Changing the sensitivity of a night light?

Discussion in 'The Projects Forum' started by peterjd, Apr 17, 2012.

  1. peterjd

    Thread Starter New Member

    Apr 17, 2012
    3
    0
    Hi,

    I would like to reduce the sensitivity of a simple commercial night light so that it operates when there is a reduction of light below a threshold rather than onset of dusk. I am using the 'spare' electricity generated by our solar photovoltaic panels to heat water in our hot water cylinder using its electric immersion heater. I'm manually switching the heater on and off when at home according to what I perceive as the level of (sun)light. I know there are automatic ways of switching but I am happy at the moment to have a simpler method eg having a visual or audible warning of 'low' light level.

    I have looked at the circuit of a low-cost nightlight and wondered whether the sensitivity could be easily reduced to achieve my requirements. I have seen the suggestion in a posting on these forums to put sunglasses material in front of a photodetector and if necessary I will try this but it seems a 'clunky' way of doing it.

    I have included what I think is the circuit of the nightlight although I wouldn't guarrantee that I have got it completely right. The photodetector is a 2-lead device but doesn't look like the usual LDR. I did some electronics many years ago but am very 'rusty' so an explanation of the existing circuit would be helpful. Look forward to receiving your advice and comments.

    Thanks

    Peter

    Market Harborough, UK
     
  2. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    as a rule of thumb this forum (i believe) doesnt really like mains topics, in case someone tries.. fails.. and sues.. or tries to.. but we can work on the detection and signal side? and i would recommend a wall adapter? similar to the blackberry charger?
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Here is the circuit posted by the op:

    [​IMG]


    MCR100-6 is a thyristor. The diode shown would be a diac.
    To reduce the sensitivity (delay when the light comes on) you can experiment with putting a large value resistance across the light dependent resistor (LDR). Start off with something like 10MΩ and reduce this until you get the desired result.
     
  4. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    might be worth using a potentiometer for summer/winter? you might want it to come on slightly earlier in winter? and in summer itll be lighter during the night..
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Any electrical supply house or hardware store should have an adjustable photo cell used for outdoor lighting that could be used off-the-shelf.

    There are also TONS of schematics all over the internet for a relay controlled by a LDR sensor. Simply google "LDR relay schematic"
    like this
    http://pcbheaven.com/circuitpages/Light_Dark_Activated_Relay/
     
  6. peterjd

    Thread Starter New Member

    Apr 17, 2012
    3
    0
    Thanks for the suggestions, I will try a few experiments (putting the nightlight on a mains isolating transformer just to be on the safe side :eek:).

    mcgyvr asks why bother with so much available off the shelf? Answer - a lifetime habit of taking things apart to see how they work and getting satisfaction from repairing equipment and/or using redundant equipment in a new application:)

    Thanks again

    Peter
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    I respectively disagree, and I think using a "filter" is the most elegant solution. It requires no tinkering with the circuit and should be very cheap and easy to implement, and to modify after field testing. Now, I'd use something other than "sunglasses", maybe multiple sheets of paper for instance. Frosted glass maybe. Lots of options.
     
  8. peterjd

    Thread Starter New Member

    Apr 17, 2012
    3
    0
    An interesting thought, wayneh, and it reminded me of my old Weston Master photographic exposure meter languishing at the back of a cupboard. Probably only familiar to older members of this forum but the change from low to high sensitivity was achieved by opening a perforated baffle covering the selenium photocell.
    [​IMG]

    Thanks

    Peter
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Yes, thanks for that reminder. :cool:
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Looks like a simple disk cover with holes in it is a good KISS solution.
     
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