DIY Tools—Build Your Own Logic Probe

Having a logic probe handy is always a must for an electronics workshop. But instead of buying one why not build your own for a fraction of the cost and have it working in an hour?

Why Do I Need a Logic Probe?

Oscilloscopes are extremely helpful when you're testing and troubleshooting digital circuits. But for many individuals, an oscilloscope may not be a solution because of the price tag (hundreds to thousands of dollars) or the bench space. The first oscilloscope I personally owned was an old 70s scope that was too bulky for a cramped working area.
For those without scopes, there is a (very simple) solution. This device cannot show you what a waveform looks like, but it can tell you whether a signal is

1. off (0),
2. on (1),
3. floating (Z), or
4. oscillating

The device is called a logic probe, and it is something that most EEs should have on their workbench.
The advantage of a logic probe is that it is a very simple circuit that is so small that it can be held like a pen. It is also dead cheap.
Interestingly, an oscilloscope will not always show you if a signal wire is floating. So even in terms of measurement capabilities, the logic probe has an advantage over a scope!

The Schematic

The logic probe circuit consists of a single 4001 quad NOR gate. The first circuit (U1A) is an oscillator and the second circuit (U1B and U1C) is a monostable multivibrator (aka a one-shot).

Schematic for the logic probe. Click for a larger image.

Floating Input

If the input is not connected to anything (floating), the logic gate U1A will oscillate (albeit a very small oscillation centered around VCC / 2) thanks to R1. The NOR gate is behaving as a NOT gate (as both inputs are tied together) with the output connected to the input (through R1). If the output is logic high, then the input voltage will be high as well—but if the input voltage is high, then the output voltage should be low (as it is an inverter). It is this “out of phase” setup that causes U1A to oscillate (where the frequency of oscillation is determined by the resistor R1 and the input capacitance of U1A).
So what will happen when U1A oscillates (because the probe is floating)? Because the oscillation does not go to VDD and GND (if you view the output of U1A on a scope, it will be a very small oscillation of about VCC / 2), the Green and Red LED (Hi and Low, respectively) will be off or dim depending on the size of R2 and R3. U1B and U1C are configured as a monostable multivibrator (the off period being determined by R4 and C2) with an inverting output stage (U1D) which is connected to an LED (D3). When the output voltage of U1A makes a low-to-high transition, the monostable is triggered and will turn on the LED (D3) to indicate that the input signal has changed. With U1A oscillating (as the input is floating and the feedback resistor R1 causes U1A to oscillate), the monostable is constantly triggered by U1A and thus the oscillating indicator (D3) will stay on.
For this circuit configuration, the monostable off period is approximately 0.47s.

Oscillating Signal

When the probe is connected to an oscillating signal (one that swings between VDD and GND), not only is the oscillating indicator (D3) on but so are D1 and D2.
Note: The logic probe will also give you some idea of the duty cycle of the signal under test. If the signal has a high duty signal (for example, 90% on 10% off) the HI LED (D1) will be much brighter than the LO LED (D2).

ON / OFF Signals

When the probe is connected to either an ON or OFF signal, the oscillating indicator (D3) will turn off because the monostable is not being triggered (as the input signal to the logic probe is not changing). If the input is ON, then the HI LED (D1) will turn on. If the input is OFF, then the LO LED (D2) will turn on.

Grounding

For the probe to function correctly, the ground on the logic probe and the ground on the circuit under test must be connected. This is where the 0V ref pad comes into play. This pad gives you a place to connect your probe's ground to the ground of the circuit under test.

Enclosure

Depending on your requirements, you can build the logic probe in either a box with probe connectors or as a stand-alone pen-like tool. The box version is more convenient if using generic probes because it is easier to use. The pen version will obviously save space and can easily fit in a tool box but a few issues are present with such a design:

1. You have to provide power externally with wires (as batteries would make the device too large).
2. You also need to connect a flying lead to the circuit's ground point which may make the logic probe inconvenient to use

I built both to show the difference between the two enclosure types, but I personally prefer the boxed version as it is much neater and more convenient. The internal battery and switch also make the unit independent of external power supplies, like a multimeter.

BOM—Bill of Materials

Logic Probe Circuit

Component / Part	Schematic Reference	Quantity
4001 IC	U1	1
1K Resistor	R2, R3, R5	3
2.2M Resistor	R1	1
4.7M Resistor	R4	1
100nF Capacitor	C1, C2	2
LED Green (3mm)	D1	1
LED Red (3mm)	D2	1
LED Yellow (3mm)	D3	1

Box Enclosure

Component / Part	Quantity
Project Box 100x60x25 mm	1
Banana Socket 4mm – Red	1
Banana Socket 4mm – Black	1
PCB Switch SPDT	1
Stripboard (cut to size)	1
M3 Screws 10mm (self-tapping)	4
Superglue	4

PCB Probe Version

Component / Part	Quantity
PCB (cut to size)	1
Pogo Pin	1
Red Wire	as needed
Black Wire	as needed
Electrical Tape	as needed

Construction—Box Version

Making the box version of the logic probe requires machining tools to cut the stripboard / PCB to size, drilling holes into the circuit for mounting purposes, drilling holes for the LEDs / connectors, and milling bits to make the cut-out for the switch. All of this can be done with a drill, but it's best if a drill press is used. The stripboard cut-out shown here was done using a bandsaw followed by filing to get a straight edge.

Stripboard cutout to fit the PP3 battery and the PP3 connector

Battery and stripboard fitting snugly in the case

The stripboard has a cut-out to fit the PP3 battery because the battery will not fit in the space between the stripboard and the lid of the project box. Four 3mm holes were drilled on the edges which line up with the holes in the project box (which require 3mm self-tapping screws).

The final layout with wires, banana sockets, and a switch

Logic Probe enclosure complete - testing an oscillating signal!

Construction—The PCB Edition

The PCB edition uses a single-sided PCB with all traces underneath. The small size of the PCB (75mm x 19mm) makes it ideal for handheld use.


There is an issue, however, with this design. The pins are exposed underneath and so false results are common when holding it. To get around this problem, you can use electrical tape and protect the bottom so that when the probe is held the pins do not make contact with your skin.

The single-sided PCB

The probe uses a pogo pin as a probe tip which has the advantage that you can press into a test point and the probe will retract. As there is a spring inside the pogo pin the contact that the probe makes with the test point is reliable. It would be a good idea to either use hot glue or epoxy on the soldered connection between the pogo pin and the PCB. This is because if the pin is just soldered then the only mechanical strength is coming from the adhesion between the pogo pin pad and the PCB substrate (which is not very strong).

The PCB layout - all this hard work to keep it single-sided and zero jumpers!

Using the Logic Probe

Using the logic probe is very simple:

1. Make sure the probe has power (5V to 9V).
2. Connect the probe's ground to the ground of the circuit that you are testing.
3. Probe the circuit.

The table below shows the combination of LEDs and what they represent.

Function	Red LED	Yellow LED	Green LED
On	OFF	OFF	ON
Off	ON	OFF	OFF
Oscillating	ON	ON	ON
Floating	OFF	ON	OFF

Summary

With the logic probe project completed, you can now test and debug your own circuits. Of course, this project could be expanded by designing a circuit that has multiple inputs just like a logic analyser. That way you could test multiple points at the same time and get a better understanding of what’s really going on in your circuit.