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DIY Tools Series: How to Build a Logic Probe

September 24, 2018 by Robin Mitchell

Learn about the value of a logic probe and build one to use in your future projects. 

The first electronic projects you will attempt usually require a single multimeter for debugging and testing. However, as your projects become more advanced and digital in nature, you will find that a multimeter alone is not enough. In this second installment in the DIY Tools series, we will be learning how to build a simple logic probe  capable of analyzing a digital signal.

Check out the first part of our DIY Tools Series below: 

Probing Signals 

Multimeters are handy tools and can communicate a lot about a signal including the voltage, current, resistance, and continuity. There are some more advanced multimeters that can even measure inductance and capacitance making them very useful for analog circuits. 

While multimeters can also be used in digital environments, they are somewhat inappropriate for a number of reasons. First, multimeters are designed to measure either DC or AC sources, which means that if a digital signal is switching then the multimeter will record unusual voltages. Second, multimeters are large and bulky, so using more than three on a circuit can be difficult. Third, multimeters have a common (ground) input that must be connected and using more than two can make this a difficult task.

A logic probe, unlike a multimeter, is a very simple circuit that is designed to measure digital systems. Logic probes are often only equipped with three output LEDs that indicate the following states:

  • 1 – (Digital High)
  • 0 – (Digital Low)
  • Z – (High impedance)
  • P – (Signal is pulsing / switching)

However, logic probes cannot measure analog readings, so when measuring signals, the voltage levels of the signals cannot be determined. Despite that shortcoming, logic probes are easily designed, constructed, and used with other probes. The logic probe that we’ll be constructing consists of four separate logic probe circuits combined into a single circuit that provides four logic probe inputs and four LED output displays.

Logic Probe Schematic

A single logic probe circuit

How the Logic Probe Works

The logic probe (shown above) has three main blocks:

  • U1A – A simple oscillator 
  • U1B and U1C – A monostable latch
  • U1D – An inverter

If the input is connected to nothing (i.e., high impedance), then U1A oscillates due to the feedback resistor R3. Since this oscillation is very small (100mV amplitude) and only oscillates about 2.5V neither LED D2 or D3 turn on. However, this small oscillation is enough to trigger the monostable made up of U1B and U1C, resulting in the output of U1C oscillating. This oscillation also causes the output of U1D to rapidly change and therefore the LED D1 turns on. If the input is connected to a logical high, then the output of U1A remains low which turns on LED D2 and turns off LED D3. 

Since the output of U1A is not changing, the monostable circuits output remains low which results in the output of U1D being high. The result is that LED D1 cannot conduct and therefore turns off.

If the input is connected to a logical low, then the output of U1A remains high, which turns on LED D3 and turns off LED D2. Since the output of U1A stays consistent, the monostable circuit’s output remains low, resulting in a high output for U1D. The result is that LED D1 cannot conduct and turns off. If the input is connected to an oscillating digital signal, then the output of U1A rapidly switches between VDD and 0V. Unlike the floating situation, this signal is able to turn D2 and D3, triggering all three LEDs to turn on.  

Construction of the Logic Probe 

Instead of mounting everything onto a single PCB, individual boards—each acting as a single probe— were used. This design not only makes it easier to identify the multiple probes when they’re together, it also helps to better organize it overall. Ideally, all parts would be loaded onto a single PCB, as the probes use common power and ground traces. 

The logic probe circuit has a single ground reference output, which connects to the ground on the circuit under test. You can use your choice of connector to link the probes to the PCB. 

For this project, the logic probe PCB has a 4-pin SIL header allowing the use of cheaper logic analyzer probes (these probe types also have spring loaded clips that keep them attached to a test point). Some custom probes were also made using pogo pins as the probe head. This type of probe contains an internal spring that provides a very smooth action when probing signals (i.e., applying pressure results in the head retracting slightly which improves contact). 

The circuit needs a power supply equal to the supply used by the circuit under test because the logic probe relies on having the input signal equal to the voltage level of the power line fueling the 4001 IC. For example, if the probe was powered by 5V and the logic signals being probed were 3.3V logic, there’s a chance that 3.3V would not provide enough energy to trigger the NOR gates.

A wooden housing base contains all the parts of the logic probe, while a 3D-printed front cover removes the need for machine cut-outs and other complex parts. Once you have your logic probe circuits constructed, use it to analyze the digital signals of your future projects!

The wooden housing base containing all logic probe parts

The wooden housing base containing all logic probe parts (top view)

Wooden housing unit (side view)


Robin Mitchell

Graduated from the University Of Warwick in Electronics with a BEng 2:1 and currently runs MitchElectronics.

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