LM3914N Driver Cct

electric soup said:

Hi
I'm using this IC to measure batt voltage and drive LEDS...success
Does anyone know how I can switch a relay on when one of the LEDs are
lit?

thanks

Alex[/Which ever LED you want to operate your relay_ connect one end of a 1k resistor to it's cathode, the other end to a PNP type transistor_say BC212 etc general pupose. Connect the emitter to the +v the collector two one side of your relay coil. Connect the other side to ground -V. Connect a diode say 1N4001 etc across the relay coil. Anode side to -V /circuit ground. Note supply voltage for relay coil? I haven't tried circuit with LM3914 but should work OK. Note none latching.]

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MERV said:

Alex[/Which ever LED you want to operate your relay_ connect one end of a 1k resistor to it's cathode, the other end to a PNP type transistor_say BC212 etc general pupose. Connect the emitter to the +v the collector two one side of your relay coil. Connect the other side to ground -V. Connect a diode say 1N4001 etc across the relay coil. Anode side to -V /circuit ground. Note supply voltage for relay coil? I haven't tried circuit with LM3914 but should work OK. Note none latching.]

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on1aag said:

Hi Alex,



If you want to drive a relay you'll need a saturated switch,
therefore you'll need more base current than in (normal) linear
operation. The base current to drive your transistor won't flow
through the led which means that this led will be less bright than
the other's. So save yourself a resistor and connect the base
emitter junction of the transistor in series with the led.

on1aag.

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k7elp60 said:

I believe your idea will not work, as the collector current is equal to the emitter current - the base current. The LED current is limited by the circuit of the LM3914 and that will be the maximum emitter current.

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k7elp60 said:

I believe your idea will not work, as the collector current is equal to the emitter current - the base current. The LED current is limited by the circuit of the LM3914 and that will be the maximum emitter current.

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on1aag said:

Hi k7elp60,



Are we having fun yet ?

on1aag.

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Roff said:

Did you forget that most transistors have beta >> 1?
Brain fart, maybe?

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k7elp60 said:

After reconsidering my answer, Rolf is right it....... was a brain fart. The only thing the LM3914 is going to do is limit the base current.
One thing I like about this forum is if you make a mistake you know about it
pretty quick.

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