Schematics - Terrade Electronique et Logiciels

The current consumption is equal to the single led current (less than 3mA for recent low current leds) plus the CD4011 supply current (less than 10° at 25°C). That's to say about 3 mA.

You can find the simplified schematic.

Vbat is the voltage under test. R1, R2, R3 form a voltage divider. Here, we use the trigger voltage of a CD4011 C-MOS integrated circuit to switch the different leds on and off.

1 - If Vbat is too low, under a value called Vmin :

V1 is under 2.5 volts	B=0 :	D3 is ON
V2 is under 2.5 volts	D=1 :	D1 is OFF
with B=0 and D=1	C=1 :	D2 is OFF

2 - If Vbat increase over Vmin :

V1 is over 2.5 volts	B=1 :	D3 is OFF
V2 is under 2.5 volts	D=1 :	D1 is OFF
with B=1 and D=1	C=0 :	D2 is ON

3 - If Vbat increase over a value called Vmax :

V1 is over 2.5 volts	B=1 :	D3 is OFF
V2 is over 2.5 volts	D=0 :	D1 is ON
with B=1 and D=0	C=1 :	D2 is OFF

We can resume the led lightning in a table :

Vbat	Vbat < Vmin	Vmin <Vbat< Vmax	Vbat > Vmax
D1	OFF	OFF	ON
D2	OFF	ON	OFF
D3	ON	OFF	OFF

R4 and R5 are placed to protect the CD4011 inputs since V1 and V2 can be over the suppply voltage. The value of R6 is not critical and must be choosen to give a enough light for the three colors.

The 5 volts supply can be derived from Vbat with a little 78L05 regulator.

Values of Vmin and Vmax are in relation to those from R1, R2 and R3. For the values in the exemple, Vmin and Vmax are respectively equal to 11.5 and 12.5 volts.

If you want to change the values for Vmin and Vmax, you must compute R1, R2 and R3 :

1 - First you must define Vmin and Vmax, the two values witch represent the
state of the battery under test.

2 - Then you choose a value for R3 and you can compute the values for R1 and R2.

                Vmax - Vmin
3 - R2 = R3 x ---------------       (units in volts and ohms)
                   Vmin

[ Vmax Vmax - Vmin ]
4 - R1 = R3 x [ ------- - --------------- - 1 ]
[ 2.5 Vmin ]

For information, these equations are derived from :
                           R3
          1) Vmax x --------------- = 2.5
                      R1 + R2 + R3

                        R2 + R3
          2) Vmin x --------------- = 2.5
                      R1 + R2 + R3

First: 1) = 2) gives : Vmax x R3 = Vmin x ( R2 + R3 )
-----
R3 x ( Vmax - Vmin ) = R2 x Vmin

                                     Vmax - Vmin
                         R2 = R3 x ---------------   3)
                                        Vmin

                               Vmax          R2     R1
Second: 1) can be written:   ------- = 1 + ---- + ----
------                          2.5          R3     R3

                             R2
                     where ---- can be replaced from 3)
                             R3

                             R2      Vmax - Vmin
                            ---- = ---------------
                             R3         Vmin

                             R1     Vmax        Vmax - Vmin
             then it comes ---- = ------- - -------------- - 1
                             R3      2.5           Vmin

                                         [ Vmax         Vmax - Vmin        ]
               and finally R1   = R3 x [ ------- - --------------- - 1 ]
                                         [   2.5            Vmin            ]


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