# 3.6 Volt cell phone battery meter

## Discription

This is a similar circuit to the above and provides a 4 LED bar graph
indicating the voltage of a common 3.6 volt Lithium – Ion recharable
cell phone battery. The reference voltage is provided by a TL431
programmable voltage source which is set to 3.9 volts where the TL431
connects to the 1K resistor. The lower reference for the LED at pin 14
is set with the 5K adjustable resistor.

The programmed voltage of the TL431 is worked out with a voltage
divider (10K 5.6K). The adjustment terminal or junction of the two
resistors is always 2.5 volts. So, if we use a 10K resistor from the
adjustment terminal to ground, the resistor current will be 2.5/10000 =
250uA. This same current flows through the upper resistor (5.6K) and
produces a voltage drop of .00025 * 5600 = 1.4 volts. So the shunt
regulated output voltage at the cathode of the TL431 will be 2.5 + 1.4,
or 3.9 volts.

Working out the LED voltages, there are three 390 ohm resistors in
series with another adjustable (5K) resistor at the bottom. Assuming the
bottom resistor is set to 2K ohms, the total resistance is
390+390+390+2000 = 3170 ohms. So, the resistor current is the reference
voltage (3.9) divided by the total resistance, or about 3.9/ (390 + 390 +
390 + 2000) equals 1.23 mA. This gives us about .00123*2000= 2.46 volts
for the bottom LED, and about .00123*390 = .48 volts for each step
above the bottom. So, the LEDs should light at steps of 2.46, 2.94,
3.42, and 3.9. A fully charged cell phone battery is about 4.2 volts.
You can adjust the 5.6K resistor to set the top voltage higher or lower,
and adjust the lower 5K resistor to set the bottom LED for the lowest
voltage. But you do need a 6 to 12 volt or greater battery to power the
circuit.