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Simple circuit help

Discussion in 'General Electronics Discussion' started by drmoreau, Feb 13, 2012.

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  1. drmoreau

    drmoreau

    1
    0
    Feb 13, 2012
    Hey guys, working on a prop (i attend a lot of scifi/anime conventions). It's basically a steampunk "Tesla" rifle.

    It's going to have a few LEDs and such on it and I'm concerned about one particular part of the build.

    Basically it will be 3 AA batteries in series connected to a master power switch. When the switch is thrown 2 blue LEDs in the "clip" will light up, if the trigger is pressed while the switch is set to ON 2 red LEDs in the barrel will light up also.

    I've got all of that worked out just fine, but I'd really like to add a knob somewhere on the gun that allows me to dim the 2 blue LEDs in the "clip". Basically like I'm adjusting the gun's power level.

    I was thinking something like a potentiometer (often used for guitars) or something similar? Would that trim back the voltage? If so what specs would I need on the potentiometer for this kind of setup?




    Another thing I was considering was some sequential LEDs that would light up as I turn the dial. For instance on "1" only 1 LED would light up, but by the time I turn it up to say "5" on the dial I'd have 3 LEDs lit up. Would this be difficult to accomplish?

    Thanks!
     
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    For dial-up consider using a transistor switch for each LED with a resistor and LED at the collector, and a base resistor driven by the potentiometer. For the second LED add a diode in series with the base resistor. For the third LED add two diodes to the base resistor. The design will be easier if you can select transistors for matched current gain. Try to arrange for the first transistor to go into saturation when the second transistor begins to conduct, etc., so at maximum Tesla power all three transistors will be in saturation with all three LEDs at their brightest.
     
  3. GreenGiant

    GreenGiant

    841
    6
    Feb 9, 2012
    For the dimming you can use a pot, you have to put the pot in series with another resistor with the LED in the middle line of progression would be Power->pot->LED->resistor->ground (negative)
    A 1K and a 1K pot would range the voltage from 4.5-2.25 approximately (assuming they are low voltage LED's) this should give you the dimming affect you are looking for

    Laplace's suggestion is a good one, and similar to his I would say use transistors to control the LED's that are on, use resistors to create voltage dividers that will give enough voltage at the base of the transistors to turn them on at certain points in the Pot's sweep, use the transistors as switches, have the 4.5 volts applied directly to the LED's that you want to turn on, and when the Transistor's are energized send the other side of the LED to ground through a small pull up resistor

    so for the one that you always want on, just have that LED tied to the power so no matter where the dial is its always on, for the second one say you're using a transistor that turns on below 2 volts (PNP) you would use around a 500 ohm resistor with a 1k pot so when you get to about half way with the pot it will reach the 2 volts and enable the transistor, and then for the third one you would need it up near 850/900 so when you reached the end of the pots sweep it would drop below 2 volts and enable the transistor
     
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