LED "relay" circuit

Discussion in 'The Projects Forum' started by Doktor Jones, Sep 3, 2014.

  1. Doktor Jones

    Thread Starter Active Member

    Oct 5, 2011
    58
    1
    I'm looking to "relay" a small LED into a much brighter LED array, so that when the small LED comes on, the LED array lights up.

    Use case:
    My legally blind brother-in-law has an answering machine; this answering machine has a small red LED that blinks when the answering machine has a new message. The problem is, he can't see that little blinking LED (especially since it's red, and what vision he does have is least sensitive to red), and needs something much bigger/brighter/bluer to be able to see it from any distance further than sticking his face on the machine.

    The simple solution would be just to replace that LED with a transistor that directly drives the bright LED array. The problem is, he loves his answering machine and knows how to operate it by touch, and is afraid to have me open it up and run *any* risk of possibly breaking it.

    I'm thinking I could use a photodiode that could be stuck to the answering machine's LED with double-sided tape or some other semi-permanent fixative, then use that to drive a circuit that makes the LED array blink.

    I found this circuit via Google:
    http://www.circuitstoday.com/photo-relay-circuit
    [​IMG]

    As far as I can tell, D1 would cause the transistor to switch off (disconnecting the circuit) when light hits the photodiode; to reverse this behavior, would it be safe to put D1 on the other side of the base of Q1 (between it and R1)?

    Thanks!
     
  2. Retiredguy

    Member

    Feb 24, 2007
    28
    5
    What I would do is just replace Q1 with a PNP type general purpose transistor to get the results you wish.
     
  3. Doktor Jones

    Thread Starter Active Member

    Oct 5, 2011
    58
    1
    How about this?

    [​IMG]
    MTD5010W is a photodiode that has a decent response in the 600-650nm range, BS170G is a MOSFET in a TO-92 case (which should be fine for switching ~60mA of LEDs).

    Might I need an op-amp or some other boosting circuitry to amplify the output of the photodiode to make it switch reliably with the low light output of the LED (the LED is the old-fashioned indicator-style LED, not a high-brightness LED)?
     
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