The Design Idea in Figure 1 indicates a low-battery condition in an audio test instrument that is powered by four AA cells. As the instrument was otherwise an all-discrete design, this same approach seemed more in keeping with the spirit of the project than the use of a single-sourced integrated circuit.
A garden-variety red LED serves as both the indicator and the voltage reference. A small current through R5 forward biases the LED, but its glow at this low value is barely visible, even in a dark room.
For this application, the LED proved to be a decent reference diode. It has a temperature coefficient that tracks fairly well with that of Q1, and the LED reaches its turn-on knee with less current than a zener diode needs to maintain its rated voltage. The circuit consumes only 160µA until it is triggered, at which point the LED runs at 2mA.
R5 biases the LED to its turn-on threshold, about 1.5V for a red one. This is applied through R6 to the base of Q1. R1 and R2 divide the battery voltage down to about 1.1V at the Q1 emitter. This 0.4V base-emitter differential is below the Q1 turn-on threshold.
As the battery is depleted, voltage across the LED remains constant, but the emitter voltage drops proportionally, and eventually Q1 begins to conduct. This draws base current from Q2, turning it on as well. Additional current to the LED through R4 raises the base voltage of Q1, and this positive feedback snaps the LED on smartly at the alarm threshold. When the LED lights, voltage across it rises to about 1.65V.
Read more: Simple circuit indicates a low battery