Simple circuit indicates a low battery
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.
Figure 1 Simple circuit to indicate a low battery.
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.
Circuit hysteresis is determined by the forward voltage characteristic of the LED, which "stretches" between the small bias current through R5 and the larger illuminating current through R4. But as the battery is not expected to come back to life during instrument operation, hysteresis here is irrelevant.
The RC delay imposed by R6 and C1 slows the circuit latching action. This reduces sensitivity to brief voltage dips from momentary current demands by the load. CR1 was added to discharge C1 when the power switch is turned off, preventing a false alarm should the unit be turned back on suddenly when the battery is partially depleted.
The alarm's trigger point was chosen to guard against biasing and signal headroom issues within the instrument proper. For the values shown, the LED comes on when the battery falls to about 5V. This may be changed by altering the value of R1, or by replacing it with a 100kΩ pot to afford adjustability. The "BC" series transistors are specified only because they were common to the rest of the instrument; almost any general-purpose devices should perform nicely.