Microelectronics | Logic Probe

Synopsis

A logic probe is a simple tool that allows you to see signal states on a logic board. It lights up an LED for logic high signals, another LED for logic low signals, and a third LED for oscillating signals or high impedance states.

Originally I had a logic probe based on a quad 2-input AND gate integrated circuit. But it got lost when I moved from one place to another many years ago. These days I really needed one since I had to figure out some logic signals on a computer ISA controller card. I didn't want to use the oscilloscope because it was pretty hard to get on location. I searched a little bit on the Internet and I found a simple and effective schematic. I redrew it and then produced a PCB layout. I don't own any rights on that schematic and I don't know who is the original author. I have seen it redrawn in different CAD tools by different people. So all credits go to the original Author, whoever he or she might be. My contribution to this project is that I only produced the circuit board.

Enough talking and let's get to work. It's a simple schematic and it does not require any further description. If you want to understand how it works, it is out of the scope of this article. But I am sure it is described somewhere on the Internet. At least I do remember that I have seen a very elaborate description somewhere.

Schematics

Fig. 1: Electrical Principial Schematic

Printed Circuit Boards

Fig. 2: Silkscreen

Fig. 3: Top Layer Printed Circuit Board

Fig. 4: Bottom Layer Printed Circuit Board

Fig. 5: Top Layer Printed Circuit Board - Simulation

Fig. 6: Bottom Layer Printed Circuit Board - Simulation

Bill of Materials (BOM)

The following list contains the parts that are required to assemble one SIMM.

LOGIC PROBE
Identifier	Value	Qty	Notes	Mouser Number
Printed Circuit Board	18.20	1	VER. 1.0 REV. A	Order from OSHPark
IC1	CD4001	2	Quad 2-input NOR Gate	595-CD4001BE
C1, C3	100 nF / 50 V	2	MLCC	80-C322C104M5R-TR
C2	10 uF / 25 V	1	Tantalum Capacitor	80-T350E106M025AT
R1	2.2 MΩ	1	Carbon Resistor	291-2.2M-RC
R2, R3, R5	1 kΩ	3	Carbon Resistor	291-1K-RC
R4	4.7 MΩ	1	Carbon Resistor	291-4.7M-RC
HI	5 mm Green LED	1	High Indicator	755-SLR-56MC3F
LO	5 mm Red LED	1	Low Indicator	755-SLR-56VC3F
HI-Z	5 mm Orange LED	1	High-Z Indicator	755-SLR-56DC3F
POWER	4-pin Header	1	Header	649-68001-204HLF

Construction and Pictures

Assembly is straightforward thus I won't detail how I soldered the parts on the PCB. But here are some pictures of the final logic probe assembly.

I used a Romanian integrated circuit made by (now defunct) Microelectronica corporation. As for sensing purposes, I used a small needle which I soldered on the sensing pad. Now I can probe SMD pads as well.

I quickly tested the probe and I am very happy with how it works. When idling or oscillating signal is detected, the orange LED is lit. When low signal (ground) is detected then red LED is lit. And when high (VCC) signal is detected then green LED is lit. I tested with a power supply ranging between +5 V and +12 V and it works great. However I am planning to use it to debug logic boards that work at +5 V.

Thanks for reading this, and thanks to the original Author for this simple yet effective logic probe schematic.