Need LED to fire with Solenoid Operation

Discussion in 'The Projects Forum' started by LesIzMore, Jun 30, 2014.

  1. LesIzMore

    Thread Starter New Member

    Mar 17, 2014
    Given solenoid operating on 70VDC, pulsed at 2 to 3 times per second for duration of 5 to 6 seconds. Operation varies from 3 secs. to 1 min. cycles.

    Require LED to fire with operation of solenoid.

    Solenoid driver circuit is off limits to touch with modification.

    Would a reed switch fire strapped to the external windings of the solenoid so
    that 12VDC could be used to illuminate the LED?

    Is there a cleaner way to accomplish this other than a 3.9K 1/2w resistor in series with the diode (2.5V @ 20ma) connected to the solenoid 70VDC?
    Maybe 1/2w would get too hot so 1w would be better.
  2. inwo

    Well-Known Member

    Nov 7, 2013
    Reed switch should work. Less liability than tapping the solenoid leads, which also works.

    If reed isn't sensitive enough use hall effect.
  3. MaxHeadRoom


    Jul 18, 2013
    You mentioned that the circuit is off limits to modification?
    But if you can tapoff the solenoid coil, what about utilizing the LED as a BEMF diode, If these are dispensing solenoids, I know they cannot be delayed by a normal BEMF diode, but if the series resister is of sufficiently high impedance, it may work?
    Although it shows the off condition, it is off such short duration, I would not think that matters?
    Last edited: Jun 30, 2014
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Modern high brightness LEDs will give excellent light for indication at as low as 1mA, you don't need "20mA" like a 1970's LED did.

    I would use a series resistor and decent type of LED at 70v @ 1mA, you need a resistor about 68k, 1/4W size will be fine as dissipation will only be 68mW.

    Also put a 0.33uF poly cap across the LED so it is protected from transients.