50. Door Reed Switch Selection

A switch is an electromechanical device that opens or closes an electrical circuit, controlling current flow.
Think of it as a manual gate for electricity — ON means current flows, OFF means it doesn’t.

Switches are the primary user interface in many systems, from a power ON/OFF button to a microcontroller reset pin.
 They provide:

• User input (mode select, menu navigation)
• Control (power enable/disable)
• Maintenance functions (reset, programming mode)
• Safety interlocks (door sensors, emergency stop)

Practical note: In embedded products, switches often carry logic-level signals to MCUs, but sometimes handle power switching too, which changes their rating requirements.

• Contacts: Metal surfaces that touch to close the circuit.
• Actuator: The part you press, toggle, or slide.
• Housing: Protects the mechanism.

Switch Operation Types	Image	Description
Momentary		Works only while pressed (e.g., reset switch).
Maintained (latching)		Stays in the last position until changed (e.g., light switch).

1) By Contact Configuration

2) By Actuation

Type	Image	Description
Tactile Switch (e.g., Omron B3F)		Compact, momentary, SMD or THT, used for menu buttons.
Pushbutton		Larger, panel-mount, momentary or maintained.
Toggle Switch		Lever-type, maintained, for power/mode selection.
Rocker Switch		Common in mains power panels.
Slide Switch		Mode selection in small devices.
DIP Switch		Sets configuration bits, replaces jumpers.
Rotary Switch		Multiple position selector (e.g., fan speed control).
Reed Switch		Magnetic actuation, used in door sensors.
Membrane Switch		Flat, sealed, used in keypads.
Microswitch		Snap-action, high durability (mouse buttons, limit switches).

3) By Mounting

Type	Image	Description
Through Hole Mount		Durable, replaceable, suited for power circuits
Surface Mount		Compact, reflow-solderable, for automated assembly.
Panel Mount		Designed for user-facing controls.

Electrical:

• Contact Rating – Max voltage & current (e.g., 50mA @ 12VDC for a tactile switch).
• Contact Resistance – Lower is better (<100 mΩ typical).
• Insulation Resistance – Typically >100 MΩ.
• Bounce Time – Delay before stable contact (1–20 ms).

Mechanical:

• Operating Force – Force needed to actuate (e.g., 160g for Omron B3F tactile).
• Travel Distance – How far the actuator moves before closing contact.
• Life Expectancy – Measured in cycles (e.g., 1M actuations for tactile switches).

Example FSMJ series Tactile Switch specifications:

• Choosing a low-current contact for power switching → overheating.
• Ignoring debounce → erratic readings.
• Using a non-IP-rated switch in an outdoor environment → corrosion failure.
• Poor soldering on THT switches → intermittent contact.

• Pad size & hole diameter for THT switches — follow datasheet.
• Keep-out zones for actuator clearance.
• Reinforce mounting pads for high-force switches.
• Thermal reliefs for large copper areas.

Switch Press and Release Oscilloscope Waveform:

• Panel-mounted switches can act as antenna points for ESD.
• Add TVS diodes to MCU lines.
• Use series resistors or ferrite beads for filtering.
• If it's a metal housing, ground it.

• Hardware: RC filter, Schmitt trigger buffer.
• Software: Ignore state changes for a set period (10–50 ms).
• Measurement Tip: Use a scope to capture bounce waveform.

1. Define the electrical rating needed.
2. Pick mounting style (THT/SMD/panel).
3. Choose momentary vs maintained.
4. Check the IP rating for the environment.
5. Confirm mechanical life.

Verify actuator accessibility in the enclosure.