Logic Probe

Next in Electrical we made Logic Probe testers.

                                           Logic Probe Parts List

Brass Rod (150mm long)                               2 Resistors 1KΩ
Red LED light                                                Red alligator clip
Green LED light                                             Black alligator clip
Black wire (2m long)                                     100mm Plastic tube 7mm ld
Red wire (2m long)                                       
Shrink tubing;
Black: 2.4mm diameter, 300mm long

Red: 6.4mm diameter, 175mm long


Black: 12.7mm diameter, 125mm long

Also needed for this exercise is a 12v power supply, digital voltmeter and a soldering iron.

First we viewed the wiring diagram of a Logic Probe that was given to us in our workbooks, once we had studied the diagram we were able to begin wiring it up.

The first step was stripping our long black and our long red wire so that we had enough exposed wires in order to connect up the 1KΩ resistors, which we connected by twisting them around the wire. After we had done this for each wire, we soldered the resistor to the wire to strengthen the connection. Then we grabbed the Green LED and took the positive terminal on it and wrapped it around the other end of the resistor on the red wire, we then did the same with the Red LED on the black wire, then soldered them into place.

Using the small heat shrink, we then insulated both the LED legs and the resistorsfor each wire, making sure they were as overed up as possible, then using the heat gun to shrink the wrap around them, adding even more security and protection.

We then took our brass rod's and ground a point on one end, and made a tin in the middle of the rod aswell, which will make it easier to solder onto. We insulated our brass rod with some more heat shrink for 60mm starting 10mm from the point that we made earlier on the rod. There was a gap left were we had tinned the rod beforehand and then more heat shrink was fitted in order to insulate the blunt end of the rod. We applied the heat gun at this stage to shrink the wrap.

Before we soldered the uninsulated legs of the LED's to the brass rod's tinned area, we had to cut two peices of 15mm shrink wrap and slide it over both the red and black wires together and up onto the rod. After we had got the LED's into position, we grabbed their leg's and twisted them around the rod, making them secure by soldering them to the rod. After that, we used a hot glue gun just underneath the LED bulbs to make sure they were secure.

Next we had to grab the 100mm length of 10mm diameter plastic tubing and cut a slot out of one end, big enough to fit the two LED bulbs plus an extra 5mm. We slid the plastic tube over both the wires and positioned it in the right place, making sure the LED's are both protected and there is an extension of 5mm.

To improve the appearance of our Logic Probe, we had to cut the 20mm length of the 10mm black heat shrink and slide it on from the pointed end of the brass rod, over the top of the plastic tube overhang and close to the Green LED. We then heated it up, making it shrink and holding the plastic tube in it's place. Next we cut a 90mm length of the 10mm black heat shrink and slid it over the wires at the other end of the plastic tubing and shrink it, also holding these wires in position. Cut 20mm lengths of red heat shrink and fit to both ends of the probe, pulling the black heat shrink onto the brass rod and wire tightly.

Twistin the black and red wires around each other neatly, we fitted 15mm lengths of the red heat shrink every 500mm along the wires, leaving the last 300mm of the wires untwisted as we had to still attach alligator clips to the ends of them.

To fit the alligator clips, install the correct way, making sure they are conneted to the appropriate wire. We stripped the insulation and twisted about 10mm of each wire tightly. Inserting the twisted end of the wire through the small hole in the clip, bending them and then securing the insulated section of the wire with the claws provided on the clip. Chopping off the excess parts of the wires once secured, we then soldered the joints together.

Testing:

These Logic Probes are only designed to test 24 volts of less, DC only. Connect the red alligator clip to the battery positive and the black alligator clip to the battery earth. Both red and green LEDs will light up, signifying that there is a connection. Now, touch the brass probe end to the battery positive terminal and the green LED should go out, and the red LED should get brighter. Touch the probe to the earth battery terminal and the red LED should go out, and the green LED should get brighter.