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I am trying to design a very simple system for my workplace that turns on a warning beacon light when a swinging door is opened. My current thinking is to use a non-contact magnetic safety interlock switch to actuate a relay, which will switch the circuit connected to the warning beacon light.

The image below details my current plan for the circuit design. Links to the specific components are listed below the image. V1 is standard US wall voltage at 120VAC. RLY1 is a general-purpose SPDT relay rated at 10A 120VAC. SW1 is a normally-open magnetic non-contact safety switch with 120VAC operating voltage rated at 3A max. LAMP1 is an industrial high-intensity LED beacon light rated at 90-240VAC with 50mA current consumption.

When the door is open, the magnetic safety switch is not actuated and therefore open, so the relay coil is not energized. Therefore the NC contact of the relay is closed, powering the light (shown in the image).

When the door is closed, the magnetic safety switch is actuated and therefore closed, energizing the coil. Therefore the NC contact of the relay is open, meaning the light is not powered (opposite condition of the image). Circuit including safety switch, relay, and warning light, on wall 120VAC power.

RLY1, general-purpose relay: https://www.galco.com/buy/IDEC/RH1B-UT-AC120

SW1, magnetic safety switch: https://www.galco.com/buy/Edwards-Signaling/111-4Y-06K

LAMP1, warning beacon light: https://www.productsforautomation.com/meltric-ms86trffr-86-red-beacon-light-p/ms86t-rff-r.htm

Is this design feasible? Do I need to add additional components? Is there a more simple approach I am overlooking?

I am worried about putting only the relay coil and the magnetic safety switch in series with wall voltage. I am fairly confident that the other side of the circuit will be fine (wall+120VAC > relayCOM > relayNC > LAMP1 > wallGND).

P.S. if this is a stupid or previously-answered question, please forgive the Mechanical Engineer that asked it. I have been incessantly researching this for the past 12 hours (even digging through various old textbooks) and am at my wit's end.

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  • \$\begingroup\$ This is a very well worded and thought-out question. A great first post. Nice work! \$\endgroup\$
    – mhilden
    Jan 19, 2021 at 17:23

2 Answers 2

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You can make this simpler, using just a switch and the lamp connected in series. Use a magnetic non-contact switch that is normally-closed instead. Presto, no need for a relay. The 50 mA LED lamp current is easily handled by the switch. Do fuse the circuit and follow codes for wiring.

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  • \$\begingroup\$ Thank you for your response Mark! I had considered this approach but have noticed that when looking at product catalogs, certain switch manufacturers once made NC versions of a number of their switches, but I can only find the NO versions in online stores. I also wanted to build the circuit around an NO switch and a relay for added redundancy in case of a switch failure. Is there a way I could do this (turn the lights on if the switch fails) without a relay? For all intents and purposes, your approach for this system will be more than adequate. Thank you for verifying its efficacy. \$\endgroup\$
    – user2441984
    Jan 19, 2021 at 12:19
  • \$\begingroup\$ Increasing reliability is a worthwhile goal. To eliminate the switch as a single point of failure, use two NC switches wired in parallel. The bulb is vulnerable too, so you could use two of them, also in parallel. If you want maximum reliability, install two completely separate circuits, each with its own fuse, switch, and bulb, to eliminate almost all single points of failure. \$\endgroup\$
    – Mark Leavitt
    Jan 19, 2021 at 13:56
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In principle, this will work. There is nothing wrong with applying 120Vac directly to a 120Vac relay coil. Doing anything other than that would be an issue. I would put the door switch on the line side of the relay coil though, so the coil is not “hot” when de-energized. I would also consider fusing the circuit or providing a supplementary circuit breaker, in addition to any required branch circuit protection. Make sure to adhere to local codes.

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  • \$\begingroup\$ the chosen switch has NO and NC contacts so it could be used. the switch is rated for 100VA so use a 750mA fuse, or smaller. \$\endgroup\$
    – Jasen Слава Україні
    Jan 19, 2021 at 5:50

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