What Is a Push Button?
A push button is a momentary switch that changes position only while held down. When released, a spring returns it to its former position.
NO and NC Contact
Buttons come in two basic contact types: a Normally Open (NO) button conducts when pressed; a Normally Closed (NC) button breaks the circuit when pressed. Emergency-stop buttons are usually NC; in a fault/break condition the circuit falls to the safe side.
| Type | Idle | Pressed |
|---|---|---|
| NO | Open | Closed (conducts) |
| NC | Closed (conducts) | Open |
Reading With a Microcontroller
When reading a button with an MCU, a pull-up or pull-down resistor is needed so the pin doesn't float when unpressed. Otherwise the input reads random values from noise.
Contact Bounce (Debounce)
When a button is pressed the contacts vibrate for a few milliseconds; the MCU can mistake this for many presses. This "bounce" is removed with a delay in software or an RC filter in hardware.
Where Is It Used?
Reset/start buttons, keyboards, user interfaces, elevator/panel controls and emergency stop.
Guarded and Illuminated Types
Industrial buttons come with a protective cover (mushroom head), latching (push-lock) or illuminated types with a LED inside. Illuminated buttons also serve as a status indicator; red mushroom-head buttons are standard for emergency stop and are usually released by twisting.
Common Mistakes
- No pull resistor: an open input is read wrong from noise.
- Skipping debounce: a single press is mistaken for many.
- Mixing NO/NC: the wrong contact choice is risky in safety circuits.
Frequently Asked Questions
Button or switch?+
Why is NC used?+
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