Fleapower flasher draws less than 50 µA

Some applications require a circuit to indicate that a battery's voltage has fallen below a certain value. However, if you don't frequently check the indicator, the low-battery indicator itself can easily discharge the battery. The circuit in Figure 1 indicates when the battery voltage has dropped below a preset value. The circuit draws less than 50 µA, regardless of whether the indicator flashes. IC_1A operates as a simple comparator. IC₂, a low-current voltage reference, supplies the reference voltage to Pin 2 of comparator IC_1A. Resistor R₁ provides an adjustable trip point. A potentiometer setting of 124 kΩ yields a trip point of approximately 10.3V. When the power-supply voltage is above the trip point, IC_1A's Pin 1 is high, forward-biasing diode D₁ and holding the flasher in the off state. R₂ provides approximately 300 mV of hysteresis for the detector. IC_1B provides the flashing of the LED. C₁ charges through R₃, and, when its voltage exceeds the voltage at IC_1B's Pin 5, Pin 7 pulls low, discharging the capacitor through the LED. Resistors R₄ and R₅ provide a voltage reference, and resistor R₆ provides hysteresis of approximately one-third of the supply voltage.

The circuit's current consumption is 45 µA at 10V, climbing to 48 µA at 12V. The state of the flasher does not affect current consumption, because the LED receives its power via the capacitor. You have to make a number of trade-offs to achieve this low current consumption:

For example, discharging the capacitor through the LED allows the circuit to reuse the capacitor's charge, instead of dumping it to ground. It also eliminates the current spikes that occur when driving the LED from the supply rail. However, the charge on the capacitor limits the resulting LED brightness, so you should use high-efficiency LEDs when possible.

Another trade-off is that the LED affects the minimum operating voltage because of the forward voltage drop of the diode. In the prototype, a red LED works down to a lower operating voltage of 4.3V. A yellow LED in the same circuit operates down to 6.4V. Additionally, the high resistances on the board, most notably the resistors attached to IC_1B's Pin 5, require careful attention to board cleanliness. Small leakage currents can significantly affect circuit operation and current drain. You can reduce the values of these resistances to improve manufacturability at the expense of higher current consumption.

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