Why delay() Makes Your Button Miss Presses (and the Fix)

You press the button and nothing happens. You press it again, harder, and the LED finally changes. The wiring is right, the code compiles, and yet the button feels unreliable. If your sketch has a delay() in loop(), that is very likely the reason, and the fix is small.

Mokxi's Circuit Doctor knows this problem. When a sketch reads a button and also waits for a long time, it shows a tip with this title: "delay(N) in loop() can miss a button press." This post explains what that tip means, shows the bug in a real circuit, and fixes it step by step.

The circuit

You need an Arduino Uno, a pushbutton and an LED with a 220 ohm resistor.

From	To
Button pin 1a	Uno pin 2
Button pin 2a	Uno GND
Uno pin 13	one end of the resistor
Resistor, other end	LED anode (A)
LED cathode (C)	Uno GND

The button connects pin 2 to ground when pressed. The sketch turns on the Uno's internal pull-up with INPUT_PULLUP, so the pin reads HIGH when the button is up and LOW when it is down.

The sketch with the bug

This sketch toggles the LED each time you press the button. It looks fine, and it compiles.

{{SKETCH:p6bad.ino}}

Here is the idea. loop() reads the button, toggles the LED on a new press, remembers the button state, and then waits one second with delay(1000). The program is simple, and the bug is hiding in that last line.

Why it misses presses

A delay() does not make the Uno do anything useful while it waits. The chip just sits there counting. During those 1000 milliseconds, nothing reads pin 2. If you press and release the button inside that window, the sketch never sees it. The pin went low and came back high between two reads.

So the program only looks at the button once a second, like a camera that takes a photo each second and ignores everything in between. A quick tap is a blink the camera missed. A long press, longer than the delay, would be caught, which is why the button seems to work "sometimes".

I checked this in the Mokxi simulator. I held the button for 300 milliseconds, well within the one-second gap, and the LED never changed and nothing was printed. A second, shorter tap of 50 milliseconds was missed as well.

What the Circuit Doctor says

The Doctor checks your code and wiring in the browser. For this sketch it reports a tip on the delay(1000) line: delay(1000) in loop() can miss a button press. Its explanation is that while delay() waits, nothing reads the button, so a press shorter than the delay can be lost, and that the better way is to time things with millis() and to debounce the button by ignoring changes for about 50 ms.

The Doctor only flags it when the sketch calls digitalRead() in loop() and also has a delay() of 200 milliseconds or more there. A delay of 10 milliseconds is short enough that you are unlikely to notice a miss.

The fix: use millis() instead of waiting

millis() returns how many milliseconds have passed since the Uno started. Instead of stopping the program, you check the clock and keep going. The loop runs thousands of times a second, and the button is read every time.

{{SKETCH:p6good.ino}}

Walkthrough, line by line

Line 4 sets the debounce time. Debouncing means ignoring the rapid chatter of a switch. A real button does not close cleanly: its metal contacts bounce, and the pin flickers for a few milliseconds. The Mokxi pushbutton imitates this. Its description says a press chatters for 1 to 5 ms on the typical setting and up to 20 ms on the worst one. Waiting 50 ms is a safe margin.

Lines 6 to 9 are the memory. stableState is the button state we trust. lastReading is what the pin said on the previous loop. lastChange is the time of the last flicker. ledOn is the LED's state.

Lines 11 to 15, setup(), turn on the pull-up, make pin 13 an output, and start the serial monitor.

Line 18 reads the pin. This happens on every pass through loop(), thousands of times a second, because nothing blocks.

Lines 20 to 23 notice a change. If the reading differs from the last one, we store it and note the time. While the contacts chatter, this keeps resetting the clock.

Line 25 is the heart of the fix. millis() - lastChange >= DEBOUNCE_MS asks whether the pin has held still for 50 ms. The second half, reading != stableState, asks whether this is a real change from what we trusted before.

Lines 26 to 31 run once per real press. We accept the new state, and if it is LOW, the button went down, so we flip the LED and print on or off.

The result: the button is checked all the time, a bounce is ignored, and one press makes exactly one toggle. I ran this version with the same 300 ms press and the LED turned on, then a second press turned it off. The Doctor reported nothing for it.

A note on millis() and its types

Keep every time you take from millis() in an unsigned long, as lastChange is on line 8. Using an int is a common mistake. On an Uno an int tops out at 32767, so a time stored in one overflows after about 33 seconds and your timing silently breaks. The Doctor has a separate check for this and says so.

An unsigned long lasts much longer: millis() itself wraps back to zero after about 49 days. The subtraction pattern on line 25 survives that. Because of how unsigned math wraps, millis() - lastChange still gives the right elapsed time across the rollover. A check like millis() > lastChange + 50 does not, which is why the Doctor suggests the subtraction form.

What to try next

1. Add a blinking LED. Blink pin 12 with a second millis() timer, the same way. With delay() you could not do both jobs at once. Now both run together.
2. Count presses. Add a counter and print it with Serial.println().
3. Detect a long press. Record the time when the button goes down and compare it to millis() when it comes up.
4. Break it again. Put a delay(500) back into the good sketch and watch the Doctor's tip return.

For more on the technique, read the blink without delay guide, and the button debounce guide for the bounce side of the story.

Try it in your browser

Open the Arduino Uno simulator, build this circuit, paste the first sketch and press the button quickly. Then switch to the second sketch and press again. Seeing the difference yourself is the best way to remember it. The Uno board page lists the pins.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.