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LM3914 Bar Graph meter help?

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magman116

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Hello all!

I am working on making a circuit to replace an analog volt meter using an LM3914 and a Kingbright red 10 led bar graph display.

I have a max supply of 12v and also 3.3V to work with. The bar graph display needs to have a range of 2V to 9v and run in Dot mode with 20ma to the LEDs. I would like to be able to adjust the range to fine tune for the circuit application.

I would like to actually have two switchable ranges. One from 1V - 7.9V and 2.9V - 9.3V. I would use a toggle switch to change ranges. Right now I have it bread boarded and isn't quite working correctly. I would like to just start from scratch again and not reuse my circuit.

Please help?

Thanks!

_Chris
 
Please attach your schematic to your reply and we will fix it.
 
Meter board schematic

Here is the schematic I am currently using. VT is basically 2.9V to 7.8V, but I need two ranges and more adjustment. I do have 12V I can use if the 3.3V isn't enough.

-Chris
 

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  • Meter board sch.pdf
    15.1 KB · Views: 220
yes you need to power the LM3914 off 12 volts not 3.3.

I still need an adjustable low end voltage and high end voltage. Can you offer any suggestions?

What schematic is the calculator based on? What is the other sides of R3 and R4 connected to?

Thanks,

Chris
 
Last edited:
What schematic is the calculator based on?
None; it just helps determane the resistors.
What is the other sides of R3 and R4 connected to
Thay are set up as voltage dividers. The bottom would be V- and the top is what ever voltage you put in the box next to it.
Why cann't you just do 0 to 10 volts? to do ranges you would need a muiltible poll switch and two sets of resisters.
 
None; it just helps determane the resistors.

Thay are set up as voltage dividers. The bottom would be V- and the top is what ever voltage you put in the box next to it.
Why cann't you just do 0 to 10 volts? to do ranges you would need a muiltible poll switch and two sets of resisters.

Here is an updated schematic. I looked again at the datasheet and saw it was just a voltage divider. For this circuit, one range is good enough. I will add a switch and a couple more trimmers to add another rage for a different project. At least with this circuit I can adjust the low and high end to my liking.

This is going into a custom circuit to control the sweep waveform of an AM/FM radio. Each band has its own voltage range and I would like to have full band coverage with one display. I will probably use a switching IC to switch from one range to the other depending on the band selection. That way I wont have to add a toggle switch. I could just do a dual display to cover the largest voltage range and one circuit would do it. It would just take up more room.

Thanks for the help.

-Chris
 

Attachments

  • Meter board X2.pdf
    16.3 KB · Views: 193
Last edited:
Why does your new circuit have pot VR3 that does absolutely nothing? Pin 8 should be grounded.
The 100k value of your pots VR1 and VR4 are much too high because the voltage divider inside the LM3914 is a total of only 12k which will load down your high value pots.
 
Last edited:
Why does your new circuit have pot VR3 that does absolutely nothing? Pin 8 should be grounded.
The 100k value of your pots VR1 and VR4 are much too high because the voltage divider inside the LM3914 is a total of only 12k which will load down your high value pots.

VR3 was to adjust the brightness, but it does nothing.

What would be the recommended trimmer values for the divider in my circuit?

-Chris
 
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