Combined Indicator for Power-On and Battery Status
In numerous small battery-powered devices, it’s common practice to employ an LED as a dual indicator for both ‘power-on’ status and ‘battery state.’ The prevalent circuit for this purpose, illustrated in Figure 1, utilizes an 78L05 voltage regulator, although similar principles can be applied to other low-drop voltage regulators. It’s important to note that the 78L05 necessitates a minimum input voltage of 6.5 V for optimal functioning.
In Figure 1, the LED operates at around 1.8 V, where the zener diode causes a voltage drop of 4.7 V, and the resistor manages the surplus voltage above 6.5 V. The described concepts are applicable not only to the 78L05 but also to various low-drop voltage regulators, allowing flexibility in circuit design.
Choosing a Low-Current LED for Efficiency
When selecting an LED for this circuit, it’s advisable to opt for a low-current variant due to its modest power requirement of only 2 mA. This choice ensures efficient power usage. As the battery voltage drops below the combined voltage of the zener diode and the LED, the LED is deactivated, preserving power.
Optimizing LED-Resistor Combination for Varied Load Currents
In the configuration depicted in Figure 2, for load currents exceeding a few milliamps, the current passes through the LED-resistor combination. The resistor’s value is calculated to allow slightly less than the minimum load current. Consequently, the LED assists in diverting some current past the regulator, avoiding unnecessary battery wastage as seen in Figure 1.
Using Multiple LEDs for Enhanced Indication
In Figure 3, two LEDs with different turn-on voltages are employed, enabling distinct color indications. If a low-drop regulator with a minimal voltage drop below approximately 0.1 V is used, LED D1 serves as the low-battery indicator, and LED D2 functions as the power-on indicator. To ensure proper operation, D1’s turn-on voltage must be approximately 0.2 V higher than that of D2. LEDs in both Figure 2 and Figure 3 can be either 2-mA types or the more prevalent standard 20 mA ones. It’s crucial to note that the current flowing through a typical LED should not surpass 50 mA for optimal performance and longevity.