This circuit works as a voltage comparator with a current mirror reference, turning OFF the green LED, and turning ON the red LED when the tested battery voltage drops too low.
Drop the voltage of the right 1.5V supply, which represents a 1.5 V battery slowly to see the circuit operate.
1. The Constant Current Reference (Left Side)
The leftmost section sets up a stable reference point that does not shift dramatically with small voltage changes:
• The Voltage Divider & Diode: A 9 V main supply pushes current through a 33 kΩ resistor and a diode connected to the base of an NPN transistor. This establishes a stable bias voltage of around 1.36 V.
• The Diode-Connected Transistor: The NPN transistor at the bottom left has its base and collector tied together. This configures it to act exactly like a standard signal diode, creating a fixed voltage drop (around 680 mV) to establish a bias point. Using a transistor here is simply to demonstrate that if you tie the base and collector of a transistor together it creates a single diode.. You could swap it for a diode if you wish the outcome of its operation will remain the same.
2. How the Threshold and LEDs Work
The relationship between the green LED and the red LED determines the status of the tested battery.
A. When the Tested Battery is Healthy (e.g., 1.5V)
• The 1.5V battery supply on the right side provides enough voltage to bias the base of the far-right NPN transistor.
• This turns that rightmost NPN transistor ON, allowing it to sink current down its branch.
• This current flows directly through the green LED, turning it bright green (20.7 mA) to signal that the battery is good.
• Because the rightmost branch is fully conducting and sinking current to ground, the voltage at the node above it drops. This keeps the base of the middle-left transistor low enough that the red LED branch remains deactivated.
B. When the Tested Battery Drops Below the Threshold
• As the battery voltage falls below the required threshold to overcome the transistor's base-emitter junction (plus its emitter loading), the rightmost NPN transistor begins to turn OFF. When the voltage falls to approximately 1.3V the GREEN LED is completely extinguished, and the RED LED glows brightly.
• As it shuts down, current stops flowing through the green LED, causing it to dim or turn off completely.
• Because the right side is no longer sinking current, the voltage at the steering nodes rises. This diverts the bias current from the 9V supply over to the base of the transistor controlling the red LED.
• The red LED transistor turns ON, lighting up the red indicator to signal that the battery has failed the test.
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