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Build a 3.6V Cell Phone Battery Meter with LEDs
Description
This circuit resembles the one previously presented and demonstrates a 4 LED bar graph that indicates the voltage of a standard 3.6 volt Lithium - Ion rechargeable mobile phone battery. The reference voltage is generated by a TL431 programmable voltage source, configured to 3.9 volts. The TL431 connects via a 1K resistor to establish this reference. The lower reference voltage for the LED at pin 14 is controlled using a 5K variable resistor.
The programmed output voltage from the TL431 is determined through a voltage divider comprising a 10K and a 5.6K resistor. The point of connection between these two resistors consistently maintains a voltage of 2.5 volts. Consequently, if a 10K resistor is connected from this junction to ground, the current flowing through it will be 2.5/10000 = 250uA. This identical current traverses the upper resistor (5.6K), producing a voltage drop of 0.00025 * 5600 = 1.4 volts. Therefore, the regulated output voltage at the cathode of the TL431 will be 2.5 + 1.4, equaling 3.9 volts.
To calculate the LED voltages, three 390 ohm resistors are arranged in series with another variable resistor (5K) at the bottom. Assuming the bottom resistor is set to 2K ohms, the total resistance becomes 390 + 390 + 390 + 2000 = 3170 ohms. The resistor current is then calculated by dividing the reference voltage (3.9) by the total resistance, resulting in approximately 3.9 / (390 + 390 + 390 + 2000) = 1.23 mA. This value of current flows through the upper resistor (5.6K), yielding a voltage drop of 0.00123 * 5600 = 2.46 volts for the bottom LED. Additionally, the current through each of the step resistors (390 ohm) produces a voltage drop of 0.00123 * 390 = 0.48 volts. Consequently, the LEDs will illuminate at voltages of 2.46, 2.94, 3.42, and 3.9 volts. A fully charged mobile phone battery typically provides a voltage of 4.2 volts. The 5.6K resistor can be adjusted to modify the top voltage, and the lower 5K resistor can be adjusted to control the bottom LED's minimum voltage. However, a power source of 6 to 12 volts or greater is required to operate this circuit effectively.
Circuit diagram
