20 Segment Expanded Scale Voltmeter
Created "Nov 07 2002"
This is an Expanded Scale Voltmeter based on the National LM3914 chip.
It is a 0.25 volt range between each LED.
I specifically designed this unit for my spare battery in my RV Truck to continuously monitor its voltage. I did the orgional in a rush and have now changed a few values for ease of adjustment. Also added a few capacitors to corrected a few minor hum problems that affected some adjustments.
This Voltmeter it set for a range of: "10.25 to 15 volts".
The 20K resistor across the LED is required to null out the #10 LED.
Leaving this resistor off will cause #10 to light Faintly when LED's above #10 are lit.
This resistor is soldered to the copper side, due to space limitations!
Currently this unit is designed to operate from a 12 volt battery, over the range
specified above. But it can be changed for Other Voltages and/or Ranges.
1) Set your supply Voltage to 12.500 Volts.
2) Trimpot VR1 is set to create a 1.200 volt drop between pins 4 and 6 on the adjacent
LM3914. Test Point are provided on the board. This Must be set VERY ACCURATELY.
3) With the Supply Set Precisely at 12.500 volts
Using Trimpot VR3, set the voltage at pin 5 of the LM3914's to 6.250 volts
(both pin 5's are connected together)
4) Now Trimpot VR2 is adjusted to set the #10 LED to "JUST BARELY LIGHTING",
(This occurs when Both 9 and 10 are Lit) Representing a voltage at midscale
of the range. (Typical = 12.50 Volts)
This is the LED marked #10 on the Schematic.
The Full Scale should now be calibrated from 10.25 to 15 volts!
Calibration Voltage Settings must be Very Precise with a good Digital meter..
Currently this circuit is designed to run in "Dot Mode" to reduce current consumption.
Breaking the trace between pin 9 and 11 on the top IC and connecting pin 9 to the supply rail will give a "Bar Display".
"But as the bar length increases, SO DOES THE CURRENT, CONSIDERABLY".
I can supply the LED displays, listed in my parts area. Look at "LED's, Bar Displays"
They are $1.50 each plus postage. All the other parts are available also.
Postage cost on these parts depends on weight and where you live.
I can also supply the PCB for $5.00 plus postage cost of only $1.00.
It fits in a regular envelope.
Copyright by Bill Bytheway, K7TTY February 2012