Logic Probe Construction

The task we had to do was to make a component known as a logic probe, this component is a pen sized tester used to test particular points in an electronic circuit, the logic probe is used in finding faults and testing circuits. The logic probe that I made has a main use of dealing with an automotive application and testing electronic circuits on vehicles as the probe is rated to 24v.

This logic probe is a basic one and relatively simple, this description will give a step by step instructions on how it was made.

Step 1: There are two L.E.D's (light emitting diodes) a green one and a red one, and there are two resistors which are rated at 1000 ohm's (1.0kOhm), you then connect one of the resistors to the green l.e.d's long leg which is the positive side, this connection is made by doing a western union twist in which you inter twine the leg of the l.e.d and the resistor by wrapping the legs around each other (it does not matter which end of the resistor you use). It is however different for the red l.e.d you will do the western union twist with the short leg of the red l.e.d which is the negative side. You will then get two wires a red one and a black one around around 1000mm (it does not have to be exact). You will then trim off around 20mm of the insulator on both wires, and wrap the red wire around the free end of the resistor connected to the green l.e.d you will then wrap the back wire around the free end of resistor connected to the red l.e.d (you dont need to do a western union to attach the wires). At this point you will then solder together all the connections that you have made, so you will solder the twist that you made at the resistor and l.e.d and the connection that you made at the resistor and the wire that you wrapped around the resistor. You will then slide some heat shrink up both wires each piece of heat shrink will be around 50mm long, which you will slide up till it touches the l.e.d this should cover all the soldering connections that have been made. Using a heat gun shrink the heat shrink over the soldering connections that you made to create some insulation.

Step 2: You will then get a brass rod around 160mm long with a pointed end, at this point get two pieces of heat shrink one which will be about 60mm long and the other 75mm long, now getting the shorter heat shrink slide it over the pointed end leaving about 10mm from the point then getting the larger piece of heat shrink slide that over the non sharpened end of the brass rod leaving around a 10mm gap in the middle between the two heat shrinks. Then using the heat gun shrink the heat shrink on the brass rod making sure that the pointed end and the middle of the rod is left exposed. Now that there is heat shrink on the rod the part in the middle that was left exposed now must tined with solder, this is done to make it easier when connecting the l.e.d's which will be described later on. Once you have soldered or tined the middle part of the brass rod get the two l.e.d's and wrap the leg that has not got anything on it around the middle of the brass rod when you do this remember to keep the red l.e.d infront of the green one. Both l.e.d's should be standing verticle and in line this is done mostly for aesthetic purposes. Now with the two wires hanging below the brass rod bend them up so that they are in line with brass rod. Then using some wider heat shrink cut two pieces around 20mm long and slip one end up the wires until you reach the l.e.d's and then using the other piece slide that up until you reach the the end of the brass rod, and heat shrink them down, this is done to support the wires and stop them moving around.

Step 3: Now with the two l.e.d's standing up use a hot glue gun on the sides and underneath the l.e.d's to give support and stop them from moving and potentially breaking. Once the l.e.d's have been supported by the glue get some 10mm diameter clear plastic tubing and cut a length around 100mm long, then cut a rectangular grove which needs to fit around the two l.e.d's plus allow another 5-10mm to extend past the l.e.d's. Then slide the tube up the wires and have the cut out groove fit around the l.e.d's make sure that it fits securely and that the grove extends past the led's.

Step 4: This step is about improving the appearance of the logic probe get some 10mm diameter black heat shrink cut a 20mm length, then slide it over the pointed end of the brass rod and have it go over the clear plastic tubing that is extending past the l.e.d's now using the heat gun shrink it over the front of the brass rod.(it will not shrink tightly onto the rod at this point) Now cut a 90mm length of 10mm diameter black heat shrink and slide it over the wires till its hard up against the green l.e.d now use the heat gun to shrink the heat shrink over the back of brass rod with the clear plastic tubing, now cut two lengths of red heat shrink around 20mm long and slide one up the pointed end of the brass rod until it is touching where the clear plastic tubing is but not covering it. Then do the same for the other end, slide the heat shrink over the wires until it reaches the clear plastic tubing but not covering it, then use the gun to shrink the ends of the heat shrink onto the brass rod. Then you need to begin twisting the red and black wires together neatly then use red or black heat shrink every 400-500mm to stop the wires from unravelling, do not twist the last 300mm of wire as this is where the alligator clips will go.

Step 5: Get two alligator clips and remove the plastic covering off of them, an easy way to do this is to clip the alligator clips on to something small like the handle of a small spanner. Then strip the insulation off the end of both wires around 10mm, then twist the exposed wire as tightly as you can and put it through the small hole in the clip and then cut off the excess wire and solder the wire to the alligator clips, once the clips have cooled down slide the plastic back over the clips and you will have a completed logic probe to use in an automotive application.

This will give a circuit which indicates whether your connected to a negative side of a battery or positive, one way of testing this is to put the red wire on the positive battery post and the black black wire on the negative battery terminal. If the circuit has been wired up correctly then both l.e.d's will glow as a circuit has been created through both l.e.d's. Now put the probe or brass rod on the positive terminal and the black wire on to negative or ground and the red l.e.d will glow and the green one will go out, the red l.e.d glows brighter when the probe touches the positive terminal because more current is able to flow through the l.e.d than when both are connected up because there is now less consumers(l.e.d's and resistors) using up the current meaning more current can be provided to the red l.e.d. The green l.e.d goes out because a circuit has been created through only the red l.e.d current cannot flow through the green l.e.d as it is stopped because an l.e.d is like a one way valve current can flow one way but not the other. Now place the red wire on the positive battery terminal and put the probe onto ground this will make the green l.e.d glow and the red one will go out. The resistors that are attached to the l.e.d's reduce current flow as a 12volt battery would provide far to much current to the l.e.d's and burn them out almost immediatly. The resistors reduce the current to allow the l.e.d's to operate under there normal low power rating and the resistors allow for the probe to be used on circuits up to 24 volts, this is why the 1.0kohm resistors were used.

The finished logic probe should now look like the one in the image below.