The moment comes for everyone: you finish soldering and power it up, and nothing happens, or it behaves strangely. If you fidget with this and that out of impatience, the problem gets worse. If you set an order and check one thing at a time, most problems turn out to come from a few common causes. This article covers a checking order split into before and after power-up, how to use a multimeter, and a table of causes by symptom.
Step 0: Turn the power off and calm down
If anything seems wrong, unplug the power first. If an IC gets hot or you smell burning, switch it off right away. Touching parts with power on risks shock and shorts, and the longer an abnormal state lasts, the more parts are ruined.
Step 1: Look with your eyes (power off)
With bright light and a magnifier, slowly scan the solder side.
- Is there any place where neighboring pads are joined by solder (a bridge)?
- Is there any cold joint with a rough or bead-shaped surface (Soldering Basics)?
- Do any part leads touch a neighboring pad or wiring?
- Is the orientation of every IC and polarized part (LEDs, electrolytic capacitors, diodes) the same as the layout diagram?
- Is any wiring or soldering missing?
Keep the diagram you drew in Patterndo beside you and check it line by line. Drawing a line over each connection on the diagram with a colored pencil as you check it is effective.
Step 2: Check for shorts with a continuity test (power off)
Set the multimeter to continuity (beep) mode and check the following. How to use it is in How to Choose a Multimeter and Basic Measurement Order.
- Between +V and GND: it must not beep. If it does, there is a short, so do not apply power. Suspect a bridge near the power rail, a part inserted backwards, or an electrolytic capacitor with reversed polarity. (Seeing a brief low resistance while a large capacitor charges is normal.)
- Between neighboring signal lines: if they beep to each other, it is a short.
- Between pins that should be the same: for example, check that an IC's VCC pin is connected to the +V rail.
Step 3: Power up and measure voltages
If possible, use a bench supply with current limiting, or put a fuse or a small resistor in series with the power line for the first power-up (Choosing a Bench Power Supply). After applying power, check in this order.
- Voltage at the power entry: measure whether it is the value you expect (5 V, for example).
- Voltage at each IC's VCC pin: measure directly at the IC pin. If the rail is normal but the IC pin is 0 V, the wiring between them is broken.
- Check for heat: touch ICs, regulators, and resistors very briefly with a fingertip. If something is hot, cut the power at once.
- Power LED and current draw: check that the power indicator LED lights and whether the current shown on the current-limited supply is larger than expected.
Step 4: Follow the signal
If the power is fine, follow the signal one stage at a time from input to output.
- Measure whether the input pin voltage changes (High ↔ Low) when you press the switch.
- Check the input and output voltages of each IC to find at which stage it differs from what you expect. The section where the problem appears is where the cause is.
- Do not try to look at everything at once; check by halving (if it is normal at the halfway point, look at the later part; if not, the earlier part).
Table of causes by symptom
| Symptom | Possible causes | How to check |
|---|---|---|
| Power LED does not light | Bad power connection, LED reversed, resistor too large | Measure the +V voltage, check LED polarity |
| Gets hot soon after power-up | Short, IC backwards, electrolytic capacitor reversed | Power off and do a continuity test, recheck orientation |
| Only one IC is hot | That IC is backwards or its outputs conflict | Check the notch and pin 1 |
| LED does not light | Reversed polarity, resistor too large, broken current path | Check the LED in diode mode, measure voltages |
| Occasionally stops or resets | Cold joint, bad contact, supply noise, reset pin left open | Recheck solder, add 100 nF, handle the reset pin |
| Digits display strangely | Segment wiring order mix-up, confusing common cathode and anode | Check against the FND pin table |
| No response when pressing the switch | Wrong tact switch pins, missing pull-up | Use diagonal pins, check the pull-up |
| Blink rate differs | Electrolytic capacitor tolerance | Measure the value, trim with a potentiometer |
| Works in reverse | Mixing up pull-up and pull-down, logic mistake in circuit | Compare against the schematic |
Top 5 frequent mistakes
- Plugging the IC in backwards (Reading DIP IC Pin Numbers)
- A missing ground connection or missing shared ground
- Cold joints and bridges (Soldering Basics)
- Mistakes in the orientation of polarized parts (Capacitor Types and Polarity)
- Leaving input pins floating (Pull-Up and Pull-Down Resistors)
Replacing a suspect part
For a suspect IC, the fastest way is to pull it from the socket and replace it with a known good part. If you used a socket, this takes a few seconds. When you need to remove a soldered part, see Desoldering and Rework.
A habit of keeping notes
Briefly writing down what you checked and the result keeps you from checking the same place repeatedly and helps a great deal when you meet a similar problem later. For a design made in Patterndo, resolving the warnings in the diagnostics panel first also helps a lot (Reading Diagnostics Messages).
Summary
Keep to this order: power off → look with your eyes → check for shorts with a continuity test → power up and measure voltages → follow the signal. Most problems are one of orientation mistakes, cold joints or bridges, missing ground, or polarity mix-ups. We have collected more common mistakes, so see also The 10 Mistakes Electronics Beginners Make Most Often.