1. circuit
ideas
Battery-level indicator edi
s.c. dwiv
 Aniruddh K.S. makes it easier to recognise the
voltage level on the basis of the
N
ormally, in mobile phones, calibration made. Red LEDs (LED1
the battery level is shown in through LED3) indicate battery initially set it at 3V. Slowly adjust VR1
dot or bar form. This lets you capacity of less than 40 per cent. until LED1 glows. Now, increase the
easily recognise the battery level. Here Orange LEDs (LED4 through LED6) input voltage to 15V in steps of 1.2V
we present a circuit that lets you know indicate battery capacity of 40 to until the corresponding LED (LED2
the battery level of a device from the less than 70 per cent and green through LED10) lights up.
number of LEDs that are glowing. It LEDs (LED7 through LED10) indi- Now the circuit is ready to show
uses ten LEDs in all. So if three LEDs cate battery capacity of 70 to under any voltage value with respect to the
glow, it indicates battery capacity of 100 per cent. The brightness of the maximum voltage. As the number of
30 per cent. Unlike in mobile phones LEDs can be adjusted by varying the LEDs is ten, we can easily consider one
where the battery-level indicator func- value of preset VR2 between pins 6 LED for 10 per cent of the maximum
tion is integrated with other functions, and 7. voltage.
here only one comparator IC (LM3914) Diode D1 prevents the circuit Connect the voltage from any
does it all. from reverse-polarity battery con- battery to be tested at the input
The LM3914 uses ten comparators, nection. The tenth LED glows only probes of the circuit. By examining
which are internally assembled in the when the battery capacity is full, the number of LEDs glowing you
voltage divider network based on the i.e., the battery is fully charged. can easily know the status of the bat-
current-division rule. So it divides the When the battery is fully charged, tery. Suppose five LEDs are glowing.
battery level into ten parts. relay-driver transistor T1 conducts In this case, the battery capacity is
The circuit derives the power to energise relay RL1. This stops the 50 to 59 per cent of its maximum
supply for its operation from the charging through normally-open value.
battery of the device itself. It uses (N/O) contacts of relay RL1. Assemble the circuit on a general-
ten LEDs wired in a 10-dot mode. For calibration, connect 15V vari- purpose PCB. Calibrate it and then en-
The use of different coloured LEDs able, regulated power supply and close in a box. 
8 4 • D E C e m b e r 2 0 0 8 • e l e c t ro n i c s f o r yo u w w w. e f y m ag . co m