Reed Switch Symbol
Definition: The Reed Switch symbol represents a magnetically operated, hermetically sealed contact with two terminals (A, B), drawn per IEC 60617 as a switch contact enclosed in an elongated capsule (rounded-rectangle envelope) with two overlapping ferromagnetic reed blades inside — the envelope signifying the sealed glass tube and the operating field coming from an external magnet or coil.
Also known as: reed switch, magnetic reed switch, reed contact, hermetic switch, door/window sensor switch, proximity reed, Form A reed switch.
What the Reed Switch symbol means
The reed switch symbol denotes a pair of thin ferromagnetic (nickel-iron) blades sealed in a glass capsule filled with inert gas, whose free ends overlap with a tiny gap. Bring a permanent magnet (or an energised coil) near the capsule and the blades magnetise, attract, and snap together, closing the circuit; remove the field and blade springiness reopens the contact. Because the contacts live in a hermetic atmosphere they never oxidise or collect dust, giving reed switches extraordinary reliability — often specified for 10⁸ to 10⁹ operations at signal levels — with zero standby power, which no semiconductor magnetic sensor can match.
The symbol's envelope matters: an ordinary switch contact drawn without the capsule is mechanically operated, while the capsule announces 'sealed, field-operated device'. Reed switches come in Form A (normally open — by far the most common), Form B (normally closed) and Form C (changeover); the drawn contact arrangement inside the capsule reflects which form is meant. A reed switch wrapped in a coil, drawn together, is precisely a reed relay.
How to identify the Reed Switch symbol
Look for an elongated pill or rounded-rectangle outline with a lead entering each end and two thin overlapping lines meeting (or nearly meeting) in the middle — the reeds. A Form A device shows the blades separated or just touching; Form C adds a third stationary contact for the changeover. Some drawings add a small magnet symbol or the letters 'RS' nearby, and sensor datasheets may instead use the generic proximity-switch symbol with a magnet qualifier.
IEC 60617 treats it as a contact with an envelope and, where needed, the magnetic-operation qualifier; ANSI/IEEE 315 practice is equivalent, and North American alarm-industry drawings often simplify door contacts to a switch symbol labelled 'magnetic contact'. Distinguish it from the Hall-effect sensor (a three-terminal semiconductor block requiring supply voltage) and from a thermal reed/thermostat symbol (temperature qualifier instead of magnet).
Function in a circuit
In circuit terms a reed switch is a perfect two-terminal switch that a magnetic field operates at a distance, through glass, plastic or aluminium. The classic application is the door/window sensor: the reed mounts on the frame wired into the alarm loop, the magnet on the moving door; while the door is closed the field holds the contact closed, and opening the door breaks the loop. Because operation is proximity-based and sealed, the same principle serves in float switches (magnet in the float, reed in the stem), flow meters, anemometers, laptop lid sensors and bicycle speedometers.
Sensitivity is specified in ampere-turns (AT) — the coil excitation needed to close the contact — typically 10–40 AT; lower AT means the switch closes at a greater distance from a given magnet. The strict limits are electrical: reed contacts are small, so exceeding the rated switching power (commonly 10 W) or hot-switching capacitive/inductive loads arcs and welds the blades. Contact protection (series resistor, RC snubber or flyback diode) is required for anything beyond clean resistive signal loads.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 draws sealed magnetically operated contacts as a contact within an envelope; device performance is standardised by IEC 62246 (reed switch specifications and quality assessment). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 uses an equivalent enclosed-contact form; industry practice (EIA/NARM standards, e.g. RS-421-style reed relay specs) governs North American reed switch and reed relay characterisation. |
| Key difference | Both standards depict a contact inside a sealed envelope; differences are notational only. In building-security and floor-plan drawings, North American practice frequently degenerates the symbol to a labelled generic contact ('MC' for magnetic contact), while IEC installation drawings retain the capsule form. The Form A/B/C contact arrangement is drawn the same way in both. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
Typical values
Typical small glass reed switches (e.g. 14 mm body): switching capacity 10 W maximum; maximum switching voltage 100–200 V DC; maximum switching current 0.5 A (carry current up to 1–1.2 A); contact resistance 50–150 mΩ initial; insulation resistance >10¹⁰ Ω; operate sensitivity 10–40 AT; operate time 0.1–0.5 ms (bounce included), release ~0.1 ms; life 10⁸–10⁹ operations at signal load, dropping to ~10⁵–10⁶ at rated load. Larger power reeds handle 100 W/3 A. Operating distance with a typical alarm magnet is roughly 10–25 mm.
Where the Reed Switch symbol is used
- Door and window sensors in burglar-alarm and smart-home security loops (reed on frame, magnet on sash)
- Float switches and liquid-level sensors, where a magnet-bearing float slides past a reed sealed in the stem
- Bicycle speedometers, anemometers and flow meters counting magnet passes per revolution
- Laptop/tablet lid-close detection and appliance interlocks (sealed against dust and moisture)
- Reed relays for low-level signal switching in test equipment, ATE and telecom line circuits
- Position sensing on pneumatic cylinders and proximity detection through non-magnetic housings
Example
In a window-alarm loop, the Reed Switch symbol's Terminal A connects to the alarm panel's zone input and Terminal B to the zone return through a 2.2 kΩ end-of-line resistor; with the window shut, the sash magnet holds the Form A reeds closed and the panel sees the EOL resistance, but sliding the window open removes the field, the blades spring apart, and the open loop triggers the zone — the panel can also distinguish a cut cable (infinite resistance) from a legitimate open.
Key facts
- The symbol is a switch contact drawn inside an elongated sealed capsule; the envelope is what distinguishes a reed switch from an ordinary mechanical switch contact.
- Operation is by external magnetic field — permanent magnet or coil — acting through the glass; no electrical control input exists.
- Contact forms: Form A (normally open, most common), Form B (normally closed), Form C (changeover); a reed plus an operating coil drawn together is a reed relay.
- Hermetic sealing in inert gas gives 10⁸–10⁹ operation lifetimes at signal levels and immunity to dust, moisture and contact oxidation.
- Sensitivity is rated in ampere-turns (AT), typically 10–40 AT; lower AT closes at longer range from a given magnet.
- Electrical limits are small — commonly 10 W, 0.5 A switching, 100–200 V — and exceeding them arcs and welds the thin blades shut.
- Reed switches need zero standby power, unlike Hall-effect sensors, making them ideal for battery-powered door sensors lasting years.
- The glass capsule is fragile: bend the leads with two pliers, never against the glass seal, or the hermetic seal cracks.
Frequently asked questions
How does a reed switch work?
Two thin nickel-iron blades are sealed in a glass tube with their ends overlapping across a small gap. An approaching magnetic field magnetises the blades with opposite poles at the gap, so they attract and snap together, completing the circuit through terminals A and B. Removing the field lets the blades' spring force reopen the gap. Everything happens inside a hermetically sealed, inert-gas-filled capsule, so the contacts never corrode.
What is the difference between a reed switch and a Hall-effect sensor?
Both detect magnets, but a reed switch is a passive mechanical contact — zero supply current, works at any voltage within rating, naturally isolated, but with finite mechanical life and some contact bounce. A Hall sensor is a semiconductor needing a supply (typically 3–24 V), giving unlimited operations, no bounce, and fast, vibration-immune switching, but it consumes standby current and outputs a logic-level signal rather than a dry contact. Battery-powered door sensors favour reeds; automotive and high-speed sensing favours Hall devices.
What do Form A, Form B and Form C mean for reed switches?
They describe the contact arrangement: Form A is SPST normally open (closes when the magnet approaches) — the overwhelming majority of reed switches; Form B is SPST normally closed (opens with the magnet), usually built with a bias magnet; Form C is SPDT changeover, with a common blade that transfers from a normally closed to a normally open stationary contact. Alarm loops typically use Form A held closed by the door magnet, so opening the door opens the loop.
Why did my reed switch fail stuck closed?
Almost always contact welding from switching too much power. Inrush into capacitive loads (long cable runs, LED drivers), inductive kick from relay coils, or simple overload arcs across the tiny blades and micro-welds them. Stay well inside the rated 10 W / 0.5 A, add a series resistor for capacitive loads, a flyback diode across DC coils, or an RC snubber for AC — or switch the load with a transistor/relay and let the reed handle only signal current.
How far away will a magnet operate a reed switch?
Typical alarm-grade combinations operate at 10–25 mm gap, but the real answer depends on the switch's ampere-turn sensitivity, the magnet's strength and orientation, and any surrounding steel. Lower-AT switches and larger magnets extend the range; ferrous mounting surfaces shunt the field and shorten it. Datasheets specify operate/release distances for a reference magnet — always test the actual pairing, and remember release distance is greater than operate distance (hysteresis).
Related symbols
- Float Switch symbol
- Hall Effect Sensor symbol
- Limit Switch NO symbol
- PIR Motion Sensor symbol
- Push Button symbol
- Relay SPDT symbol
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