Open collector output

Discussion in 'Homework Help' started by Tigerterry, Nov 7, 2013.

  1. Tigerterry

    Thread Starter New Member

    Nov 5, 2013
    I wish to interface the output of one Device(A) with another Device(B) that requires a metallic contact closure to start Device B. Device A has two outputs:

    1. open collector
    2. common held at 0v

    The supply voltage is of device A is +5v and 0v

    I do not know anymore technical info about device A

    Device A is operational when a push button code is input by keypad. At this point the open collector should change state. Can anyone suggest a test procedure to determine whether the open collector is working properly.

    Once I know Device A is working I need to feed this output into an interface circuit input that will cause a relay to operate and provide a contact closure to turn on Device B.

    I have put together an interface circuit using an NPN transistor (BC547C), a relay - Maplins 2A DPDT 5vdc (coil resistance 125 ohms/operate current 40mA), a diode (IN4007S) and a base input resistance of 6 Kohms.

    The transistor collector is connected to one side of the coil and +5v to the other side of the coil. The emitter is connected to 0v and the diode is connected across the coil to protect the transistor against switch off transient voltages.

    I have connected the base via the 6Kohm resister to the open collector output. I have connected the common to my 0v of the interface circuit. However, Device A's open collector output does not operate the relay in the interface circuit when I key in the correct code into Device A.

    I have tested the interface circuitry in stand alone mode and when a resistive (6kohms) +5v is applied to the transistor (BC547C) base the relay operates with the contacts closing accordingly.

    So, I would be grateful for any advice regarding testing the Device A's open collector circuit to prove it works. I would also appreciate any suggested adjustments that may be needed to my interface circuit to enable it to work with this open collector output.

    Terence Sutton (tigerterry)
  2. MrChips


    Oct 2, 2009
    Open collector means that the collector is open, i.e. it is not committed and therefore left to the circuit designer to provide an external load.

    Connect a 1kΩ resistor from the collector to +5V.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Are you sure you even a relay?

    Tell us some more about device B. Operating voltage? operating current? Is the relay contact in the positive lead or in the negative lead to device B?
  4. Tigerterry

    Thread Starter New Member

    Nov 5, 2013

    Mr Chips, thanks for your response . I have not yet tried connecting the open collector output via a 1Kohm resister to +5v to see if I can see the collector output change with my multimeter, but hopefully this will give me enable me to prove Device A is working.

    Mike, Device B is gate controller board. I do not have a diagram of the input circuitry but know a metallic contact closure across two input terminals will operate the gate. Hence the need for a relay in my interface to provide the metallic closure across these terminals. Until I know a little more about the open collector output, I am unsure as to the final make up of my interface circuit.
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    If you connect the 1k resistor as MrChips suggested and determine the output is working, then the relay circuit should work. But you may find it works in reverse.
    Open collector outputs typically activate devices by sinking current to ground, the opposite of what you might expect
    That's why they don't show any change without a pullup resistor to the V+ supply of the device.
    Last edited: Nov 10, 2013
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Is one of your two input terminals directly connected to circuit ground? If so, the open collector can probably drive the other terminal without the need for a relay, as MikeML said.