Logic Probe Construction

Logic Probe components

A 12 volt power supply
Digital voltmeter
A soldering iron
Brass Rod 150 mm long
Red LED    
Green LED 
Black wire 2 meters long

Red wire 2 meters long
2 resistors 1k ohm
Red alligator clip
Black alligator clip
100 mm plastic tube 7mm ld

Shrink tubing:Black 2.4mm diameter, about 300mm long
Red 6.4mm diameter, about 175mm long
Black 12.7mm diameter, about 125mm long

Soldering the parts:

The LED's, 1 kΩ resistors, and wire are then soldered together





Using the small heat shrink insulate both resistors and LED legs,(from the Black and Red wires to as

 possible to the LED)

Before soldering the uninsulated legs of LED's to the brass rod, cut two 15mm pieces of the blue heat shrink and slide them both over the red and black wire and up onto the rod, get the LED's into approximately the correct position then heat the heat shrink.This will support the weight of the wires and prevent the insulated legs of the LED's breaking off.

Position the LED's to sit just above the rod,making sure that the insulation protects the insulated legs of the LED's from the brass rod.Twist the uninsulated legs of the LED's individually around the brassrod and solder to the tined section.

Fitting the plastic tube:

Select the 100mm length of 10mm diameter plastic tube and cut a groove out of one end. This groove needs to be as long as the two LED's are in line, plus 5mm that when fitted will extend past the LED's.

Improving the appearance: 

To improve the appearance cut 20mm length of the 10mm black Heat Shrink.Slide it on from the pointed end of the brass rod. It may be necessary to squeeze the plastic tube together so the heat shrink will slide right up close to the green LED.

Cut 20mm lengths of red heat shrink and fit to both ends of the probe ,this will pull the black Heat Shrink onto the brass rod and the wire tightly.

Fitting Alligator clips:

 

Remove the plastic covers from both the clips and install the correct way around on the appropriate coloured wires. Strip the insulation and twist about 10mm of each wire tightly. Insert the twisted end of the wire through the hole in the clip, bend and secure the insulated section of the wire in the claws provided.Cut off the excess twisted wire and solder joints.

 

 

Testing:

This tester is only for 24 volts or less,DC volts only. Connect the red clip to battery positive and the black clip to battery negative.Both red and green LEDs will light. This tests the connections and that the tester is properly connected. Touch the brass probe end to the positive battery terminal and the green LED goes out,and the red led gets brighter.Touch the probe to the earth battery terminal and the red LED goes out,and the green LED gets brighter.