Keyswitch with auto-ejector? - SOLVED

[Zero999]

I have a device (a high power UV lamp) which needs to be activated by a momentary pushbutton and must only be used by someone who's been authorised to do so.

Is there a momentary key/token switch, which doesn't retain the key, so the user can't leave it in, allowing an unauthorised person to use it?

There appears to be plenty of momentary key switches, but I can't find one which will not retain the key?

Ideally it should be able to switch mains voltages, 230V at 1A, so I don't have to use a relay.

[ConKbot]

Not exactly what you're looking for, but some industrial safety gear has 2 key switches with a common key. Access control to the hazardous area, and enable of hazardous device, so to gain access to the area, you have to pull the key from the device.

The lock to the area, of course needs a switch to confirm the door/gate is closed, or at least be key retaining if not properly locked, to prevent the door being propped open.

Not sure if this could be applicable to your system.

[Zero999]

I think I'll use a safety interlock transponder system. The sensor will be fixed to the device and the user will carry the transponder. My only concern is it might be too difficult to use, which could encourage the user to bypass it. I do trust them not to tamper with it. It's a means to prevent accidental use.

[sparkydog]

I guess you're looking for a single component, not a system, so this probably doesn't apply...

Based on your description, I might be tempted to use an RFID keycard system; no issues with key (token) retention because the card is never inserted into anything. Depending on how long of a momentary pulse is acceptable, you might even be able to use a COTS system without additional components... except you'll likely need that relay you don't want because COTS access control systems (ACS) almost all switch 12V or 24V, not mains. (If the ACS pulse is too long, and/or or doesn't have a momentary-high output, a 555 one-shot should get you an acceptable pulse, though in that case you definitely need a relay...)

[Nominal Animal]

Is there a momentary key/token switch, which doesn't retain the key, so the user can't leave it in, allowing an unauthorised person to use it?

Wouldn't a spring-loaded momentary key switch (that always returns the key to the normal unlocked position) work?
If the user leaves it in, there is nothing stopping someone else just pulling the key off.
Similarly, a key switch where the key can be pulled out in either position, would allow anybody grabbing the key if the authorized person leaves it in.
All technical solutions you devise can be worked around by human users.  If it is security sensitive, you need to do like the military and intelligence services do it, and make the human responsible for the key.  If they lose it, it is exactly the same thing as if they gave it to an unauthorized person.

If this is about safety and not security, use a fingerprint reader instead, one that can hold a sufficient number of prints.  Mouser has many different types available.  Humans tend to be more attached to their fingerprints than external keys.  However, for a security measure, fingerprints are too easy to duplicate and fake.

[Zero999]

I guess you're looking for a single component, not a system, so this probably doesn't apply...

Based on your description, I might be tempted to use an RFID keycard system; no issues with key (token) retention because the card is never inserted into anything. Depending on how long of a momentary pulse is acceptable, you might even be able to use a COTS system without additional components... except you'll likely need that relay you don't want because COTS access control systems (ACS) almost all switch 12V or 24V, not mains. (If the ACS pulse is too long, and/or or doesn't have a momentary-high output, a 555 one-shot should get you an acceptable pulse, though in that case you definitely need a relay...)

Yes, I'm looking for something off the shelf.

Is there a momentary key/token switch, which doesn't retain the key, so the user can't leave it in, allowing an unauthorised person to use it?

Wouldn't a spring-loaded momentary key switch (that always returns the key to the normal unlocked position) work?
If the user leaves it in, there is nothing stopping someone else just pulling the key off.
Similarly, a key switch where the key can be pulled out in either position, would allow anybody grabbing the key if the authorized person leaves it in.
All technical solutions you devise can be worked around by human users.  If it is security sensitive, you need to do like the military and intelligence services do it, and make the human responsible for the key.  If they lose it, it is exactly the same thing as if they gave it to an unauthorized person.

If this is about safety and not security, use a fingerprint reader instead, one that can hold a sufficient number of prints.  Mouser has many different types available.  Humans tend to be more attached to their fingerprints than external keys.  However, for a security measure, fingerprints are too easy to duplicate and fake.

The idea is the key must never be left in the lock. A spring to eject it, would be ideal.

It's not security sensitive, but a safety issue. The UV LED is powered from a constant current power supply, with a dimming input, which accepts a 0 to 100k resistance to set the intensity between zero (under a few kOhms) full (100k). The PSU is controlled with a normally closed pushbutton switch, in parallel with a 100k potentiometer. It can be installed in a separate room from the LED.

The problem is, if the user forgets to turn off power supply off, whilst they're near the LED, it might misfire, due to a loose connector, or someone else might push the button. The LED outputs >15W, concentrated in a small area, which is enough to cause severe burns and blindness. This requirement came about because the user forgot to remove the dust cap, turned on the LED, which caused it to melt and smoke. They then cleaned the filter, without turning off the power. Fortunately I was there and immediately pulled the plug and no harm was done, but it has been removed from service, until it's fixed.

The system needs to ensure it'll only work, when the authorised user is at the control box. They still have to ensure no one is near the LED, but there will be a camera and screen, enabling them to check. An ordinary key switch is no good, because the key can easily be left in.

Anyway, I've decided I'm going to use a safety switch and transponder. The receiver will be inside a plastic box mounted on top of the controller. It will drive a safety relay, which will switch the main power to the LED power supply. The user will have the transponder on a key ring. Yes, I know it can be defeated, by taping the transponder to the box, but this is to prevent the user being injured due to their forgetfulness, rather than deliberate sabotage.

Something like this aught to do the job. It can be locked to a specific transponder and we don't have anything else like it, on site, which also minimises the risk.
https://uk.rs-online.com/web/p/non-contact-safety-switches/6192773?gb=s

[sparkydog]

The PSU is connected to a normally closed switch? That seems like a very odd setup.

The system needs to ensure it'll only work, when the authorised user is at the control box.

Well... a great way to do that is to make the user push and hold a button (or some sort of dead man's switch, anyway). In which case, you'd be better off with one-shot access control such that the access token gets immediately returned to its resting position (e.g. the user's pocket, or can be on a lanyard that never leaves contact with the user). Also, you can get COTS token authenticators for dirt cheap. (You might even consider using a fingerprint reader, as Nominal Animal suggested.)

You'll need two relays, though. One can be as dirt-cheap of a 12V SPST as you like, the other needs to be able to control your LED but can also be SPST. (SPDT is fine, you just won't use the NC contact.) And the momentary switch does need to be dual pole (DPST or DPDT).

Hook it up like this.

The "black box" (with "in" and "out") is the control box of a generic, COTS Access Control System (ACS). I checked this time, and you should easily be able to find one that has both NC and NO terminals, i.e. one that momentarily interrupts voltage and one that momentarily supplies voltage. It will also give you the 12V from mains that you need for the relay logic. The first/upper momentary switch is the token reader. The effect of this in real use is that "out" goes briefly to +12V when an access token is used, with a delay (usually a few seconds) that can be adjusted by a pot on the controller. The lower pair of switches are linked, though this isn't visible in the schematic; they're the dual pole momentary switch used to turn on the LED.

The way this works, when the ACS is activated by a valid token, voltage is briefly supplied to the first relay's coil. The momentary switch is connected in such a way that pressing the switch will "latch" the relay by connecting the relay's NO back through the coil. Thus, if the ACS is supplying power, holding the momentary switch will keep power connected as long as the switch is held. The relay also supplies power to the second pole of the switch, which in turn drives the second relay. This means that, if the first relay is open, pressing the momentary switch does nothing... and the momentary switch can't close the first relay, it can only keep it closed. The only way to close the first relay (and thus, to turn on the LED) is for the ACS to supply the initial voltage pulse as when presenting an access token.

Thus, the LED can only be turned on if an authorization token is presented, and while a user holds down the momentary switch. The moment the switch is released (unless still within the few seconds of authorization), the system requires the token to be re-presented to turn the LED back on.

Now... you could build this with the part you linked... the very expensive part that requires the token be left in place during operation... or you can use a COTS RFID ACS that costs a fraction of the price. 🤷

Of course, this assumes that you need the LED turned on for however long the user presses the button. If you're okay with it turning on for a fixed time interval, you might (depending on what interval you need) be able to dispense with everything but the ACS and second relay. And you might be able to use a timed relay if you can't get a suitable interval out of just the ACS.

Oh, and, if you're feeling clever, you might go ahead and get an ACS package that comes with an electromagnetic door lock and hook that up in parallel with the second relay coil, such that the room with the LED can't be entered while the LED is turned on. 🙂 That might also allow using a fixed-on interval even if you otherwise couldn't.

[sparkydog]

Question: are the control box and LED in the same room? Is it practical to make the area with the LED physically inaccessible while the LED is on?

If "yes", there are easier ways to go about this... hence I'm guessing the answer is "no"?

[jc101]

Could you use a key switch with a circular/tubular key, as they tend to not stay in the lock?  You have to push them in against the pins.

Something like this https://www.soundimports.eu/en/060-216.html ? or https://www.ebay.co.uk/itm/182254276272 or https://www.onlinegaragedoorspares.com/product/universal-electric-keyswitch-round-key-type/

[LaserSteve]

I work with lasers. The rule for laser safety world wide is simple. Key is not removable in the on position. Two safety interlock switches in series make sure the PSU covers are on.  Another two sense if the lid is on the laser head. Emission indicators are required that can be seen if the safety glasses are on.
Simple matter to sense if the cap is on/off.

The rule says keys are to remind operators of their responsibilities. 

It is the responsibility of the operator to attend the unit and ensure safety. Unattended operation is usually allowed.

External jacks are provided for external interlocks such as switches  on the lab doors. A  shorting plug is required
if the external loop is not in use.

If an interlock is breached the restart must be manual unless the radiation is 100% contained.

Usually there is a 7 second start delay, and audible or visual start warnings on Class IIIb and up.

If a bystander can enter and access more then the legal AEL without shielding, redesign the system.  Interlock removable shielding.



Interlocks are always normally closed current loops.

Defeat switches for repairs are
allowed,  provided the cover cannot close if the defeating mechanism is in place.

In the US, 21 CFR 1040.1.J covers non ionizing radiation exposures. The responsible agency is CDRH.

Warning signs, warning labels, emission indicators outside the lab, training, and keeping the doors locked from outside entry goes a long way.

Don't overthink it.

Steve

[Zero999]

Thanks for all of the helpful comments.

I've settled on Pilz transponder system and a safety relay.

Just to answer some of the comments.

The power supply to the LED is not switched at the moment. It's enabled by a normally closed switch connected across the 100k potentiometer on the dim input. I will change this so the power supply is switched via a safety relay, connected to the transponder system.

The UV LED is currently in a separate room to the controller.

It will be used in other locations. The idea is to add interlocks to the doors, but it's not practical to do so at this stage, due to time constraints.

It's not acceptable to use general purpose finger print recognition, or RFID systems because they don't satisfy the required performance level. Ordinary relays are also out of the question because they're not reliable enough. A special safety relay, with force guided contacts is required.

I'll update this thread later.

[Zero999]

I thought I'd post an update.

Here's the idea I went with.

U1 = Pilz 540150 An RF transponder safety switch.
U2 = Pilz 750104 A dual channel safety relay.
U3 = Pilz 774330 A single channel safety relay.


U1 will be mounted inside a plastic enclosure, large enough so it won't activate when the user places the RF transponder on top of it. They'll have to hold it next to the enclosure, whilst pushing the NC momentary switch, which bypasses the 100k resistor on the LED driver's dimming input. U2 monitors the safety transponder switch as well as activating U3, which switches the mains power to the LED driver.

The reason for using two safety relays, rather than one is it reduces the risk of a hazardous failure and it'll make it easier in future it will be interfaced with an external door interlock.