Bipolar LED as fuse indicator

Discussion in 'The Projects Forum' started by faceless, Feb 11, 2013.

  1. faceless

    Thread Starter New Member

    Feb 11, 2013
    Hi all

    I'm working on a design for a 12V power distribution board, my criteria being
    • circuit can be switched on manually or via a relay, as in a multiway switch in a hallway.
    • green LED on shows circuit is on, red LED shows fuse is blown.
    • can set the relay from a microcontroller and read status of the circuit at the same.
    • if the microcontroller or relay fails, I can still turn the switch on or off by hand.
    • 24 circuits, so minimal parts and complexity.

    There are plenty of "fuse blown" indicator circuits out there but I haven't seen any with a schmitt trigger like this, so as I haven't used one before I thought I'd run my design past you, dear reader. See attached - note led is red/green bipolar, ie current flows from A to B for green, B to Ground for red. I didn't have the right part in eagle.

    The switch and relay are wired together as a multiway switch, and I don't see any issues here (standard 87/87A automotive relay). R2 and R3 are both 0.5W, SENS1 goes to a microcontroller and is high for "circuit on", and TRIG1 comes from a ULN2803A darlington controlled by the microcontoller and is high at 12V. D1 is to guard against power spikes by dumping anything above 15V to GND.

    The theory is this:

    • If the fuse is good and the circuit is live, the input to the trigger is high and output at point B is ground; the LED has current flowing from A to B so lights green, and SENS1 is 3.9V
    • If the fuse is good and the circuit is open, the output of the trigger at B is low and point A is pulled low via R4. LED is off and SENS1 is 0V
    • If the fuse is bad, the output of the trigger at B is high (5V) and A is pulled low via R4. LED is red and SENS1 is 0V.

    My questions are - well, will it work I suppose, but more specifically can I use the 74HC14N to both source and sink current (20ma) like this, and do I need a diode between the fuse and the input to the trigger? I've seen a some similar circuits with this in, but I can't see why it's required.

    Thank you for your consideration...

    Cheers... Mike
  2. faceless

    Thread Starter New Member

    Feb 11, 2013
    Bah - no sooner than I post do I see the problem, if the fuse blows the LED will not light. Also in my edits to the diagram points A and B got lost.

    Revised diagram attached. So now:

    • Circuit is live: B is GND, SENS1 is 3.4V and flows through LED to point B. LED is lit green
    • Circuit is off: B and SENS1 are both 0V, LED is off
    • Fuse is off: B is 5V, path to GND through R3 will light red LED.
    Last edited: Feb 11, 2013
  3. faceless

    Thread Starter New Member

    Feb 11, 2013
    No replies, but for posterity I'll finish this thread myself. I tested the circuit and with a small modification it worked (the schmitt trigger input signal must be reduced to 12v).

    The working circuit is attached, and the light goes green when the circuit is active, off when the circuit is not and red when the fuse is removed.