Tutorial - Reed switch

Reed switch was invented in 1936 by Walter B. Ellwood in the Bell Telephone Labs. Reed Switch consists of a pair of ferromagnetic (something as easy to magnetize as iron) flexible metal contacts typically nickel-iron alloy (as they are easy to magnetize and doesn't stay magnetized for long) separated by only a few microns, coated with a hard-wearing metal such as Rhodium or Ruthenium(Rh, Ru, Ir, or W) (to give them a long life as they switch on and off) in a hermetically sealed (airtight) glass envelope (to keep them dust and dirt free). The glass tube contains an inert gas (An inert gas is a gas that does not undergo chemical reactions under a set of given conditions) typically Nitrogen or in the case of high voltage it is just a simple vacuum. 

In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed. Green-colored Infrared-absorbing glass is frequently used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The glass used is of a high electrical resistance and does not contain volatile components such as lead oxide and fluorides which can contaminate the contacts during the sealing operation. The leads of the switch must be handled carefully to prevent breaking the glass envelope. 

When a magnet is brought in close proximity to the contacts, an electro-mechanical force field is generated and the stiff nickle iron blades become magnetically polarized and gets attracted to each other, completing the circuit. When the magnet is removed the switch returns to its open state.

Since the contacts of the Reed Switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch makes them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard.

A Reed Switch has very low resistance when closed, typically as low as 50 milliohms hence a Reed Switch can be said to require zero power to operate it.


Components
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For this tutorial we need:
- Reed Switch
- 220Ω Resistor
- 100Ω Resistor
- LED
- Multi-meter
- Battery
- Breadboard
- Arduino Nano
- Magnets and

- Few Connecting Cables


Demo
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Using a multi-meter I am going to show you how a Reed Switch works. When I bring a magnet close to the switch the multi-meter shows a continuity as the contact touches each other to completing the circuit. When the magnet is removed, the switch returns to its normally open state. 


Types of Reed Switches 
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There are 3 basic types of Reed Switches:
1. Single Pole, Single Throw, Normally Open [SPST-NO]   (normally switched off)
2. Single Pole, Single Throw, Normally Closed [SPST-NC] (normally switched on)
3. Single Pole, Double Throw [SPDT] (one leg is normally closed and one normally open can be used alternate between two circuits)

Although most reed switches have two ferromagnetic contacts, some have one contact that's ferromagnetic and one that's non-magnetic, while some like the original Elwood reed switch have three. They also vary in shapes and sizes.


Connecting Without Arduino
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Lets first test the Reed Switch without an Arduino. Connect a LED in series with the Reed Switch to a battery. When a magnet is brought in close proximity to the contacts, the LED lights up when the nickle-iron blades inside the switch attracts each other, completing the circuit. And, when the magnet is removed the switch returns to its open state and the LED turns off.


Connecting Reed Switch to Arduino
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Now, lets connect the Reed Switch to an Arduino. Connect the LED to the pin 12 of the Arduino. Then connect the Reed Switch to the pin number 13 and ground the other end. We also need a 100ohm pull-up resistor connected to the same pin to allow a controlled flow of current to the digital input pin. If you want, you can also use the internal pull-up resistor of the Arduino for this setup.

The code is very simple. Set the pin number 13 as Reed_PIN and pin number 12 as LED_PIN. In the setup section, set the pin-mode of the Reed_PIN as input and LED_PIN as output. And Finally in the loop section, turn on the LED when the Reed_PIN goes low.

Same as before, when a magnet is brought in close proximity to the contacts, the LED lights up and, when the magnet is removed the switch returns to its open state and the LED turns off.



Reed Relay
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Another widespread use of Reed Switch is in the manufacturing of Reed Relays.
In a Reed Relay the magnetic field is generated by an electrical current flowing through an operating coil which is fitted over "one or more" Reed Switchs. The current flowing in the coil operates the Reed Switch. These coils often have many thousands of turns of very fine wire. When the operating voltage is applied to the coil a magnetic field is generated which in turn closed the switch in the same way the permanent magnet does.

Compared to armature-based relays, Reed Relays can switch much faster, as the moving parts are small and lightweight (although switch bounce is still present). They require very less operating power and have lower contact capacitance. Their current handling capacity is limited but, with appropriate contact materials, they are suitable for "dry" switching applications. They are mechanically simple, offer high operating speed, good performance with very small currents, highly reliable and have long life.

Millions of reed relays were used in telephone exchanges in the year 1970s and 1980s.


Areas of Application
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Just about everywhere you go, you'll find a Reed Switch nearby that’s quietly doing its job. Reed switches are so pervasive that you’re probably never more than a few feet away from one at any given time. Some of their areas of application are in:

1.  Burglar alarm systems for the doors and windows.
2.  Reed switches puts your laptop to sleep/hibernation when the lid is closed
3.  Fluid level sensors/indicator in a tank - a floating magnet is used to activate the switches placed at various levels.
4.  Speed sensors on bicycle wheels/ DC electric motors
5.  In the spinning arms of dishwashers to detect when they jam
6.  They keep your washing machine from running when the lid is open
7.  In the thermal cut-offs in electric showers, to stop the water heating to dangerous levels.
8.  They know if the car has enough brake fluid and whether or not your seat belt is fastened.
9.  Anemometers with rotating cups have reed switches inside that measure the speed of the wind.
10. They are also used in applications which make use of their extremely low leakage of current.
11. Old keyboards, in vehicles, industrial systems, Household appliances, telecommunication, medical appliances, Clamshell phones and more......

On the relays side they are used for automatic cut sequences.



Life
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The mechanical motion of the reeds is below the fatigue limit of the materials, so the reeds do not break due to fatigue. Wear and life are almost entirely dependent on the electrical load's effect on the contacts along with the material of the reed switch. Contact surface wear occurs only when the switch contacts open or close. Because of this, manufacturers rate life in number of operations rather than hours or years. In general, higher voltages and higher currents cause faster wear and shorter life.
The glass envelope extended their life and can be damaged if the reed switch is subjected to mechanical stress. They’re cheap, they’re durable, and in low-current applications, depending on the electrical load, they can last for about a billion actuation.




Thanks
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