What is Reed switch and reed switch working

Reed Switch or reed contact is an electrical switch which closed by external magnetic field. Reed switch composed of two ferromagnetic flexible metal strips enclosed in glass or plastic envelop with inert gas filled. Reed switches can carry small current and limited to low contact voltages. However, these are much more long life than normal electromechanical relays. Reed switch also called magnetic switch or magnetic reed switch. Types of Magnetic switches are described as under.

Reed Switch Applications

reed switch as sensors

When an electrical status of a magnetic reed switch is used as input signal instead of power switching, then it is called reed sensor. Some common applications of reed switch sensor are described here.
Reed switch sensor open or close circuit upon application of external magnetic field. This is low-cost solution to sense nearby magnet while no electrical connection between moving parts and switch circuit. Here are some magnetic reed sensor applications

Water tank level sensor

A magnet tied with floating ball which moves around vertical axis with water or liquid level. At certain level fixed reed switch closes upon arrival of floating ball containing a magnet. Thus, reed float switch works to sense water tank level. Further logic circuit energizes magnetic contactors to start or stop motors and valves. This system is called magnetic float sensor for water level controller. Level water sensor also use for water tank level indicator, using multiple reed sensors fixed on different levels. Logical circuit indicate water level by LED’s or numeric display according to reed sensors outputs.

Water flow sensor, water flow switch

In instant water geysers, a small magnet moves with water flow in copper pipe and return back upon no water flow due to its weight. A reed switch at outer side of copper pipe operates when magnet move towards reed switch with water flow. Geyser controller circuit starts heating when this water flow reed switch is closed. And stop heating when reed switch is open. This flow sensing method is useful in high pressure and high temperature pipelines, due to no special assembly requirement for sensor.

Magnetic proximity/position sensor

Reed switches often used as magnetic proximity sensor and magnetic limit switch. For example, conventional snap switch or snap action switch and limit switches are replaced with magnetic reed switches and optical limit sensors in CNC/CMM machines, 3D printers, LASER cutter, printers etc. In industry sometimes safety door or cover position, conveyer belt or Rotary table position, detected by reed switches with an external magnetic field interrupt.

Door Opening alarm

For Kids security and safety door opening alarm is considerable option. For example, water pool, garage door alarm is often use to restrict kids and toddlers visit alone to outside or water pool.
To avoid any incident Door Opening alarm is used simply made with normally closed reed switch and a magnet attached with door. Reed switch through a supply voltage to alarm-bell. Thus, magnetic door alarm works without any complex circuit.

Reed switch types

NO reed switches

Normally open when no external magnetic input. It also called Form A Reed switch. It consists of two ferrous strips enclosed in glass capsule with inert gas.

NO reed switch symbol

NC reed switch

Normally closed reed switch passes current when no external magnetic input applied, and turns open when external magnetic field is applied. Normally closed reed switches are constructed with a small magnet attached which keeps reed switch closed. Whenever external reverse magnetic field is applied reed switch goes open circuit. another type of NC reed switch is three pin SPDT reed switch. SPDT reed switch is not requires internal magnet, because NC pin contact is non-ferrous and attached with common pole blade by spring tension of blade itself. So SPDT does not require internal magnet and also not polarity requirements

NC reed Switch symbol

MEMS Reed Switches

Micro reed switch or MEMS reed switches are manufactured using photolithography semiconductor fabrication process and micro-electroplating for micro metal object growth. In micro reed switch Rhodium and Ruthenium metal are used as magnetic blade. Ruthenium is more suitable as it can bear more contact current. Following photo shows Coto Technology® MEMS reed switch structure with 1500*200*25-micron Ruthenium blade can carry 100mA. With 4 microns contact gap its breakdown voltages are 200. And contact resistance is low as 3Ω. Expected Life span is 125million operations.

Reed switch Parameters

Reed switch important characteristics are these.

Operate and release range in Tesla

Magnetic flux thresh-hold value is required to operate and hold reed switch. Less than thresh-hold value reed switch blade will release to normal position.

Operate and release time

Time required to operate and release reed switch blades upon magnetic flux application or removal. Bouncing also occurs when reed switch blades are going to close circuit position.

Contact Voltage

Due to small contact gap, reed switch voltage limited up to 250V. Although some specially designed for high voltage reed switches are commercially available. For example, HYR-1559 (by Aleph, USA) is high voltage reed switch, its switching voltages are 500VDC and breakdown voltages are 1500VDC.

Switching Current

Contact current carrying capacity of reed switch without melting or spot-weld of reed switch blades. Normally reed switches are available with 100mW to 3A switching current. High current reed switches are available with up to 150W power switching capability.

Contact resistance

Contact resistance of reed switch is important parameter for power dissipation and signal path impedance calculations. Contact resistance is measured at typical magnetic flux input described in datasheet.

Expected Life

Expected life of a reed switch is explained by number of successful ON/OFF operations. Commercially available Reed switches offer more than 100million operations life.

Shock and Vibration Resistance

These parameters define bearable range of shock and vibration in which a reed switch works fine. MEMS based reed switch can works up to 1000G shock.

Reed switch & Arduino

Here is explained that how to connect reed switch with an Arduino board. With program example

int LED_PIN = 14; // LED pin Arduino UNO pin 14 as output
int reedswitch = 2; //reed sensor pin Arduino UNO pin 2
int reedstatus; // reed switch value

void setup() 
{
  pinMode (LED_PIN, OUTPUT);
  pinMode (reedswitch, INPUT);
}

void loop()
{
  reedstatus = digitalRead(reedswitch); // read the value of digital interface 2 and assign it to reedstatus
  
  if (reedstatus == HIGH) // LED glow when a magnet is placed near the reed switch
  {
    digitalWrite(LED_PIN, HIGH);
  }
  else 
  {
    digitalWrite(LED_PIN, LOW);
  }
}