherever it was installed leaving all of the home-run wires coming through the hole in the wall.

Depending on the installer, some technicians put the resistor inline with the door/window contact sensor (where it should go) while other technicians prefer to wire them inline at the control panel. So such a device that I am thinking of would quickly determine if there is a resistor in the line and the approximate size of it.

Typically the tolerance of most alarm systems will allow for the length of the wire run so I am not looking to detect an exact dead-on amount of resistance. I figure that if I can get the LED to illuminate if it can detect...say.. anywhere between 900 to 1200 ohms. Then it would be safe to assume that I have a 1000 ohm resistor on the line. Detecting the difference between 2K and 2.2K might not be as easy without a meter so I am not too concerned about that. But if I could devise a way to test the difference between 0, 1K, and 2K while allowing an additional 100 or so ohms of 'error' then that would be great.

Thanks!

 

You could use an LM3914 dot/bar display generator. It lights LEDS in proportion to the voltage at the input. It can be set to light the LEDs in succession (bar mode) or one LED at a time (dot mode). It can do up to 10 LEDs but you can just use three of the ten and leave the others open if you like.

The LM3914 sensitivity can be set between 1.2V to 12V full-scale and it will operate down to a 3V supply voltage.

A simple way to generate the signal is just to have a resistor across the input (say 1kΩ), apply voltage to the one of the line pair being measured, and connect the other line pair to the circuit input. Then if there is no resistor in the circuit, the voltage will be full-scale, if there is a 1kΩ resistor, it will be about half-scale, and a 2kΩ resistor will give about 1/3 scale.

You could use a 9V battery for power, but for longer battery life I suggest using 3 AA or AAA batteries in series to give 4.5V nominal.

 

Thanks! That sounds exactly like what I need. I was sitting here thinking about how I can arrange a set of resistors together to do the same thing but I guess that's part of the reason why that chip was invented.