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

 

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+.

 

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?

 

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.

 

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.

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).

 

Elegant solution, Ron!

 

Elegant solution, Ron!

Thank you.

 

Thanks all for your ideas i will give them a try and let you know.