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Simple LED timer circuit

Discussion in 'LEDs and Optoelectronics' started by wingnut, Oct 16, 2015.

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  1. wingnut

    wingnut

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    Aug 9, 2012
    Hi all
    I wanted a LED to come on when a bell switch is pressed, and then it must automatically switch off after 2-5 minutes.
    I would appreciate you taking a look at the attached circuit. When the switch is pressed, a capacitor is charged, which feeds the base of an NPN transistor, which switches on the diode. It works fine, and the LED slowly fades till its off. The size of the capacitor determines the time on.

    If you can suggest improvements to the circuit, maybe to use less current, or have the LED maintain brightness and then snap off, it would be appreciated.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Basically this circuit should work. One improvement: place LED and resistor in the collector leg of the transistor.
    The timing will not be very precise and the LED will finally not switch off hard but slowly fade.

    If you want more precision, lok into 555 timer circuits (see e.g.our ressource section).
     
  3. wingnut

    wingnut

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    Aug 9, 2012
    Thanks Harald.
    The reason I did not use a 555 timer is because it will keep switching on and off. I want a device which switches itself off when not in use, to conserve the battery.
     
  4. dorke

    dorke

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    Jun 20, 2015
    "Bell switch" ,is this a bell- door project?
    If it is ,why operate from a battery?


    Do you do this with devices you have on-hand?
    If you are,
    any Si diodes or any Zeners about 3V ?
     
  5. wingnut

    wingnut

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    Aug 9, 2012
    Sorry, when I said "bell switch" I simply mean one that springs back.

    This LED sits above the toilet bowl for when I get up at night and don't want to switch the bathroom light on. It needs to be battery operated. I had a regular on/off switch at first but the trouble is that being half asleep, I keep forgetting to switch the LED off. Hence the need for automation. Apologies if this is too much info :)

    Ps. I do have diodes and 3V zeners if you have any suggestions.
     
    Last edited: Oct 16, 2015
  6. dorke

    dorke

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    Jun 20, 2015
    O.K a "push-button".

    What parts have you laying around?
    resistors,si diodes,zeners about 3V.
    The circuit can be improved and ,meet your needs...
     
  7. wingnut

    wingnut

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    Aug 9, 2012
    I collect components, so hit me with you best shot thanks.
     
  8. Martaine2005

    Martaine2005

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    May 12, 2015
    Just a side note...
    The 555 doesn't have to oscillate. It can be used as monostable..
    But see what Dorke has up his sleeve!

    Martin
     
  9. wingnut

    wingnut

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    Aug 9, 2012
    I forgot the 555 had a monostable mode (which is also why I forget to switch the LED off in the first place).
    I stand to be corrected, but the 555 would continue to draw current even when no output??
    I know there are super-efficient, low power 555's out there. Thanks Martin.
     
  10. Martaine2005

    Martaine2005

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    May 12, 2015
    Yes, you are right. They are rather power hungry! The CMOS ones are a lot better but there output is tiny in comparison.
    I wasn't saying to use one! Just that they don't have to "flash" constantly.

    Martin
     
  11. dorke

    dorke

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    Jun 20, 2015
    Your two requirements:
    1.max Battery life
    2.no dimming of led light

    the solution:
    Use a CMOS Schmidt trigger inverter.
    The best results for long battery life will be achieved with the NC7SV17 Tiny single inverter in a pack.
    I think a 3.6V rechargeable battery(in a socket) will be a good choice.

    Look at the diagram:
    R1*C for setting time(voltage on C from 3.6V down to input negative TH).
    note ,you can use much higher values for R1 than with the transistor.
    C=1000uF and R1=220K will give about 3.6 minutes.
    R2 for led current and light intensity(depends on led used).
    R3(47 ohm or about) to reduce initial charge current from the battery

    If you still want to use 6V,
    go with 74HC14 (tie the unused inputs to either GND or VCC- extra quiescent current).

    led circuit.JPG
     
    Harald Kapp and wingnut like this.
  12. wingnut

    wingnut

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    Aug 9, 2012
    Thank you so much for this super circuit which I will try as soon as possible.
    On hand I do have a CD4069UBE CMOS hex inverter which I will try.

    Don't you think that R3 should be between C and R1 so that the capacitor can charge up as quickly as possible, and then R3 will limit the current to the inverter, just as a thought?

    I will probably order the 74HC14 because it comes in a through-hole type which I can use in a breadboard. Soldering is not my strong suite - now welding, that is a different story ;)

    Thanks so much again for all your trouble. I will update you as to how it goes.
     
    Last edited: Oct 17, 2015
  13. GPG

    GPG

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    Sep 18, 2015
    6v is close to the wind datasheet wise.. The unused inverters can be wired in parallel to provide increased current capability IE one for the time and the others as drivers. The input/output transitions will be fast enough to not worry the chip.. A 555 could be used to switch itself off after timeout.
     
  14. Colin Mitchell

    Colin Mitchell

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    All the above circuits are absolute RUBBISH.
    You need the first circuit wired as a SUPER ALPHA pair with two transistors.
     
    Martaine2005 likes this.
  15. Martaine2005

    Martaine2005

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    May 12, 2015
    I can't help but love your replies! I used to find them offensive..
    Now they crack me up! Absolutely hilarious...They remind me of my childhood....Mr Men:)

    Martin
     
    Harald Kapp likes this.
  16. dorke

    dorke

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    Jun 20, 2015
    Wingnut,
    The CD4069UBE has very limited output drive capability ,hence it isn't a good choice.
    Assuming a 5ma LED ,
    you should wire all 6 inputs and 6 outputs in parallel .

    About R3,
    It is in the correct location as it's purpose is to limit the initial current drown from the battery
    at the first moment.
    Assume the Cap is fully discharge,
    at the first moment you close the switch the battery feels a short on it's terminals(limited only by a very low internal resistance of the cap-practically a short).
    That is not a good thing for the battery and it's life time...

    Yes,It will slower the charging time , but nothing you could notice:

    Assuming C=1000uF and R=47 ohm, we get a time constant of T=0.047 seconds
    the cap will be 99% charged in about 5*T=0.25Sec,
    which is less than the time a person presses the push-button...
     
    wingnut likes this.
  17. dorke

    dorke

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    Jun 20, 2015
    Colin,
    "RUBBISH",
    care to explain or just be rude?
     
  18. dorke

    dorke

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    Jun 20, 2015
    Wingnut,
    Have you got any 74HC series "combinatorial " (Gates ,MUXs etc.)chip on-hand?
    If you do ,
    the circuit can be slightly modified for them to work properly .
    Almost any can do a better job than the 4000 series.
     
  19. GPG

    GPG

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    Sep 18, 2015
    You mean darlington? Szilaki might be useful.
     
  20. wingnut

    wingnut

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    Aug 9, 2012
    I do have on hand TL074CN.

    I have 741 op amps - in fact many different op amps and comparators in my pill-boxes at home.
    These include 741, LM339N, LM393, LM10393, CD4069UBE, BA10393, TL074CN, LM331, CD4060BE and HEF40106BP hex Schmitt-triggers if any of these will do?
     
    Last edited: Oct 17, 2015
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