Choosing a machine safety sensor usually starts with one question: how should the guard open before you let the machine run again? If the guard is tied to a rotating or moving part, a stop motion detector with an interlock release is often the direct answer. If the guard position just needs to be monitored without contact, a magnetic or non-contact safety sensor may be the better fit. The four units here span from $100 to $218.47, and the right pick depends on how many release paths you need, whether you prefer new or used condition, and what output your control circuit expects.
Quick take
- The KEYENCE GS-M51P is the non-contact interlock switch in this group.
- The Schmersal BNS 33-12ZG is the magnetic sensor option here.
- The 4-Way stop motion detector carries one more release path than the 3-Way.
- The 3-Way stop motion detector is the lower-priced prewired detector in the same family.
Listed price comparison
| Product | Listed Price | Price Bar | Condition |
|---|---|---|---|
| Prewired Stop Motion Detector Safety Machine 3 Way Interlock Release | USD 185.52 | Used | |
| Prewired Stop Motion Detector Safety Machine 4 Way Interlock Release | USD 218.47 | Used | |
| 1PC BNS 33-12ZG Schmersal Magnetic Safety Sensor | USD 153.00 | New | |
| Keyence GS-M51P Safety Interlock Switch | USD 100.00 | Used |
Decision matrix
| If you need... | Best fit | Why |
|---|---|---|
| A prewired stop motion detector with three release paths | Prewired Stop Motion Detector Safety Machine 3 Way Interlock Release | Prewired and built around a three-way interlock release |
| One more release path than the 3-way | Prewired Stop Motion Detector Safety Machine 4 Way Interlock Release | Four-way interlock release in the same prewired detector family |
| A new-condition magnetic sensor | 1PC BNS 33-12ZG Schmersal Magnetic Safety Sensor | New unit with the BNS 33-12ZG model code |
| A non-contact interlock switch for a DC digital circuit | Keyence GS-M51P Safety Interlock Switch | Non-contact design with DC current and digital output |
Product notes
Prewired Stop Motion Detector Safety Machine 3 Way Interlock Release
This detector is the straightforward choice when your guarding setup is built around a three-way interlock release. It arrives prewired, so you are not adding separate connector hardware before wiring it into a stop-motion circuit. The used condition and US manufacturing origin are clearly identified, and the unit shares a 25-inch width and 8-inch height with the 4-Way version. If you only need three release paths, this is the lower-priced way into that prewired detector family. The obvious limitation is the used condition, meaning the prior service history is not a new-sensor guarantee. Directly compare it with the 4-Way model if you think one more release path will be needed later.
Prewired Stop Motion Detector Safety Machine 4 Way Interlock Release
If your machine guard needs to be released from four positions, this is the one to weigh against the 3-Way version. Like the 3-Way detector, it carries Allen Bradley model information, and it shares the same 25-inch width and 8-inch height as that unit. The prewired configuration plus the four-way release path adds an extra release option within the same detector family. At the top of the price range, though, you are paying for an extra release path and used condition rather than for new-condition hardware. That makes the 4-Way less attractive if your current machine never uses more than three release points. Choose it only when the fourth path is a real part of your guard layout.
1PC BNS 33-12ZG Schmersal Magnetic Safety Sensor
This is the only new-condition sensor in the group, and it is built around the BNS 33-12ZG model code. The magnetic sensor design is aimed at machine safety control, and the title identifies it as a Schmersal unit. At the price, it lands below the two prewired stop motion detectors, which makes it a sensible choice if you want new hardware rather than used. The main thing to keep in mind is quantity: the title starts with 1PC, so this is a single sensor. A machine with multiple guard doors or monitored positions will need one sensor per position. The main advantage is the new condition, but the per-position quantity is the limitation to plan for.
Keyence GS-M51P Safety Interlock Switch
The KEYENCE GS-M51P is the lowest-priced unit in this set and the non-contact interlock option to check first. The title says non-contact safety sensor, and the unit includes DC current, digital output, and the GS-M51P model number. It is also marked as made in Japan and used, having been removed from a working automation system. For a control circuit that expects a DC digital signal, this is a direct match among the four. The limitation is the used condition; you are getting an interlock switch with service history rather than a factory-fresh unit. If the non-contact function and the low price are what matter, start here and pair it with your existing automation controller.
Final recommendation
For a replacement in a non-contact guard monitoring circuit, the KEYENCE GS-M51P gives you a direct way into a DC digital interlock switch. If you prefer a magnetic sensor, the Schmersal BNS 33-12ZG covers that use case. For existing stop-motion detectors, choose the 3-Way Prewired Stop Motion Detector when three release paths are enough, and step up to the 4-Way version only when the fourth path is required. Match the release paths and output type to what your machine already uses.
