Transistor Current Circuit

[StudentSA]

Hi.

I have a circuit which uses the LM3914 as a Voltage level meter. I would like to be able to vary the brightness of the LED`s using a circuit as attached.

Pins 7 and 8 (REF OUT and REF ADJ) of the LM3914 has a constant voltage source of 1.25V. According to the datasheet the amount of current drawn from this pin is proportional(10x) to the LED brightness.

With this in mind I would like my signal of 0-10V (V2 in schematic) to cause a current of 0 - 1mA (i.e. through R2) to be drawn out of the 1.25V source.

Is this ciruit idea correct and how will I choose the values of the resistors?


Any advise is appreciated.

Regards,
StudentSA

 

[ericgibbs]

hi,
The LED current is approx 10 times the current flowing between pins 7 and 8.
But changing this current changes the Hi Vref out voltage and will change the LED sequence. OK.?

 

[crutschow]

Yes, the current to ground from pin 7 determines the LED current. This current change will not have a significant effect on the LED sequence trip points.

Make R2 about 1kΩ to limit the current from the reference.

The value of R1 will vary depending upon the gain of the transistor. Start out with about 1MΩ and go from there.

 

[StudentSA]

hi,
But changing this current changes the Hi Vref out voltage and will change the LED sequence. OK.?

Not in my case, I have pins 7 and 8 solely for LED brightness the Hi Vref and Low Vref are set with external voltages.

The transistors I have availible are a 2N2222 and BC337-25. I will try using the prescribed resistor values, Thanks crutschow

 

[ericgibbs]

hi SSA,
Post a circuit diagram showing how you plan to connect this 'control' circuit, depending on how its connected it will effect the Vref Hi point.!
Also advise what your Vref High and low will be.