Looking for a circuit that detects DC mV & turns on

Discussion in 'The Projects Forum' started by doug08, Jun 22, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    a transistor's base to let current flow from the collector to the emitter to turn on an LED. I'd like the circuit to activate an LED when 50mV or higher is detected.

    Thanks
     
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Search around for a "comparator" like an LM339. They switch at whatever level you decide by manipulating their input voltages. 339's are about 50 cents each, you get 4 in a pkg.
    Most comparators can't switch much current by themselves (like 5-6ma). If your LED will glow at those currents you'll be all set. If not you'll have to use a 3904 or something. Most comparators have open collector outputs too.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    We really could use some more information.

    1) How do you plan to light the LED? (battery or other power source)
    2) What is the device you are using to bias the transistor?
    2) What is 50mV or higher, specifically the "higher"? (1V, 10V, 100V...)

    A circuit diagram would be helpful as well.
     
  4. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    a really sensitive ionization chamber I have. The circuit will be using 9vdc. I'd like an LED to light up when the radiation detected rises to 50mv or higher...50mv is not critical, but between 50-75 is desired.

    Thanks
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    The comparator is the way to go for simplicity, since you could sink the LED's current (say 5mA, plenty bright) directly without a transistor. Using the LM339, you could even have up to 4 LEDs, to indicate different levels of input. You'll need a voltage reference, divided down to the appropriate range, to set up the comparisons. You could just use the power supply, but that will change as the battery drains. The LEDs will trigger at increasingly lower voltages.
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
  7. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Sweet amp! You wouldn't really need the comparator at all if you use this op-amp. It's also capable of powering a 5mA LED, so you could just wire it as a comparator and you're done.
     
  8. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90

    Back in the day, this was vacuum tube territory. We were ecstatic when MOSFETs came out and we spent ages designing a discrete component MOSFET input op amp. Now they're $5.
     
Loading...