Breadboard Tip: Simple LED Substitute for a Logic Probe

Return to the breadboard guide for additional construction tips.

The LED is a valuable troubleshooting tool especially when prototyping digital circuits. This post presents a simple construction technique that can save you valuable time.

The idea is very simply. All it takes is a square-body LED and a 1 kΩ resistor. Solder the resistor into the cathode leg and you have a visual indicator that can be installed into the breadboard in seconds. Since the resistor is integrated with the LED, we no longer need to use a separate breadboard tie point.

Figure 1 shows a few LEDs installed in a microcontroller circuit. Note that the square LEDs are preferred for this application as they can be installed next to each other within the 0.1 inch (2.54 mm) spacing of the breadboard as show in the lower left side of Figure 1.

Figure 2 presents a close-up image of the assembly constructed using type SSL-LX2573GD LED and a type CFR-25JB-52-1K resistor. The resistor is installed in the LED’s cathode leg.

Figure 1: Close up picture of the LED & Resistor assembly in a microcontroller breadboard circuit.

Figure 2: Picture of the completed LED Resistor assembly.

Tech Tip: For a new LED, the cathode may be identified as the shorter of the two legs. The cathode may also be identified by looking at the LED’s structure. The LED’s semiconductor die is typically installed on the larger of the two elements. The larger anvil (cathode wire) is easily seen as the upper element in this picture.

Figure 3 shows the setup for constructing the LEDs. It’s certainly not the most elegant setup, but it is fast.

Additional design constraints

The square LED is an important design constraint. The narrow profile allows many such LEDs to installed in close proximity. To illustrate, please consider Figure 4. This picture shows many such LED assembles being tested on a breadboard. The design accommodates the 0.1 inch (2.54 mm) spacing.

The resistor placement is important. While it’s true that we could have associated the resistor with either the anode or cathode it vital that all assembles be constructed with the same polarity. With speed of prototyping and troubleshooting as the prime consideration, eliminating the need to consider which way to install the LED assembly is important. With this design all you need to remember is that the resistor goes to ground.

The resistor value isn’t critical. However, the 1 kΩ values does accomplish a few things for us:

  • it provides relatively small current consumption for 3.3 and 5.0 VDC logic. This allows many LEDs to be added without the need to worry about overloading the logic circuits.

  • it allows the LED to be used in higher voltage systems. For example, we could use two LEDs to indicate the presence of the voltage rails for a +/- 12 VDC op-amp circuit.

Parting thoughts

This simple idea works. As a former educator, I can tell you that students take to the idea very quickly. They were first introduced to the idea in their digital logic class. The techniques follow them to the microcontroller classes and even clever applications in analog projects.

Remember there are limitation. This is no substitution for a true logic probe or logic analyzer. Yet, it serves its purpose proving a fast easy to use indicator.

Kindly, share your prototyping tips.

Best Wishes,


Figure 3: Clamp use to hold the LED as the resistor is soldered to the cathode leg.

Figure 4: The square-body LED profile is an important design consideration as it allows many such LED assemblies to be placed close together on a breadboard.

Return to the breadboard guide for additional construction tips.

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P.S. LEDs are available with integral resistors. This list contains a selection of 5.0 VDC devices for your consideration. In the picture below you can see the internal resistor inside the LED body of the Lumex LED.

Note that this option is not popular as the internal resistor limits flexibility. It locks in a particular current and resulting brightness. Based on device availability, it is apparent that the majority of circuit designers prefer to use an external resistor. Still, there are specialized application where minimizing the number of BOM (Bill Of Material) line items is important.

As counter example, a LED with an internal resistor would not be usable for application such as multiplexing where increased but pulsed current are important.

Best Wishes,


The internal resistor can be seen inside the LED body.