This document describes a novel voltage-level indicator which causes a single bi-color light-emitting diode to glow green when the supply voltage is above a preset level, red when the voltage falls significantly below that level, and various shades of yellow-orange during the slow transition period which is characteristic of a battery discharging below the preset level.

Problem Solved:

During the development of the PicturePerfect image capture system, it was necessary to devise a means of monitoring the device's rechargeable battery level, and to offer the user some degree of visual feedback on which an estimation of remaining operating time could be based.

Two-color LEDs are frequently used for this purpose, but ordinarily measures are taken to insure against a metastable region in the voltage comparator's operating curve. This design actually takes advantage of this often-undesirable characteristic.

Regulator U1 supplies a constant operating voltage of 5 volts to comparator U2 and inverter U3. The regulator's input, as well as the voltage to be sampled, is derived from the main 12 volt Vcc supply. R2 and R3 derive a fixed 2.5 volt reference which drives U2's non-inverting input (B), while R1 allows the sampled voltage to be divided down to match the 2.5 volt reference at the non-inverting input (A) when the Vcc supply crosses the desired comparison threshold. When the Vcc voltage is at one end or the other of its range, the comparator's output is a steady DC potential which reflects the differential state of its two inputs. Consequently, the LED element pair driven in complementary phase by inverters U3A/U3B produces either a solid red ("low") or green ("normal") indication based on the Vcc level.

As the Vcc voltage declines (in the case of a battery discharge cycle) through the comparator's threshold value, the comparator's lack of hysteresis compensation gives rise to a brief period of metastability, which results in a rapid and flicker-free toggling of the LED element pair. The duty cycle of this toggling effect is dependent on the exact input voltage differential. This results in a smooth "color fade" effect from green through orange to red.

A more complex but more easily controlled implementation would toggle between the LED elements with a variable-duty-cycle voltage-to-frequency converter. A circuit such as this would find use anywhere a slowly changing voltage level must be monitored without sacrificing either quantitative feedback or a large amount of front-panel space.