F3F TIMER PROJECT by Jon Edison
Created November 2018

Expanded Scale Voltmeter

It is useful to know the state of charge of the Lipo battery used for powering the F3F Timer described elsewhere. A Lipo has a useful voltage range of 3.1v to 4.2v per cell. Thus the 3S lipo used for the Timer can be considered to be flat at 9.3v, and fully charged at 12.6v.
This voltmeter uses a LM3914 IC, and by modifying the input configuration, the IC can be made to represent any voltage range. In this case it is arranged to show 9.0v to 12.6v.

          

Components required

Item
Name
  
R1
 4K7 0.6w Metal Film  
R2
 1K2 0.6w Metal Film  
VR1
 5K Multiturn Pot  
VR2
 5K Multiturn Pot  
C1
 10uF x 35v Tant Bead  
LED1
 3mm Red LED  
LED2
 3mm Yellow LED  
LED3 to LED10
3mm Green LEDs  
ICI
 LM3914  
SK1
 JSH XH 4pin 90deg  


Pcb size is 2.0" x 0.88" ( 50mm x 22mm ). Click above image for actual size print. Photo etch the board and drill all holes 0.8mm. Open up the holes for the two potentiometers and the JST socket to 0.85mm. Assemble in any order to suit your own preferences. Check for solder bridges etc, before applying power.

To setup the display, apply 12.6v to the input. Adjust VR1 until the RHS Green LED10 is just lit. Reduce the voltage to 9.0v and adjust VR2 until the LHS Red LED1 is lit. Check adjustments several times until the LED's light as required.

NB this voltmeter can be set as a 'DOT' display, ie only one LED is lit at any one time, or a 'BAR' display where an increasing number of LED's are lit as the voltage increases. The DOT display keeps the load on the battery down to a minimum, and by default is set to DOT display.
Adding a link across the pads at point 'A' will convert it to a BAR display.

Disclaimer

I have no connection with any of the companies listed above, any links given for information only, and for use at your own risk. All information is provided in good faith and without liability.

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Jon Edison
Nov 2018