Design ideas for phase control circuit

Discussion in 'The Projects Forum' started by Josato, Sep 26, 2011.

  1. Josato

    Thread Starter New Member

    Mar 8, 2011
    Hi all!

    My friend is terrified of the dark and wanted me to design a 'night light' for her with the following specifications:
    Has to look pretty (rolls eyes)
    After you turn it on, takes an hour or so to dim to dim linearly
    It can't make any noise, such as a relay switching etc.

    I have decided to have a tilt switch in it to there are no 'ugly' buttons on the outside of the thing. From there, I was going to create some sort of inverse ramp, which I am sure I can do with a current source and capacitor (will it really be ok over an hour?)

    So I am hung up on the best design to do. (I have not really done this before). I thought there would be some sort of electrically-variable potentiometer that I could just have in series with the bulb, but I can't find anything like that. The other option looks like a SCR to control the phase, don't know how easy that would be to control. Or a TRIAC? Or there is something with BJT's or that ilk, since I have some experience there. Or maybe a high-power op-amp? As I said, I do not know.

    So I would appreciate ideas as how best to implement this. Also perhaps ideas for things to scavenge, since I am sure it is something I can hack.

  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
    The rules of this board prevent anyone telling you how to make dimmer switch that is operating mains connected lamp ....
    A low voltage lamp, after an isolation transformer would be OK, from an advice point of view I mean.

    If you are stuck with mains then buying a voltace controlled dimmer, vellerman do one, would be an option.
    All you would need then is a slowly falling signal, with some sort of reset button, perhaps you could follow the cap voltage in a 555 based timer circuit, add an op-amp to scale it without mucking up the timing and then feed that to the dimmer module, which is commercial and therefore considered no more dangerous than anything else you might buy and plug in.

    If you can use a low voltage lamp then PWM is the way to go and there a a raft of ways to do that. you don't need precise control so again 555 and or op amps

    Generate a triangle / or sawtooth wave, or at least something close.
    Compare that to a DC level to get a pulsed output.
    Now if you change the DC offset of either signal the duty cycle of the output will change will change.

    Hope that helps
  3. mcasale


    Jul 18, 2011
    What amount of power are you thinking of?

    For a night light, one or two LEDs might be sufficient. Maybe even battery operated.

    In a dark room, even LEDs seem bright.

    The dimming can be accomplished with a PWM or the like coming from a 555.

    If you get real inspired, maybe a small MPU with a PWM output to a FET drive would be a good idea.
  4. Josato

    Thread Starter New Member

    Mar 8, 2011
    Thanks you guys!
    Oh, do the rules forbid such things because I might build something that is a danger to myself or others? Weird, since people readily advised me on synchronising generators together, which is also quite dangerous (not that I am complaining) :p

    I have never done anything with 555s, mostly because they don't teach it at my uni. But it sounds interesting. I have found a build-it-yourself kit from velleman that looks pretty much identical to my design, here:
    Saves me melting tons of triacs and is much more safe, at not much cost, so I am happy! How did you know to point to there?

    It needs to be built to attach to the lamp provided, I am afraid.

    So I just have to somehow vary the (varying) voltage on the diac. I was going to use a tilt-switch to get the process started (so she can just nudge the lamp to turn it on).

    Actually, looking further, there might be the cheapest solution that does almost everything here:
    Makes me sad; too simple to be interesting. But, I am an engineer and a student and thus am unwilling to spend more than neccessary.
    Looks like you won't be seeing my solution on hackaday.