What is a Logic Probe

Logic probes are cheap and easy to use forms of digital testers being able to test logic levels of slow moving signals.

Logic Probe Tutorial Includes:
Logic probe basics    

Logic probes are very cheap and easy to use as simple digital testers in many applications. Logic probes can provide a simple way of testing slow moving digital logic levels and signals.

Being very cheap to buy, these digital logic probes are ideal for experimenters, but they will seldom be found in a professional electronics laboratory because of their limited measurement capability and the availability of more advanced test equipment.

What is a logic probe

The logic probe or digital tester is normally a low cost handheld probe contained within a pen-like tube with indicator lights to show the state of the line being probed.

Image of a simple logic probe used for testing logic levels.
Simple logic probe

Typically logic probes are used to test digital circuits like those using TTL or CMOS logic. They often have three indicator lights on the body to indicate the state of the line. As such logic probes are very basic forms of digital testers, only able to test the state of a single line, but they can be useful in many applications.

The logic probe is normally powered by the circuit under test - there are normally leads with crocodile / alligator clips that can be attached to the ground and supply of the circuit under test.

Logic probe measurements

A logic probe is restricted in the number of measurements it can make when compared to other test instruments, but it can nevertheless be used for a variety of digital measurements:

  • Logic high state:   The logic probe / digital logic tester is able to detect lines that are at the digital or logic high state. The logic probe will indicate this typically with an LED which is often coloured red.
  • Logic low:   The logic probe also is able to indicate a logic or digital low. A common indication is with the use of a green coloured LED.
  • Digital pulses:   The logic probe may incorporate some form of pulse detection circuitry. When the line is active and being pulsed a third colour, possibly amber will be indicated. The logic probe may well incorporate circuitry to detect very short pulses and in this way indicate when the line is active. Sometimes the length of the pulses may be indicated by the brightness of the LED.
  • Line tri-stated:   Some logic probes may also be able to detect when a line has been put into a tristate option. This is when the output device has its output turned off and no real logical state is defined. Many logic probes are able to indicate this state and they may do this by having all indicators turned off.

Logic probes vary from one manufacturer the next and therefore it is necessary to check exactly what measurements can be made and how the results are indicated.

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