Summary of Voltage indicator transitions between colours
This article presents two low-cost battery discharge indicator circuits using a dual-color LED and minimal components. The first version uses four parts to transition from green to red as voltage drops between 7.1V and 5.8V. The second version replaces a resistor with a Zener diode for a sharper transition at a specific voltage point, though it has limitations regarding standard Zener voltages and soft knee characteristics.
Parts used in the Battery Discharge Indicator:
- Dual-color LED
- Transistor Q1
- Diode D2
- Red LED (D1)
- Resistor R1
- Zener diode
This Design Idea gives two versions of an indicator light that changes from green to red as a battery discharges. There are many circuits that do this sort of thing, but all the ones I have seen are too complex and costly for my taste. This DI shows a method that uses an absolute minimum of low cost parts: a dual-color LED and four other parts.
Figure 1 shows the simplest version. With 9V on B+ only the green LED is lit because the voltage at the base of Q1 is 1.58V and the voltage at the emitter is the diode voltage VD (1.8V typ.), keeping Q1 off. As the battery voltage drops, the voltage across D2 stays nearly constant, but the voltage at the base drops to the point where Q1 starts conducting. This starts shunting current into D1, the red LED, which receives an increasing portion until all the current goes to the red LED.
This transition is centered around a voltage that is VD minus VBE; about 1.2V (most of the change in VD with temperature is also subtracted out). Taken together, a change on B+ from 7.1V to 5.8V will cause a change from green to red.
Different LED combinations will have different voltage differences, which may not be large enough to effect a complete color transition, but you can still use this design by putting a diode in series with D2.
Figure 2 replaces R1 with a Zener diode, creating a circuit with a much narrower transition. There is no longer a divider effect; a change of 0.25V on B+ will change color. The transition point will be 1.2V + VZ (about 7.2V for the parts shown).
A disadvantage is that you are limited to standard Zener voltages. This is further complicated by the fact that low voltage Zeners can have very soft knees, making it hard to predict what VZ will be at the low current in this circuit. A variation to avoid this problem would use a resistor in series with the Zener to allow some adjustment of the voltage at the expense of a somewhat greater transition voltage.
For more detail: Voltage indicator transitions between colours
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What is the main advantage of the circuit described in this article?
The circuit uses an absolute minimum of low cost parts consisting of a dual-color LED and four other components. -
How does the simplest version determine when to switch colors?
The transition occurs around a voltage equal to the diode voltage minus the base-emitter voltage, approximately 1.2V. -
What voltage range causes the color change in Figure 1?
A change on B+ from 7.1V to 5.8V will cause a transition from green to red. -
How can the design be adapted if different LED combinations do not provide enough voltage difference?
You can use the design by putting a diode in series with D2. -
What component replaces R1 in the second version of the circuit?
A Zener diode replaces R1 to create a circuit with a much narrower transition. -
How much voltage change is required to switch colors in the Zener diode version?
A change of 0.25V on B+ will change the color in the second version. -
What is a disadvantage of using a Zener diode in this circuit?
You are limited to standard Zener voltages, and low voltage Zeners can have very soft knees making VZ hard to predict. -
How can the problem of unpredictable Zener voltage be avoided?
A variation would use a resistor in series with the Zener to allow some adjustment of the voltage.
