How to convert this circuit to run on lower voltage?

Discussion in 'The Projects Forum' started by Johnnz, Mar 19, 2009.

  1. Johnnz

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    Dec 31, 2008
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    Last edited: Mar 19, 2009
  2. haran

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    Apr 6, 2008
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    its currently running on 12V n u want to make it a 5 volt isit?
     
  3. thatoneguy

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    Feb 19, 2009
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    [​IMG]

    The input side would be the same, as they are both voltage dividers to compare against the LDR.

    Where the changes would need to be made are in R5 and R6, and the use of a 5V relay coil.


    For LED, change R6 to 1kΩ, or down to 220Ω for a much brighter LED, but for an indicator, 1k will give adequate brightness.

    For a 75Ω Relay Coil, 5v-67mA. The BC106 will need to have 5mA base drive to fully switch on.

    After viewing the datasheet for the LM311, I'm not sure if/how the circuit shown above worked, I'll explain.

    If I'm reading the LM311 Datasheet correctly, it shows that it is an open collector / open emitter output. Which means the output voltage/current needs to be supplied externally. Move R5 (1kΩ) Between Pin 7 and Vcc (+5V), instead of between Pin 7 and the Base of Q1 (BC 106), basically R5 is rotated 90 degrees from current position. Connect base of Q1 (BC 106) directly to pin 7 as well.

    That should make the circuit work for you at 5V, with the change in R6 and Relay mentioned above.
     
  4. kubeek

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    The only thing that really needs changing is the resistor for the diode (or not if you don't care about the lower light). Other parts will work the same at 5V.
     
  5. thatoneguy

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    I read what I wrote and it didn't make sense, so here's a hack n slash new schematic.....

    [​IMG]

    Modified for 5V
     
  6. thatoneguy

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    Look at the LM311 datasheet, I see no source to drive the transistor base. :confused: Am I going insane :eek:
     
  7. Johnnz

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    Dec 31, 2008
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    Thanks for that info. In building a voltage regulator circuit I want to make it similar to this one:

    http://www.dse.co.nz/dse.filereader?49c4185501c87dee273fc0a87f3b06ed+EN/catalogs/SUP1000036

    However, the store didn't have any more LM7805, so I bought a LM7905 instead. Can this still be used to get a 5volt supply for this circuit? I guess that the only change to make is the connection to the rectifier will need to be reversed (as well as the Electrolytic capacitors polarity)?
     
  8. Johnnz

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    Dec 31, 2008
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    Wow, you are really on the ball. Yep, sure enough after constructing the circuit as shown in the original diagram, the result was - a second round of dissapointment (see my clap switch saga...). However, after following your suggestion and rerouting the 1k ohm resistor to Vcc this little circuit works perfectly!!! Thankyou very much Thatoneguy:D:cool:
     
  9. Johnnz

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    Dec 31, 2008
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    I built this circuit to use as part of a circuit to go into an alarm clock to make it automatically dim at night time, however, I think that it is likely to wear out the relay (and be noisy with chatter) when the light is mid brightness. I think I will need some kind of hysteresis circuit to latch the relay into a state of either on or off for say 30 minutes to give the morning or evening time to establish itself. What is the simplest way to achieve this?
     
  10. kubeek

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    I didn´t look at the datasheet, I simply assumed the original circuit worked already.
     
  11. thatoneguy

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    I'm glad you verified that! I was wondering about it, glad I checked back. Definitely an interesting schematic typo they had.

    As far as the 7905 as a positive regulator, Theoretically it could be done by re-defining Ground as +5V, and the -5 output as Ground, but that doesn't quite work and still have all the protection and regulation.

    Get a 7805, and save the 7905 for when you make a circuit needing a dual supply for an op-amp. The risks of using the wrong part aren't worth the small price. When you do use the 7905, remember to look at the datasheet for it! The pinout is NOT the same as the 7805.
     
  12. thatoneguy

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    Try a 100k Resistor from Pin 7 to Pin 2 (the + Input) of the LM311. If that isn't enough, try a 68k. That's the simplest solution, the full solution is several more parts, but you can try this to start.

    How is this connecting into the alarm clock? There might be a better way to dim instead of a noisy relay, at worse, a silent solid state relay would be good, once the hysteresis is set up ok.

    kubeek: We are all human, I've been guilty of skimming and wrong answers as well. This wasn't exactly one that needed the datasheet, since it was published, but I was wondering why they were using 1k for a base current limiter. :)
     
  13. Johnnz

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    Just gave that a go. It does seem a bit better, but still chatters loudly if moving the LDR slowly from light to dark (trying to simulate sunrise/set). I thought about using a 555 timer IC to somehow interface with the relay, but I think that would only be able to do half the job since I will have the problem when Q1 is about to be switched off with fading light, since I wont be able to trigger a 555 with an 'off' signal...

    The dimming circuit will be hooked up to the alarm clock as described in this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=20284

    I found that resistors worked well enough to dim the display without using a PWM design based on a 555.
     
  14. thatoneguy

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    Go down as low as 47k, it should still switch. Try a potentiometer to adjust to see if there is a working simple feedback for hysteresis, then measure it for value.
     
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