Breadboard Logic Probe

I needed a simple logic probe to aid in troubleshooting some of the circuits I build, so I decided to simply make one. A basic probe is what I was needing, nothing fancy. A schematic search on the internet yields all sorts of designs. I settled on one using NOR gates and modified the design for a simpler version, with no frills, just an LED for LL1 and an LED for LL0, and the two LEDs will alternate when testing a clock pulse.

 Half-size breadboard, x1

CD4001, x1

2.2M resistor, x1

1K resistor, x2

100 nF (104) monolithic disc capacitor, x2

5mm green LED, x1

5mm red LED, x1

Solid core 22 AWG wire, I used red, black, and yellow.

Stranded 22 AWG wire for leads, Dupont pins on one end and hook probes on the other end for power connections, and one lead with Dupont pin on one end and a test probe on the other end. Feel free to use prefabricated leads or you can fabricate yourself.  

Wire strippers and cutters.

There is only 8 parts plus wires to add to the breadboard, as this is a really easy build. I built this on a half-sized breadboard as this is will be used once in a while and it stores easily in my workbench drawer when not in use.

Position the breadboard as shown and insert the parts and add the wires. The white wire shown is used as the probe.

Only 1 of the 4 gates is used but I do like to add a decoupling cap on the power lead of U1, and I like to add a decoupling cap to the input power as well.

The reason for using a CMOS IC is the power that be used, 3 to 15 volts, but I have only tested this with 5 and 9 volts.

To use this, I connect the top + and - rails on the right side of the breadboard to the power points on the circuit you are going to test. I have tested this with +5 and +9 VDC but the CMOS IC is rated from +3 to +15 VDC. The probe lead is white and connected to U1 pin 1.

In the first photo, I have connected the probe lead to the + rail to indicate a LL1 and the green LED lights.

In the second photo, I have connected the probe lead to the - rail to indicate LL0 and the red LED lights.

In the third photo, I have connected the probe lead to an unused terminal, to show what a floating, or unconnected, test point looks like, showing no indication on red or green LEDs, as expected. The reason for R1 across U1 pins 2 and 3 prevents erroneous readings on the LEDs when the probe is not connected, which is a distraction when taking readings.

When reading a clock pulse, the red and green will flash alternately, the faster the flashing, the faster the clock, for a fast clock, both red and green will be on.