Want LED to stay on once it is triggered

Discussion in 'The Projects Forum' started by jazell zee, May 3, 2015.

  1. jazell zee

    Thread Starter New Member

    May 2, 2015
    Have built the Seattle Univ cell phone detector to locate hidden harassment devices around my home, especially the auto tracker presently installed somewhere in my car.

    Circuit works well, and LED comes on when it receives a cell signal (don't use the buzzer). However, I can't drive and keep an eye on the circuit LED at the same time - especially when investigating the trunk, engine area, and under body as possible hiding places.

    So, I'd like to insert a flip flop, SCR?, etc? in line with the LED so when the circuit receives a cell signal and the LED lights up, it will stay lit until I get a chance to look at it, then I'll pull the power and turn it off.

    All help welcome - thanks!

  2. #12


    Nov 30, 2010
    How about a diode from pin 5 to pin seven with the positive stripe facing pin 7.
    Whenever the second amplifier goes to ground and lights the LED, it drags pin 5 to ground through the diode and keeps itself there.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012

    Or, @jazell zee could add diode and capacitor between pin1 and pin5 to make a sample-and-hold circuit. Add a switch to allow reset of the charged cap on the sample-and hold. Also, move the LED on pin 7 between pin 7 and ground (instead of pin7 and supply).
  4. eetech00

    Active Member

    Jun 8, 2013
    Use "D" FF (with reset button) connected to output (pin 7) of opamp, drive the led with output of FF, press button to reset?