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I have designed a PCB for mouth guard device and also its enclosure is ready in which PCB will be placed when its not in use. I want to use a UV blue light for UV sterilization inside the box. Now the function I want to add is when PCB is put into enclosure and enclosure is closed then blue light should turn on for a minute and then it should turn off. I tried to use 555 timer with LDR in mono stable mode. But the issue is when enclosure is closed then LDR resistance is too high until the enclosure is open again (Trigger pin2 can not pull up due to LDR resistance too high). I am unable to run my LED for 1 minute. Kindly suggest me some circuit with IC as I don't want to use MCU.

Original link from where the circuit is taken: https://www.buildcircuit.com/modes-of-ne555-monostable-and-astable/ Ciruit

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    \$\begingroup\$ I wouldn't rely on an LDR to determine when the box is closed. Use a physical switch, ideally operated by a peg going into a hole so it can't easily be operated by accident. Not that a 555 will be able to drive enough current through an LED to do much real sterilisation anyway. \$\endgroup\$
    – Finbarr
    Commented Aug 17, 2023 at 21:41
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    \$\begingroup\$ I've added an answer with various suggestions. For interest, where are you located? How many of these do you intend to make. \$\endgroup\$
    – Russell McMahon
    Commented Aug 22, 2023 at 11:49

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I wouldn’t use an LDR for the lid because if you open it at night which I’m assuming you will it may already be dark. Use a limit switch. UV-C will probably destroy you PCB and mouth guard unless the material is UV stable. Don’t damage your eyesight by messing around with UV-C. You can also get skin burns similar to a sun burn. That’s all an assumption that you mean UV-C by blue light disinfection. Probably just use the recommendations your dentist gave you for disinfection.

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  • \$\begingroup\$ If i use Limit switch at Trigger pin then Once the Limit switch is triggered(Enclosure is closed) the LED timer of 1 minute wont work as Trigger pin will always be low until enclosure is closed(Limit switch is pressed). Any suggestion for that? \$\endgroup\$ Commented Aug 18, 2023 at 6:30
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If i use Limit switch at Trigger pin then Once the Limit switch is triggered(Enclosure is closed) the LED timer of 1 minute wont work as Trigger pin will always be low

Correct. You have to add one R and one C to act as a differentiator. This turns the steady-state closed signal from the switch into a pulse at the Trigger input. Here is a thread on this topic:

555 timer, one shot trigger

In your case, I would use these component values:

R1 - 100 K (large value reduces battery drain)

R2 - 47 K

C1 - 0.1 uF to 1.0 uF

enter image description here

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A CD4060 timer IC ha an internal clock which can be RC controlled. It can be used to create a monostable (which you want here) and many other timer related functions. Because it has an internal divider chain it can use small R & C values to create very long delays.

Here is an example of applying the CD4060.

There are many other circuits on the internet. Here is an image search result with a large number of pages that may be of use. Each image links to a related page.

Others have pointed out that an LDR that relies on external light levels will not work as desired if light levels are too low.
An alternative is a magnet plus a reed switch. These can be physically very small. You could also use a Hall sensor but the quiescent current may be an issue if the device is battery powered. (You do not say if it is). An alternative switch would be a "tactile switch" which can be very small. Here are 64 tactile switches less than 3mm x 3mm in stock at Digikey.

Here are 3000+ of various sizes - smallest is 2.1 mm x 1.65 mm (!)

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  • \$\begingroup\$ I will not use 555 then. CD4060 looks good to me. I'll try that \$\endgroup\$ Commented Aug 23, 2023 at 8:36
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    \$\begingroup\$ @ZunuranNasrullah What do you intend to use for a switch? While automatic is good, a manual switch press may be acceptable. \$\endgroup\$
    – Russell McMahon
    Commented Aug 23, 2023 at 10:57
  • \$\begingroup\$ I actually changed the design. Now I have somehow managed to integrate ESP32-C3 SoC on my PCB along with vcnl4040 Proximity and light sensor. As using non programmable IC was not reliable as per the feedback from this forum. Can vcnl4040 work to detect dark ? datasheet says that its an ambient light sensor so I am not quiet sure whether it can work to detect dark or not. Firmware engineer can work on code to adjust the sensor but not very sure about its working in dark. \$\endgroup\$ Commented Aug 27, 2023 at 10:40
  • \$\begingroup\$ If you think i should start new topic regarding this update then i can close this question and will post a new question for it \$\endgroup\$ Commented Aug 27, 2023 at 11:12
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    \$\begingroup\$ @ZunuranNasrullah You should be sure about this question first. Also, as it has 4 answers closing it would not be very useful to others. || NOTE feedback did NOT say that a non programable part would be unreliable. Their is nothing wrong with them in this application - the stated issue was in using a light sensor when there may be little ambient light. Your new proposal is more complex, very probably more expensive, requires programming AND has nothing that makes it liable to work any better. \$\endgroup\$
    – Russell McMahon
    Commented Aug 27, 2023 at 11:27
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The values of R1 and C1 in your diagram are intended for a very short monostable, according to the page you have linked to. The values will need to be adjusted a lot for 1 minute.

The LDR is meant to be high resistance in the dark and low resistance in the light. You should adjust VR1 to make the triggering reliable. The 555 timer's trigger pin is active low - pull it down to 0V to trigger it.

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    \$\begingroup\$ The problem (and the original question) is that the trigger input will stay low as long as the lid is closed, holding the output low continuously. The timer will never "time out". This is because the standard 555 monostable circuit is not a true monostable; it has no gating to make the output period completely independent of the Trigger input state once it is triggered. \$\endgroup\$
    – AnalogKid
    Commented Aug 22, 2023 at 13:34

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