When it comes to debugging a microcontroller circuit, there aren't a lot of simple options. Since a microcontroller circuit might have multiple things going on at the same time, measuring voltages with a DMM isn't an option. Using an computerized In-Circuit-Debugger solution is expensive and elaborate, and time-consuming to set up. People often resort to connecting banks of LEDs to the circuit, but this also takes time and if wired incorrectly, might mislead you about the issues you are trying to solve.
Enter the Microcontroller Debug Module (MDM). It is a device which simply transplants the microcontroller (MCU) to a separate unit, away from the breadboard. It is connected by a 40-pin ribbon cable to a small PCB which takes the place of the MCU on the breadboard, transposing each pin directly. No circuit modifications are required for the operation of the MDM. On the module are 74LS541 8-bit Line Driver ICs which drives indicator LEDs for each of the 8 bits on each of the data ports on the MCU. This chip delivers current for the LEDs without affecting the pin of the MCU itself. The pin and the breadboard circuit have no idea the chip is there.
My microcontrollers of choice are PIC18 family 28 and 40 pin PICs which I have designed this unit to be pin-compatible for. Your unit can be designed for Arduino Atmel chips or any MCU you wish to use.
This Instructable is mostly intended to inspire ideas and some circuit pieces to help you develop a MDM for your own favorite family of MCU.
Also Please note the schematic and board layout are regarding Revision 0.3, it is far more advanced and has numerous corrections/features added to it, compared to Rev 0.1, which is photographed.