Optocoupler Problem

Discussion in 'The Projects Forum' started by JDR04, Jan 22, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    Hi Folks, would appreciate your help on this one please.

    I'm using a SFH618A-2 optocoupler which is connected to a 555 IC output via a 1K5 resistor. When the 555 pin 3 output goes high I get 1.24 V at pin 1 of the optocoupler. Pin 2 of the optocoupler is connected to ground.

    I have a camera connected to the "other side" of the optocoupler as per my drawing. The idea is when the 555 is activated, the camera shutter is activated. The camera voltage onto the optocoupler is 3.4Vdc at 9mA when the shutter is activated.

    However, the camera side of the optocoupler keeps "blowing". I've done a diode measurement with my meter on pins 1 and 2 and get 1.02V. As far as I can understand on the datasheet I'm within specification. So why is this happening?

    I've attached the optocoupler datasheet and a crude drawing of my circuit.

    I must be missing something so your guys help will be appreciated.

    Many thanks - JDR04
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    What is the voltage of the power supply for the 555?

    Are you sure that there is not a current surge on the optocoupler when the shutter operates that is too fast for your meter, but is in excess of the capabilities of the optocoupler?
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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  4. Markd77

    Senior Member

    Sep 7, 2009
    I've had success with just an NPN transistor with my Pentax DSLR. It hasn't had much use (I built an IR trigger to replace it) so I don't know if it would eventually stop working like yours.
    Emitter and collector connected to camera. Emitter connected to ground of your circuit and base connected to the 555 with a resistor, which should probably be a few hundred ohms.
    It should be safe for the camera, there isn't any current flow from the circuit to the camera unless the transistor fails in a spectacular way.
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  5. JingleJoe


    Jul 23, 2011
    sounds like too much current for the coupler, add a resistor on the collector.
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  6. Adjuster

    Late Member

    Dec 26, 2010
    Could the camera input be inductive, leading to overvoltage when the transistor turns off?

    Something like say a 12V transient suppressor in parallel with the transistor might help.
    JDR04 likes this.
  7. Papabravo


    Feb 24, 2006
    The usual reason for trouble is too much current. We need to know what kind of trigger circuit is inside the camera, since it is unlikely there is a power source on the camera side you could use to power a relay.

    The Vcc for the 555 can be literally anything allowed by the datasheet. It is a pretty wide range from like 3 to maybe 18 volts.
    JDR04 likes this.
  8. Adjuster

    Late Member

    Dec 26, 2010
    I wonder if there is some unknown factor here, such as a wiring error. If there is really a 1.5kΩ resistor in series with the source, the LED current should be acceptable (≈6mA @ 10V input). Unless the current transfer ratio is remarkably high, it's a bit hard to see how this could give an excessive output current.

    You're not trying to do this with some old electro-mechanical camera are you? Some of those things had pretty brutal arrangements inside them which were only suited for use with switches or relays.
  9. JDR04

    Thread Starter Active Member

    May 5, 2011
    Thanks to all those members who tried to help me.

    After much checking of the circuit, I found I have made a complete fool of myself......

    1. The optocoupler infact was not "blown". Discovered this when I figured out a better way of testing them:mad:

    2.I forgot to take into account that the camera enters a "standby" mode and this is when the circuit cannot trigger it via the remote cable!!!! Ho ...hum.

    I appreciated your replies as they got me thinking on a more logical path.... and sorry for wasting your time. JDR04