How a magnetic reed switch works.
More pictures of reed switches

A reed switch consists of two or three springy metal reeds having plated, long-life contacts at the tips and encapsulated in a sealed glass tube. The two-reed type has normally open (NO) contacts which close when operated, and the three-reed type is a changeover, i.e. it has a pair of normally open (NO) and a pair of normally closed (NC) contacts. Reed switches are actuated by the field from an external permanent magnet or electromagnet placed in close proximity. This field causes the reeds to become magnetic, the ends are attracted and the contacts either open or close. Removal of the magnetic field allows the springy reeds to restore the contacts to their original state. Reed switches have only a momentary action - their contacts revert back to their original state as soon as the magnetic field diminishes. Ref. Reed Switches by James Johnston

Note that reed switches are fragile, low power devices!

Another reed switch with NO/NC contacts. (SPDT)

Closing the reed switch will cut on the triac. The idea is to use a small low-power switch to control high power devices such as motors or heaters. The danger is here is the high voltage AC is on the switch.

Added July 31, 2011

• Using Hall Effect Switches
• Using Ratiometric Hall Effect Sensors
• Using Hall Effect Sensors with the Arduino-ATMEGA168
• Connecting the Arduino to the TMP37 Centigrade Temperature Sensor

  » About me   » Bristol VA/TN   » E-Mail   » Hobby Electronics   » Arduino Microcontroller

• How to connect batteries in Series/Parallel
• 12AV6 Vacuum Tube AM Radio
• Coils for Highly Selective Crystal Radio
• Adding a Push-Pull Output Stage to a LM386 Audio Amplifier

• Read this safety warning and Disclaimer
• Tutorial: Transistor-Zener Diode Regulator Circuits
• Tricks and Tips for the LM78XX Series Voltage Regulators
• Bi-Polar Power Supplies
• Build a Thermocouple Amplifier
• Build a Potato battery
• Build an Adjustable 0-34 volt power supply with the LM317
• Testing a Diac
• Basic Triacs and SCRs
• Solid State AC Relays with Triacs
• Light Activated Silicon Controlled Rectifier (LASCR)
• Basic Transistor Driver Circuits for Micro-Controllers
• Opto-Isolated Transistor Drivers for Micro-Controllers
• Build a H-Bridge Motor Control with Power MOSFETS
• Series/Parallel batteries
• Using a CdS Photocells
• Voltage Comparator Information And Circuits
• Resistive Humidity Sensors
• Reed Switches
• Basic Power Transformers
• Power Supply Rectification
• Diodes and Rectifiers

You Tube Videos

• Power Supplies Part 1
• Power Supplies Part 2
• Power Supplies Part 3
• Using a Load Lamp
• Using and Testing Hall Effect Sensors