When reading security system wiring diagrams, industrial limit switch schematics, or PCB layouts, identifying the correct reed switch symbol is critical for determining whether the circuit is normally open (NO), normally closed (NC), or changeover (SPDT). Because reed switches are magnetically actuated hermetically sealed components, their schematic representations differ distinctly from standard mechanical toggle or pushbutton switches. Below is the master reference table mapping standard schematic symbols to physical pinouts across major international standards.
| Switch Type | IEC 60617 Symbol Description | ANSI/IEEE Symbol Description | Physical Pinout (Standard Glass/SIP) | Common Application |
|---|---|---|---|---|
| SPST-NO (Form A) | Two parallel lines with a gap, enclosed in a rectangular box (denoting the glass envelope). | Two contact points with a gap, enclosed in a box; sometimes a diagonal arrow indicates magnetic actuation. | 2 Pins. Polarity insensitive. Pins act as a simple open circuit until magnetic field closes them. | Door/window security alarms, laptop lid sleep sensors. |
| SPST-NC (Form B) | Two parallel lines touching/overlapping, enclosed in a rectangular box. | Two contact points with a diagonal slash crossing the gap, enclosed in a box. | 2 Pins. Requires a bias magnet to hold open; removes field to close. (Rare in standard glass, common in molded). | Tamper switches, safety interlocks where a broken wire must trigger an alarm. |
| SPDT (Form C) | Three lines: one common wiper touching the NC line, with a gap to the NO line, all in a box. | Three contact points: wiper rests on one, with a gap to the other, enclosed in a box. | 3 Pins. Center pin is Common (COM). Outer pins are NO and NC. Requires a bias magnet for NC state. | Direction sensing, dual-circuit alarms, fluid level changeover. |
| Latching (Bistable) | Form A symbol with a small permanent magnet symbol or double-headed arrow adjacent to the box. | Form A symbol with a 'box-in-box' or specific latching indicator notation. | 2 or 3 Pins. Contains a bias magnet. Requires a specific polarity magnet to set, and reverse polarity to reset. | Pneumatic cylinder position tracking, pulse counting. |
| Mercury-Wetted | Form A or SPDT symbol with a small droplet or pool indicator inside the envelope box. | Similar to IEC, often with a distinct mercury hazard or tilt-switch notation. | Multi-pin. Must be mounted vertically. Contains a drop of mercury to bridge contacts. | High-reliability aerospace, high-current legacy industrial (largely obsolete due to RoHS). |
Decoding the Rows: What Each Symbol Means in Practice
The most critical distinction in reed switch schematics is the envelope box. Unlike a standard mechanical switch, which is drawn as bare contacts, a reed switch symbol almost always encloses the contacts within a rectangular box. This box represents the hermetically sealed glass or metal tube, usually filled with nitrogen or a noble gas to prevent contact oxidation. If you see a switch symbol without this envelope box, you are likely looking at a standard mechanical limit switch, not a reed switch.
Form A (Normally Open): This is the default state for 90% of commercial reed switches (like the standard Meder MK15 or Littelfuse 59177 series). The contacts remain open until a magnetic field of sufficient Ampere-Turns (AT) pulls the ferromagnetic nickel-iron blades together. In security wiring, Form A is used for 'closed-loop' alarm zones where the panel expects a continuous short circuit when the door is closed.
Form B (Normally Closed): True Form B glass reed switches are exceptionally rare because they require a permanent bias magnet built into the assembly to hold the contacts apart. More commonly, a Form B function is achieved in a PCB design by using a Form A reed switch in conjunction with a solid-state inverter or a pull-up/pull-down resistor network. If you see a Form B symbol on an alarm schematic, verify whether it represents a physical NC reed switch or a logically inverted NO switch.
Form C (Changeover / SPDT): This requires a 3-pin physical component. The internal wiper rests against the NC contact by default (often held by a tiny bias magnet) and swings to the NO contact when an external magnet approaches.
Rows People Get Wrong
- The Latching Symbol Confusion: Engineers frequently mistake the latching (bistable) reed switch symbol for a standard latching relay. A latching reed switch does not have a coil; it relies on a built-in permanent magnet. You must use a specific magnetic polarity (North vs. South pole) to toggle it. Applying a standard electromagnet coil without checking the datasheet for the required magnetic polarity will result in a switch that only sets, but never resets.
- Form C Common Pin Mapping: On a schematic, the 'Common' wiper is drawn in the center. However, on a physical 3-pin SIP (Single In-line Package) reed switch, the Common pin is almost always the middle physical lead. Wiring the NO or NC pin as the common feed will result in a circuit that never completes or is permanently shorted, depending on the magnet's position.
- Envelope Box vs. Relay Coil: In dense IEC schematics, the rectangular box around the reed contacts is sometimes mistakenly identified as a relay coil block. Remember: a coil is drawn as a series of semicircles (ANSI) or a rectangle with a diagonal line (IEC). A plain rectangle around switch contacts specifically denotes the reed glass envelope.
Regional Standard Variants: IEC vs. ANSI vs. Old UK
If you are troubleshooting legacy equipment or working with imported machinery, the schematic standard dictates how the reed switch is drawn. Misinterpreting a regional variant can lead to incorrect wiring of safety interlocks.
| Standard | Region / Dominance | Visual Hallmark for Reed Switches | Magnetic Actuation Indicator |
|---|---|---|---|
| IEC 60617 | Europe, Global, Modern UK, AU | Contacts enclosed in a strict rectangular box. Lines are perfectly orthogonal. | Often omitted, or shown as a small 'm' or magnet symbol outside the box. |
| ANSI/IEEE (Y32.2) | United States, Canada, Japan | Contacts enclosed in a box, but lines may have slight angles. NC uses a distinct diagonal slash. | Frequently shown with a diamond or a specific looped line pointing at the box. |
| BS 3939 (Legacy) | Old UK (Pre-1980s equipment) | Often lacks the envelope box entirely; relies on a specific 'reed' notation text tag (e.g., 'RS1'). | Indicated by a dotted line connecting the switch to the actuating solenoid or magnet. |
According to the Electronics Tutorials reference on reed switches, modern global manufacturing has largely consolidated around IEC 60617 for PCB design software (like Altium or KiCad), but ANSI remains heavily prevalent in US-based industrial control panel wiring diagrams and NFPA 79 compliant schematics. If you are working on a UK legacy alarm panel (e.g., an old Texecom or Scantronic board), expect to see the older BS-style textual tags rather than graphical envelope boxes.
Physical Pinouts and Wiring the Unmarked Glass Tube
Bare glass reed switches, such as the widely used 14mm OKI ORT551 or the Littelfuse 59024, are often sold in bulk without clear markings. The painted bands on the glass (which sometimes indicate the Ampere-Turn sensitivity or the NO/NC state) frequently fade, chip, or are omitted entirely by OEM suppliers. When you have an unmarked glass tube and need to integrate it into a circuit, you must map it manually.
Never bend the leads of a bare glass reed switch closer than 3mm to the glass body. The GTM seal is highly sensitive to mechanical stress. Bending the leads at the base will cause micro-fractures in the glass, breaking the hermetic seal. This allows the internal shield gas to escape and oxygen to enter, leading to rapid contact oxidation, increased contact resistance, and eventual failure. Always use a lead-forming tool or hold the wire with needle-nose pliers at the 3mm mark while bending.
Step-by-Step Multimeter Mapping for Unmarked Switches:
- Set your DMM to Continuity/Resistance mode. Ensure the meter reads 'OL' (Open Loop) or infinite resistance when the probes are apart.
- Test 2-Pin (Form A) Switches: Touch the probes to both leads. The meter should read 'OL'. Bring a small neodymium magnet (e.g., an N42 grade 10mm disc) within 10mm of the glass center. The meter should beep or read < 0.1 ohms (typical for Littelfuse and Standex rhodium or ruthenium plated contacts). Remove the magnet; it must return to 'OL' instantly. If it sticks, the glass is cracked or the contacts are welded from a previous over-current event.
- Test 3-Pin (Form C) Switches: Identify the three physical leads. Label them 1, 2, and 3. Test continuity between 1-2, 2-3, and 1-3 without the magnet. One pair will show continuity (this is the Common and NC pair). The Common pin is the one shared between the continuity pair and the open pair.
- Verify with Magnet: Hold the magnet to the glass. The previously open pair should now show continuity, and the previously closed pair should go to 'OL'.
Understanding Ampere-Turns (AT) Sensitivity: When replacing a faded reed switch, physical size is not the only metric. You must match the AT rating. A 10-15 AT switch is highly sensitive and will close with a magnet from 20mm away. A 20-30 AT switch requires the magnet to be much closer (typically < 10mm). If you replace a 15 AT door sensor switch with a 30 AT switch, the door will have to be nearly slammed shut to trigger the alarm, causing intermittent 'fault' codes on the security panel. Always consult the Standex Electronics reed switch definitions to match the AT pull-in and drop-out thresholds to your specific magnet geometry.
