What's the most reliable way to check if a door of a cabinet is closed?

Thread Starter

redgear

Joined Oct 17, 2019
136
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
 

DickCappels

Joined Aug 21, 2008
6,546
Maybe a little simpler mechanically would be a reed switch and a magnet but your two switches sound good too. You probably want to make sure it is difficult to defeat the safety circuit, especially with easy tricks such and taping or gluing things in place, jamming toothpicks and such is places.

Edit: To make is more resistant to defeat, some sort of wireless detection circuit such as RFID chips in one or more places on the door that line up with small sensing coils. If chips are glued directly to the door that interlock would be very difficult to fool.
 

joeyd999

Joined Jun 6, 2011
4,425
What's the most reliable way to check if a door of a cabinet is closed?
I was going to be silly and say, "your eyes". But then:

I am building a UVC Sanitizer Cabinet.
So, no, that wouldn't be the best way!

It's not a matter of checking that the door is closed: If the UV is powered while the cabinet is open, damage may have already been done.

You need an interlock. Make it impossible (to the extent possible) that the UV will not receive power unless the door is closed.

A non-tamperable microswitch on the door (or integrated into the door latch) would be your primary defense (think: microwave oven). You can also added multiple secondary sensors/switches. Take it as far as necessary for the situation in question.
 

Thread Starter

redgear

Joined Oct 17, 2019
136
Personally, I think your 2 switch idea is fine as long as you stay far away from the switches maximum ratings.
Thanks!

Maybe a little simpler mechanically would be a reed switch and a magnet but your two switches sound good too. You probably want to make sure it is difficult to defeat the safety circuit, especially with easy tricks such and taping or gluing things in place, jamming toothpicks and such is places.

Edit: To make is more resistant to defeat, some sort of wireless detection circuit such as RFID chips in one or more places on the door that line up with small sensing coils. If chips are glued directly to the door that interlock would be very difficult to fool.
Great idea, I will keep that in mind.

Have a look at safety relays, they use a fault detection system which is fail safe and prevents activation if there is a switch failure.
https://www.pilz.com/en-US/products/relay-modules/safety-relays-protection-relays
These cost so much. Idk, if I will be able to spend so much.

Should I connect the switches to a relay or directly to power? I will using ballasts to power the tubes from the mains voltage. If I use a relay, can I get a switch with lower power ratings?

Edit: I want to use this circuit with a timer. So, I have to have a boolean logic with this switch and timer, the UVC lamps should work if both the contact switches and the timer relay is on.
Power -> Contact Switch Relay -> Timer Relay -> Ballasts will be the right circuit, correct?
 
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Thread Starter

redgear

Joined Oct 17, 2019
136
You need an interlock. Make it impossible (to the extent possible) that the UV will not receive power unless the door is closed.

A non-tamperable microswitch on the door (or integrated into the door latch) would be your primary defense (think: microwave oven). You can also added multiple secondary sensors/switches. Take it as far as necessary for the situation in question.
Thanks, I will have a look. Something like the ones used on refrigerators for controlling the lights, correct?
 

Wolframore

Joined Jan 21, 2019
1,749
I would put a cherry switch on a latching handle that will only allow power if the door is latched properly. If this is housed correctly it would make tampering difficult unlike a car door switch.
 

Thread Starter

redgear

Joined Oct 17, 2019
136
I would put a cherry switch on a latching handle that will only allow power if the door is latched properly. If this is housed correctly it would make tampering difficult unlike a car door switch.
Thanks, hearing it for the first time. Looks like the buttons on a keyboard.
 

DickCappels

Joined Aug 21, 2008
6,546
To answer your relay question -you can use a relay in a particular application if you don't exceed the voltage or current ratings of the contacts.

A special case is when switching very low currents like hundreds of microamps, in which case the contacts might foul -use gold contacts for that, preferably bifurcated.
 

Thread Starter

redgear

Joined Oct 17, 2019
136
To answer your relay question -you can use a relay in a particular application if you don't exceed the voltage or current ratings of the contacts.

A special case is when switching very low currents like hundreds of microamps, in which case the contacts might foul -use gold contacts for that, preferably bifurcated.
Thanks.


I have now decided on the dosage and the switches. I need to start making the main board. I will using two micro switches to detect if the cabinet door's closed, relay,timer relay and 6 ballasts(one for each tube). The ballasts are designed to work with ac mains. How do I start working on the main board? Can I just use the connectors found on the back of the smps to connect the ballasts to mains? What safety devices(fuse?) do I need? I'm also planning to make the timer with display myself. I can do it use arduino or any other microcontroller but will it be as reliable as using a IC7555 or 555?
 

MisterBill2

Joined Jan 23, 2018
6,785
A very simple and very cheap method that has been proven to work very well was used on television sets for many years. The power cord ended in a small socket that was riveted to the access cover. The plug that let power into the set was riveted to the chassis of the set in a position such that only when the set was assembled with the cover in place would it be powered. There was no easy way for the set owners to bypass the arrangement. TV service people had a separate cord, usually called a cheater cord to allow them to service the set with the back off.
A similar setup could protect your UV cabinet and be more reliable than any of the more complex and expensive methods described.
 

MisterBill2

Joined Jan 23, 2018
6,785
I had a cheater cord...and I wasn't even a TV service person...
My point being that a similar setup will be a cheap and easy way to avoid any accidental exposure. NOT FOOLPROOF, but certainly it would avoid accidentally being exposed.
Please keep in mind that it is not possible to fix truly stupid.
 
Most safety device mfg's will make an RFID coded non-contact safety sensor. With this being a life safety issue, redundancy is key as is not being easy to defeat. Depending on your budget Allen-Bradley makes some really nice and quality switches whereas if money is an issue AutomationDirect has some decent quality stuff at reasonable prices. If a safety factor is being implemented then be aware that most of your safety rated control relays will require an OSSD input from the RFID sensor. Most definitely the safest way possible to protect inadvertent contact of the contents of your control panel.
 

MaxHeadRoom

Joined Jul 18, 2013
21,275
You can also get emergency Stop Door switches that have a temper proof action, they resemble a regular limit switch but have different interior and coloured red.
Generally mounted on the enclosure door so that the circuit is broken when ever the actuator moves slightly.
Max.
 

Tonyr1084

Joined Sep 24, 2015
4,929
Do like they do with front-loader washing machines. Close the door, swing the latch. When the latch is engaged the door locks in a way that can't be accidentally opened, and then, and only then, will the UV light come on. When the cycle is finished - the lock releases and the UV shuts off simultaneously.

[edit] Let me elaborate, now that I've had a moment to think about it - the door can't be closed if the latch is not fully open. Meaning if the handle is even slightly swung closed, the door can't close. Only when the door is fully closed can the latch handle be swung into the closed position. Inside the latch receiver is a microswitch. When the door is fully closed and latched the microswitch applies power to your project. When you push the start button a solenoid can lock the latch in the locked position so that it can not be opened as long as the board is powered. When the cycle is finished, the board powers down and the solenoid falls out of position, and the latch can be opened. If there's a power failure, the door can be opened, but once opened the UV system can not be re-energized because the latch receiver microswitch is shut off. [end edit]
 
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