Four Biggest 555 Timer Circuit Mistakes

“Why isn’t my 555 timer circuit working?!” Avoid these four common pitfalls!


555 timer mistakes

You’ve found a cool 555 timer project online or in my book, procured all of the parts, wired them up on a breadboard, and are now ready to power up your newly made creation. Time for the moment of glory. You slide the power switch on and… nothing, it’s not working how you expected, or worst case scenario – the magic smoke escapes one of your parts.

It’s happened to the best of us and will continue to do so. These are the four most common errors I’ve run in to in my 555 circuits, and are common in most other breadboard projects.

Wiring/Placement Mistake

I would say this is the most common cause for error that I have experienced, especially with breadboard circuits. All it takes is for one wire or part to be off by one hole on the breadboard, or you have a particular component flipped around backward, and your circuit will probably be rendered inoperable.

Solution: Double, even triple-check your wiring. Ensure all chips and polarized components are placed in the correct orientation. Methodically follow the current flow of the schematic in comparison to your breadboard circuit.

Wrong Component

Almost a tie for the most common problem I’ve experienced is using the wrong component. An incorrect value of a resistor, capacitor, etc., is easy to do with small, sometimes cryptic labels on parts.

Solution: Similar to the wiring mistakes, carefully check each component label to make sure it’s the correct value.

Low Battery Voltage

Your circuit works perfectly once, but not the second time you test it, even without changing anything. The timing is off, LEDs are barely lighting, what the heck is going on? After checking many iterations of a circuit or leaving it on for a while, that 9-volt battery may only be at 7 volts, and things may start to get a bit wonky. The 555 is fairly immune to supply voltage differences. However, when a battery nears the lower end of its capacity, the increase in internal resistance of the dying battery reduces the available current to the point of rendering your circuit or device useless.

Solution: If you notice any of the issues mentioned above, fire up your trusty multimeter and make sure the battery voltage for your circuit is at an acceptable level.

Capacitor Leakage

After thoroughly troubleshooting every tip mentioned above, the timing cycle of your project “hangs.” An LED should turn on and then off in monostable mode but instead stays on, or your astable mode circuit doesn’t oscillate from one state to the next, etc.

If your circuit uses a large value timing capacitor (usually for longer timing periods), the culprit of the behavior described above could be a leaky capacitor. What is capacitor leakage? When basic electronics are taught using the water (hydraulics) analogy method, a capacitor is usually represented by a pipe with a perfectly sealed flexible rubber diaphragm in the middle that prevents water from passing through.

To use that analogy to describe capacitor leakage, the diaphragm isn’t perfectly sealed, and a small amount of water (current) does leak through. Large electrolytic capacitors are known for having a higher leakage current than most other types. This can cause issues for time-sensitive circuits.

Capacitor Leakage

Another way to visualize leakage current is in the diagram to the right. Imagine a resistor in parallel with the capacitor allowing a small amount of current to pass.

I have had a brand new 50-volt, 1 uF electrolytic capacitor by one manufacturer only charge up to 2 volts when 9 volts were applied due to high leakage current. All of my other caps by different manufacturers with the same ratings worked fine. It took me a while to figure out what was going on.

Solution: For more critical timing applications tantalum or Mylar and other film type caps usually work best. If you have to use electrolytic caps, at least use low tolerance, high-quality ones. You can usually find the leakage current specs on the manufacturer’s data sheet.

Wrapping Up

These are the most common errors I’ve run into while working with breadboard circuits. How about you? What problems do you tend to find in your prototype projects? Give everyone a heads up of what to look out for in the comments below!

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