What's the most reliable way to check if a door of a cabinet is closed? - Page 1
Forum section: electronics / projects

author: redgear
author_rank: Frequent Contributor
timestamp: June 24, 2020, 09:23:07 am
content: Hi all,I am building a UVC Sanitizer Cabinet. Since, UVC rays are very harmful for human beings, I want to restrict the circuit to work only when the door of the cabinet is closed. What's the most reliable way to do it? I thought of using two contact switches in series such that they are ON only when the door's closed. I would like to know about other reliable techniques that I can use.Thanks

author: sahko123
author_rank: Frequent Contributor
timestamp: June 24, 2020, 09:46:26 am
content: Large microswitches generally have screw mounting holes to place on the door in place where it can only be activated if the door is closed. Then connect the switch to power or to a relay capable of handling power

author: Gyro
author_rank: Super Contributor
timestamp: June 24, 2020, 10:07:13 am
content: It would be better to keep all of your UVC cabinet stuff in the same thread - people have more context and you have less explaining to do.

author: MarkR42
author_rank: Regular Contributor
timestamp: June 24, 2020, 10:57:26 am
content: Depends who you are trying to defend against.It's easy to put a switch (or several switches) to defend against accidental mistakes, much harder against a determined abuser.If you're looking for a non-contact solution maybe hall effect sensors and small magnets, at least two of them (to prevent it instantly turning on if the user accidentally brings a magnet close to one sensor)But to defend against a really determined abuser, I think a mechanical solution, like an odd-shaped pin going into a hole with a microswitch, would be better.

author: redgear
author_rank: Frequent Contributor
timestamp: June 24, 2020, 11:07:47 am
content: Quote from: sahko123 on June 24, 2020, 09:46:26 amThanks, connecting to a relay would be a better option if I am not wrong, since I can get a low power rated switch. Also, I would have integrate this with a timer relay, so connecting the switch to a relay seems like the correct choice.Quote from: Gyro on June 24, 2020, 10:07:13 amAlright, I will keep all further questions in the previous thread.Quote from: MarkR42 on June 24, 2020, 10:57:26 amThanks, Is there a microswitch like that in the market? Would be great if you can link to them.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 24, 2020, 11:26:14 am
content: It kinda depends what your looking for if you need high power it might be harder to get by if your powering a relay you can use almost literally anything just look on mouser or radionics or your provider of choice

author: redgear
author_rank: Frequent Contributor
timestamp: June 24, 2020, 11:28:28 am
content: Quote from: sahko123 on June 24, 2020, 11:26:14 amThanks, yes I'm planning to use a relay.

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2020, 01:21:56 pm
content: A mechanical switch embedded in the frame of the door is the common method. My next choice would be a magnetic reed switch which is commonly used by alarm systems so inexpensive and easy to acquire.

author: redgear
author_rank: Frequent Contributor
timestamp: June 24, 2020, 01:29:22 pm
content: Quote from: David Hess on June 24, 2020, 01:21:56 pmI am thinking is go with a microswitch with a mechanical protrusion on the inner frame of the door such that the switch is pressed only if the door is fully closed. What do you think? I did think of the magnet but it is susceptible to the interference from the magnets in fitness trackers and other wearables.

author: Marco
author_rank: Super Contributor
timestamp: June 24, 2020, 01:51:49 pm
content: You could go whole hog and get one of the sets of prong + receiver safety switch combos like Omron D4NS, but then you're out 30 bucks.

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2020, 02:48:11 pm
content: Quote from: redgear on June 24, 2020, 01:29:22 pmThat is how most microwave ovens do it. I like the idea of using two switches in series which prevents being able to defeat only one.Common wearable magnets are never going to be close enough to the door frame matter. And even if they are, two magnetic reed switches in series could be used so both would have to be defeated.I should add that a door interlock switch is common on anything involving UV; my UVEPROM erasers all include one.

author: wizard69
author_rank: Super Contributor
timestamp: June 25, 2020, 06:49:05 am
content: Is this for commercial use in the USA or the EU? I ask because you will need to follow local regulation. Most likely that would mean a mechanically locked door switch that is tamper proof. These sorts of witches have an oddly shaped actuator to prevent finger operation of the switch along with an electrically locked catch for that actuator. One of many manufactures is:https://products.schmersal.com/en_US/category/2411/safe-switching-and-monitoring.Depending upon the scale of the risk you may need to have redundancy in place.Your safety requirements might require the use of controlled contact relays. or series connected relays for redundancy, when it comes to control of the UV source. Oh and you may even need separate safety relays connected to the door switches.One requirement that often gets missed is that closing the doors can not restart the UV, there must be a manual reset process.As noted it depends upon the location but in the USA you have OSHA, NEC and UL requirements to meet. This might help a bit:https://www.radtech.org/images/sustainability_pdfs/UVEBEquipmentSafetyTopics.pdfMore info are here:https://www.cdc.gov/niosh/docs/73-11009/default.htmlThere are all sorts of access control hardware out there the trick is to avoid going broke buying it. UV exposure can be hazardous but unless the intensity is extremely high there are far more dangerous things in the work place. In any event redundancy is a key element in safety as switches do fail with great regularity.Quote from: redgear on June 24, 2020, 09:23:07 am

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 25, 2020, 08:07:40 am
content: A simple regular household microwave oven usually has 4 microswitches to check whether the door is closed properly.It's not just about the switches though, but also what you do with them.For example one of the switches in the microwave simply hard shorts (after a fuse) the transformer itself as long as the door is open.This gives an extra safety because it's independent from the rest of the circuit. You can have some fancy magnets with HALL switches, which go to some logic board, and even if the whole logic board (Software!) goes haywire and tries to turn the oven on, then it will just blow a fuse (and probably damage the switch). Magnets with Reed switches have been used for alarm systems singe 50+ years. The advantage of magnets is that you can have bigger mechanical tolerances.Simple circuits are usually better, because there is less that can go wrong.But with adding more complexity, you can also build in more checks.If there is a microcontroller involved, I would not put multiple switches in series, but route each switch to the microcontroller, and check the time between activation of the switches, and give some warning if one of the switches fails to switch.