need help with a SR LATCH

Discussion in 'The Projects Forum' started by arq, Jun 22, 2009.

  1. arq

    Thread Starter Member

    Nov 2, 2008
    16
    0
    Here i post a schematic of circuit implementing a window comparator output feeding a SR NOR gates latch. On the input of a window comparator resistive divider of LDR and variable resistor is connected.

    The purpose of this circuit is to turn SR latch on or off depending on the intensity of light falling on LDR. On the ouput of a latch i have connected LEDs through external pull up and pull down resistor. Circuit is working well.

    The maximam output current of LATCH in my circuit is about 44mA. which is enough to drive a 5v coil relay instead of LEDs which i want to do. But relay contacts not changing position . Relay type is dpdt i am currently using. Coil resistance is 129 ohms.

    PLZ IF SOMEBODY CAN TELL, HOE CAN I DRIVE A RELAY ON THE OUTPUT OF A LATCH?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    On the output of the latch, connect a 1k resistor to the base of a 2N2222 transistor.
    Connect the transistors' emitter to ground.
    Sink current from the relay's coil using the transistor's collector.
    Make certain to use a diode across the relays' coil (cathode towards +5v); this will absorb the reverse-EMF pulse from the relay coil when the transistor is turned off.

    You should remove R3 and R4 from your circuit. Comparators have open-collector outputs; they need pull-up resistors only to show an output. Since you have both pull-up and pull-down resistors of equal value, the outputs of the comparators will only be from near 0v to 1/2 Vcc (2.5v), the latter of which is not a valid TTL or CMOS logic level.

    Here's an example:
    [​IMG]
     
    Last edited: Jun 22, 2009
  3. arq

    Thread Starter Member

    Nov 2, 2008
    16
    0
    Thanks a lot, great help
     
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