Pages

Logic Probes

Logic Probes


Logic probes are used to analyze and troubleshoot a logical state of either 0 or 1. It is a pen-like device that is powered by either the circuit being tested or by battery. Here is a simple circuit that you can use to make your own hand-made logic probe.



Schematic Digram:




Parts List:

  • housing of the logic probe - The housing can be any plastic case to avoid any short circuit. It should be large enough to contain the circuit inside. An example would be a ball-point pen or a highlighter.
  • red and green LED
  • dual operational amplifier - Chose an op-amp that is sensitive, has good rail-to-rail characteristics, a large input impedance, and an output current greater than 20 mA. LM324 can be used.
  • resistors - see diagram for valuse
  • probe tip - This can be any metal tip like a needle, pin, paper clip, etc.
  • alligator clips - The alligator clips should be in red and black.
  • wires
  • protoboard


Assembly Instruction:

Prepare all materials needed. Remove the insides of the pen or highlighter and clean out any residue. Arrange the elements on a protoboard according to the schematic diagram. Solder them properly on the board. Another way is to solder them without the use of the protoboard. This is done in order to insert the circuit inside a narrow case like the ball-point pen. Arrange them in your convenience and and according to your own design. Use telephone wires as jumpers or connecting wires. The probe tip should not be loose and can be fixed using any adhesive like stick glue or epoxy. The LEDs should be placed in such a way that the light can be seen outside. As for the alligator clips, cut length of wires long enough to be connected for its power supply. These wires are soldered to the board at one end and connected to the clips on the other end. The red clip should be connected to the +Vcc and the black for the ground. Use soft wires for this connection.


How to use:


To test your logic probe, connect it with a power supply of +5V. Touch the probe tip to any point of a circuit you want to test. The Green LED will light if it is in a logical state of 0 meaning it is grounded, has no voltage or has a voltage of lesser than 9/20 the supplied voltage. The Red LED will light if the test point is in a logical state of 1 meaning it has a supply of voltage greater than 11/20 the supplied voltage.




Pictorials: