Since making the move to online classes, I made a YouTube video showing common mistakes made in digital systems labs:
I also wrote a post about debugging a microcontroller project that I was working on using a real-time clock (RTC) and an Arduino. It’s most relevant to microcontrollers students, but even digital students might gain some insight reading about my debug process. (This one was a doozy and took me several hours to figure out.)
Following are some of the most common errors in setting up circuits on a breadboard. Try checking these issues if you are having difficulty with your circuit.
- Broken pin on IC
- Remove the IC and check that all pins are OK
- Use a logic probe to check the pins and the adjacent row on the breadboard to ensure a connection between the two
- Incorrect IC used (i.e. NOT instead of AND)
- Check the IC number and confirm with a datasheet
- Inserting a jumper wire one row higher or one row lower than it should be
- Shorting pull-down resistors on DIP switch
- Vcc not connected to all DIP switch inputs
- Placing two connections adjacent in a single row of the breadboard
- Floating inputs on an IC (unless specified on the datasheet)
- Connections between IC outputs and Vcc or ground
- Outputs should NEVER be directly connected to Vcc or ground
- Loose connections
- If moving around the wires causes the circuit to work, there is probably a loose connection
- Incorrect resistor values
- Working with TTL devices, all pull-down and current-limiting resistors should be 220 Ω (red-red-brown)
- Otherwise all pull-up and pull-down resistors should be 10 kΩ (brown-black-orange)
- When building designs with many ICs and connections to VCC and GND:
- DO NOT coil up wires into loops!!! (Especially not GND wires!!!)
- Use bypass capacitors on each IC to smooth out the power supply. A bypass cap is a ~0.1 uF ceramic capacitor connected between VCC and GND and placed physically as close to the IC as possible. The figure below shows an example of bypass capacitors used in a circuit design.
- This article about repairing circuit boards gives a lot of tips about troubleshooting and debugging issues with printed circuit boards (PCBs).