Sunday, March 25, 2007

Simple Cable Tester

By leorick  |  3/25/2007 01:23:00 AM 11 comments

Simple DIY Cable Continuity Tester

How to use

This simple cable tester can be used to check 2 wire cable such as coax cable, telephone cable, audio cable and etc. Power the circuit using 9V battery. Plug in the cable and push "TEST" button. The dummy resistor is connected to the end of the cable which has 75ohm resistor inside. The tester will show only 3 conditions, "SHORT", "OPEN" and "GOOD".

How it works

The method is to check resistance of the dummy resistor at the "Terminator". The idea is simple. Imagine there is a short circuit cable under testing. The wire will act as nearly zero ohm resistor thus making the voltage divider near the dummy resistor to divide the 9V into 4.5V. Calculate it your self [ ( 1k / ( 1k + 1k ) ) * 9V = 4.5V ]. 4.5V is below than the lower "Comparator" limit (Vref 4.57V). It will trigger the lower "Comparator" and make "Short" LED to light up. Now for the open circuit cable. Assuming the open circuit cable has an infinite ohm. Voltage divider near the dummy resistor will give 9V. This is more than the upper "Comparator" limit (VRef 4.8V) and will trigger make "Open" LED light up. Other than that (both lower and upper "Comparator" didn't trigger) the "GOOD" LED will light up. For the op-amp, I'm using JRC4558 dual op-amp since I have it in my stock. You may use 741 op-amp or other multi-purpose op-amp. Learn more about op-amp here

Vref tuning

Before turning on the circuit (power it up), tune the potentiometer so that it will give the correct voltage reference to the "Comparator"s.

  • Upper "Comparator" = 4.8V
  • Lower "Comparator" = 4.57V

Extra - Low battery indicator

The extra circuit supplied is to check the battery condition. Low battery voltage (below than ~6.9V) will show "RED" light up. Else will show "GREEN" LED light up. The basic, if there is a small amount of current go through the zener (reverse bias), the first transistor (left) will be turned on making the "GREEN" LED turn on. Other than that the second transistor (right) will be turned on and "RED" LED will light up.

Modification - Add in buzzer

You add in buzzer to the circuit to make it give an audible alarm. The best is to alert us whenever the cable under test is not "GOOD". So, adding a resistor, transistor and a buzzer/beeper to the last NAND (near "GOOD" LED) will do the job. Use your own creativity for this. :-).

Modification - Without NAND? (update)

You can use circuit above as replacement. Only some resistors and a transistor will do. Please refer to Water Level Indicator for more info.

Testing XLR cable (update)

Since XLR cable have 3 wires, it can be tested by connecting 2 wires at a time as image above. Use crocodile clip for easy access.


Bart said...

sir.. gud day!!. im a BS ECE student in philippines, i looking for some diy projects i can make.. sir, can i know the schematic diagram and specs of parts of this project.. im interested on this SIMPLE cable TESTER ... thanks.. heres my email add... .....sir, im looking forward to it.. thanks...!!! kip posting of projects!!! Thanks!!!

Bart said...

sir.. sorry wrong email...


Bart, I've lost this project strip board layout long time ago. The schematics is up there or you can click here. The route is quite simple since the components are not more than 20 parts :-). It could be a great practice for you too. I can help you to check your layout before you start to assemble them. Parts spec? all parts should be able to handle 9V :-).

Ejudd said...

Hello leoricksimon, i know that it has been a long time since u wrote this article. But may i ask, what is the significance of the comparators plus the nand's? Is it just to provide multi indicator( open, good, short).. is there more to it? Really appreciate for ur time :)

leorick said...

The NAND can be replaced with a simple resistor + transistor circuit (added above). It just that, when I built this cable tester I've no idea on how to segreate those signal (HI, LO & NOT (HI or LO)) plus I have a lot of NANDs on my table :-). Good example of replacing this NAND can be found here at
Water Level Indicator

Spen said...

Where do you attach the cables that you are trying to check? For example, lets say you were trying tro create an XLR cable checker, would you connect a male and female on each side of the dummy resistor?

leorick said...

I can see that XLR cable have 3 wires. Meaning that only 2 wires can be test at a time. Please refer to the newly added image. Hope that would help. In your case, you can just use a crocodile clip rather than fixing a male/female XLR connector.

Anonymous said...

hi, i'll be doing a cable checker for ribbon cables and r232 cables. may i know if the schematic diagram may also applies on those types of cable? thanks!

leorick said...

The one with 9 wires am I right? Same as XLR cable/connector, just use a wire clip to test 2 wires at a time.

Anonymous said...

can u teach me how to make this projek...
please email me

leorick said...

How can I help you amirul_amirulehsan?

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