The reed switch symbol represents a magnetically actuated, hermetically sealed electrical contact. Unlike a standard mechanical toggle or pushbutton, the schematic symbol must convey both the internal contact state (Normally Open, Normally Closed, or Changeover) and the invisible magnetic actuation method. The core of the symbol is a rectangular box (or sometimes a circle) representing the glass envelope, containing two overlapping lines that represent the ferromagnetic metal reeds.
Below is the complete reference table for identifying reed switch symbols across the most common contact configurations, complete with standard designations and real-world component examples.
The Complete Reed Switch Symbol Reference Table
| Contact Form | IEC 60617 Symbol Description | ANSI/IEEE Symbol Description | Common Part Example | Max Load (Resistive) | Typical Application |
|---|---|---|---|---|---|
| Form A (SPST-NO) | Box with two overlapping lines meeting at the center. No diagonal cross. | Box with two overlapping lines; sometimes a diagonal arrow indicating magnetic actuation. | Littelfuse 59001 / MK1 | 1.2A @ 24VDC | Door/window security sensors, ESP32 wake-up pins. |
| Form B (SPST-NC) | Box with overlapping lines, plus a diagonal line crossing the reeds (indicating NC). | Similar to IEC, but the NC indicator is often a small perpendicular bar on the moving reed. | Standex KSK-1C90 | 0.5A @ 120VDC | Tamper switches, safety interlocks (failsafe). |
| Form C (SPDT) | Box with three lines: one common, one NO, one NC. The moving reed pivots between the two. | Box with a changeover contact arrangement; common line enters one side, splits to two on the other. | Meder KSR / SIP-1C | 1.0A @ 24VDC | Fluid level sensors (high/low alarms), motor direction. |
| Latching (Bistable) | Form A or C symbol with a small solid rectangle adjacent to the box (representing a bias magnet). | Form A or C symbol with a 'PM' (Permanent Magnet) notation or a hatched box next to the envelope. | ORD213 / OKI-1B | 0.5A @ 12VDC | Pulse counting, bicycle speedometers, power-saving circuits. |
Never wire a bare glass reed switch directly to 120V/240V AC mains loads. The internal contacts are typically rated for a maximum of 140V AC or 200V DC at very low currents (under 1A). Switching higher voltages will cause internal arcing, welding the contacts shut or shattering the glass envelope. Always use the reed switch to trigger a MOSFET, an optocoupler, or a contactor coil instead.
Regional Standards: IEC vs. ANSI vs. Legacy UK
When reading schematics, the region and era of the drawing dictate how the reed switch is depicted. While the underlying physics remain identical, the visual shorthand varies.
- IEC 60617 (Global / Modern EU): This is the dominant standard today. It strictly uses a rectangular box to represent the glass envelope. The contacts inside are drawn as simple straight lines. It is the cleanest and most universally recognized standard in modern datasheets from manufacturers like Standex Electronics.
- ANSI/IEEE (United States): Historically, US schematics sometimes used a circle to represent the envelope, though modern US engineering firms have largely adopted the IEC rectangular box. You will still see the circular variant in older American HVAC and industrial control prints. ANSI also tends to explicitly draw the magnetic actuation arrow, whereas IEC assumes the box-and-reed combo implies magnetic actuation.
- Legacy BS 3939 (Old UK): If you are troubleshooting a legacy British control panel, the reed switch was often drawn as a circle with a distinct 'petal' or 'leaf' shape inside to represent the flexible reeds. This standard is obsolete but frequently encountered in buildings wired before the 1990s.
Rows People Get Wrong and Schematic Pitfalls
Even experienced makers and electricians misread specific reed switch symbols, leading to incorrect wiring or software logic errors. Here are the most common pitfalls:
- Confusing Form B (NC) with a Pushbutton: The diagonal line crossing the reeds in a Form B symbol looks remarkably similar to the standard symbol for a Normally Closed pushbutton. The fix: Always look for the outer box (the envelope). If there is a box around the contacts, it is magnetically actuated. If there is no box and a mechanical actuator line (a T-bar), it is a pushbutton.
- Missing the Latching Bias Magnet: In a latching reed switch, the small adjacent rectangle (bias magnet) is often drawn faintly or omitted by lazy CAD drafters. If you wire a standard Form A logic circuit to a latching reed switch, your ESP32 or Arduino will fail to register when the magnet is removed, because the contacts remain closed. The fix: Check the bill of materials (BOM) for 'bistable' or 'latching' keywords if the schematic looks ambiguous.
- Reed Switch vs. Reed Relay: A reed switch is actuated by an external magnet. A reed relay has a wire coil wrapped around the glass envelope. The symbol for a reed relay adds a rectangular coil box adjacent to the switch box, connected by a dashed line. Confusing the two will result in you applying 5V logic to a coil expecting a dry contact, or vice versa.
Safe Interpretation When Markings Are Faded or Missing
On older home security panels or legacy industrial equipment, schematic labels fade, and physical switches are often potted in black epoxy or hidden inside plastic housings (like standard Littelfuse magnetic door contacts). When you cannot read the symbol or see the switch, you must empirically determine its form and hysteresis using a digital multimeter (DMM).
Step-by-Step Bench Test:
- Isolate the circuit: De-energize the panel and disconnect the two (or three) wires from the reed switch terminals.
- Set DMM to Continuity: Place probes on the two wires (or the COM and NO/NC terminals if 3 wires are present).
- Test Resting State: Without a magnet nearby, note the reading.
- If it reads < 1 ohm (beeps), it is Normally Closed (Form B).
- If it reads OL (open), it is Normally Open (Form A).
- If you have 3 wires, find the common (COM) by testing pairs until you find one that beeps and one that doesn't.
- Test Actuated State: Bring a strong neodymium magnet within 10mm of the switch housing. The state should invert.
- Measure Hysteresis (Crucial for Alignment): Reed switches exhibit magnetic hysteresis. The distance at which the switch closes (Pull-In, e.g., 12mm) is always closer than the distance at which it opens (Drop-Out, e.g., 18mm). When mounting door sensors, always align the magnet to trigger well within the Pull-In threshold to prevent 'chatter' from wind or door sag.
Pro-Tip for Microcontrollers: Mechanical reed contacts suffer from 'bounce' (typically 0.5ms to 2ms of rapid making/breaking upon closure). If you are wiring a reed switch to an Arduino or ESP32 GPIO pin for an interrupt, do not rely solely on software debouncing. Solder a 0.1µF ceramic capacitor in parallel with the switch, or a 100Ω resistor in series with a 0.1µF cap to ground (RC snubber), to absorb the kinetic bounce and prevent phantom interrupt triggers.
For deeper theoretical background on magnetic flux lines and contact materials (like rhodium vs. ruthenium plating), refer to the comprehensive guides at Electronics Tutorials.
