Logic Probe Construction

We were given a list of parts that would be needed to build this probe:

Brass Rod 150mm long
Red LED
Green LED
Black wire 2 meters long
Red wire 2 meters long
2 resistors 1 K ohm
Red alligator clip
Black alligator clip
100mm plastic tube 7mm Id
Shrink tubing:
*Black 2.4mm diameter, about 300mm long
*Red 6.4mm diameter, about 175mm long
*Black 12.7mm diameter, about 125mm long

We were given this diagram to follow and a list of step-by-step instructions.
Had to solder the resistors to the LED light. One to the positve leg of the Green LED and the other to the negative leg of the Red LED. Then, twisting the wires of the ends of the black and red wires, so they were the same thickness as the covered parts of the wires. Then  twisted the other leg of the green LED to the red wire and the other leg of the red LED to the black wire, symbolising red positive (green LED) and black negative (Red LED). Then to ensure insulation of the circuit, we covered the exposed wires with shrink tubing and heated to fit.

We then sanded a point to one end of the brass rod and the middle so that the solder would take to the rod easily. Then on either side of the sanded middle, shrink tubing was applied to again keep the circuit insulated. the exposed leg of the LED's then are wrapped around the sanded part in the middle of the rod. they are then soldered into place. It is important that there is no sharp points on any of the solder so it doesnt create any holes in the shrink tubing. The red and black wires then need to be pulled back to the end of the rod (without the point) and shrink tubed together (with the rod) to hold it neatly together. Then the end of the plastic support tube needs to be cut so there is a rectangle shape at the end to fit in LED lights and fit ove the brass rod. A little extra room at the end should be given so shrink tubing can be attached to improve presentation. The plastic support tube slides on from the bottom of the wires up to the rod and LED's. Once fitted into place it then needs to be shrink wrapped over as much as possible to improve the general appearance.
The wires then to be plated around one another to also improve appearance leaving approx. 30cm at the end so the wires can be seperated. Finally, the alligator clips need to be attached to the end of the wires, red to red and black to black. They must be pinched onto the wire and soldered down to ensure they are properly secured.

ref: Logic Probe Construction Practical Workbook
This tester is only for 24 volts or less, DC volts only. Connect the red Clip to the battery positive and the black to the battery earth (negative). Both red and green LEDs will light. This tests the connections and that the tester is properly connected. Now 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.

Both LED's go on when they are connected to the battery because the circuit is then a compete seires circuit, in which the current has to flow through positive to earth to be complete.
The green LED goes out when the probe contacts the battery positive because the current now flows through the probe as opposed to the first resistor which makes the circuit then shorter. The red LED then gets brighter as now there is only one resistor in the circuit as current can not flow back to the green LED.

FINISHED PRODUCT