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.

An AC Thermostat

Switch ON/OFF thermostats

Many thermostats consist of two differing metal strips bonded together. Different metals expand at different rates when heated, thus causing the bi-metallic strip to bend, either opening or closing an electrical contact. See How a thermostat works

Another type of thermostat used in home heating systems consists of a bi-metallic spring attached to a glass tube filled with mercury metal connected to a pivot and arm. As the spring bends the mercury will roll to one end of the tube or other making or breaking electrical contact depending on temperature. An on/off switch is connected in series. Click here to see an outside view, click here to see an inside view. These type thermostats could be used to trip a triac gate on/off with a proper series resistor. These are low power devices and won't carry much current. They normally operate at about 24 volts. If used with a 110/220 volt triac circuit, 110/220 volts will be present on the thermostat terminals. (Not good!)

[ My Homepage ] [ Electronics Mainpage ]

Tutorials updated/added August 2009:
• Basic Triacs and SCRs
• Solid State AC Relays with Triacs
• Basic Transistor Driver Circuits for Arduino Micro-Controllers
• Opto-Isolated Transistor Drivers for Arduino Micro-Controllers
• Multi-use demo board built in class.
• Using the ATMEGA168/Arduino with the TA8050 Motor Controller
  Demonstrates use of pulse-width modulation for motor speed control.

• Basic Capacitors
• Basic Diodes and Rectifiers
• Series batteries
• Parallel batteries
• Potato battery
• Thermoelectricity
• Reed Switches
• Bi-Metallic Thermostats

Added February 2009: Using a CdS Photo resistor. How to use photocells and touches on comparators, thermistors, relays, etc. Includes circuits to build and test.

• Voltage Comparator Information And Circuits
• Photocell with a Comparator
• Photocell with relay and Triac
• Basic Thermistor

Arduino demos April 30, 2009:
Using the ATMEGA168/Arduino with a 24LC08 Serial EEPROM
Using the ATMEGA168/Arduino with a DS1307 Real Time Clock
More to come. this will include using the MCP23016 I2C I/O Expander and several simple demo projects for robotics and power control.

Atmega168/Arduino features:
14k flash program storage
1k RAM for program memory
6 PWM outputs
6 A/D inputs
UART and SPI interfaces
2 Hardware interrupts
20 general purpose I/O pins (shared with PWM and Analog pins)
16 MHz RISC microcontroller
Open-source hardware, IDE, bootloader
Easy upgrade to more powerful hardware (Wiring)
Easy to use and learn.