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Saturday, April 30, 2011

LM3914 based voltage measurement

We were trying to investigate a simple way to make a representative LED output from a given input voltage. Basically this was intended to be a quick checker for the Battery we use in the Offline Inverters.

We found that LM3914 was a low part count Bar Graph display with Analog inputs available. And it was already in our dusty component drawer waiting for some one to use it.
This chip has a Direct Drive for the LED and contains a FLASH ADC kind of Structure for Analog Voltage to Bar Graph representation.
Internal Block Diagram (Sources: National Semi)
Pin Out Detail for the DIP Package (Source: National Semi)
We tried to use the Basic Circuit as per the Datasheet. But we did not have the exact resistances so we optimized them.
Basic Circuit for LM3914
We wanted to supply a Standard +5V and also wanted to adjust the threshold of the circuit so that we can measure different battery sizes and voltage levels.
So we modified the circuit by adding a 10k POT (BOURNS) in place of the R1 and R2.
Our Modified Design for Variable Voltage
Here the POT can be used to vary the voltage steps. In order to do this use another voltage source set at maximum of its value to be measured at J2 Signal input and then adjust the POT such that the Last LED10 is just lit. There you have the calibration done.
We also added a small filter 10K + 0.1uF to make sure we have clean input to the chip.
This is one handy circuit from our creations, hope that this would help you to develop your own application.

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