This Design Idea describes a 9V battery-voltage monitor whose total parts cost less than 34 cents (Figure 1). You configure transistor Q1 as a 10-mA current sink. LED1, a King bright WP7104IT, is on when the battery voltage is good. When the battery voltage nears the threshold voltage, the LED gradually dims.
It goes out once it reaches the threshold voltage. The threshold voltage for this design is 7.2V, which the values of D3, LED1, and R1 determine. If your application requires a different threshold voltage, you can change these three components’ values. You can reduce the PCB (printed-circuit-board) space this circuit requires by using equivalent surface-mount components.