LM3914 Help

Discussion in 'General Electronics Chat' started by AliEN556, Jul 10, 2008.

  1. AliEN556

    Thread Starter New Member

    Jul 10, 2008
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    Hi, Im look for some help with a LM3914 circuit. I need to drive a 12v relay when the last led lights (D9). I have tried opamps, opto isolators and transistor but with no luck. any help would be very much welcome. Circuit diagram (livewire) included. VR2 adjusts the reading. The ic is in dot mode
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  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, replace D9 with an optoisolator. Optoisolators such as a 4N25 generally have open-collector outputs from an NPN transistor. Unless your relay had a very low current requirement, it would not be enough to sink the current from the relay's coil. You could use the collector to pull the base of a PNP transistor (such as a 2N2907A) low via a 1k resistor. Emitter of the PNP to your V+, collector to source the current to the relay's coil, ground the other side. Don't forget to connect a diode across the relay's coil, cathode towards V+.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    There may be a simpler way to do it, but my scheme relies on LED current, so I need to know how much it is. What voltages are you trying to apply to RH and RL? and how much current are you programming the LEDs for?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    My suggestion is not elegant but should work: Use a light sensitive resistor in proximity to LED9, and thereby bias a Darlington transistor to control the relay.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It can be much simpler, if the LED current is in the normal range, and the relay current is modest (100mA, more or less). If it is on the order of amps, a Darlington could be used, or a PMOS FET (with a slightly different configuration).
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    Elegant solution, Ron!
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Thank you.:)
     
  8. AliEN556

    Thread Starter New Member

    Jul 10, 2008
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    Thanks all for your ideas i will give them a try and let you know.:)
     
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