A single LED indicates whether the battery charger in Fig 1 is delivering a trickle charge or a fast charge. During fast charges, the LED stays lit continuously because IC₂'s FASTCH output sinks dc. During trickle charges, the LED flashes because D₁ and Q₁ enable the 555 timer. The timer is an astable multivibrator operating at 60 Hz (f=1/1.4RC).

The timer remains enabled during a fast charge, but it must shut off upon disconnection of the battery. Zener diode D₁ makes that decision. With no battery connected (a condition that R₅ and an internal comparator sense), IC₂ produces a voltage at BATT+ equal to twice the number of cells for which the IC is programmed. This voltage (4V for two cells) turns on the 3.3V zener diode and disables the timer.

Connecting a battery drops the BATT+ voltage to 2.8V (1.4V per cell), thereby turning off D₁ and Q₁, and enables the timer. The timer's output drives Q₃, whose collector (in a wired-OR connection with the open-drain FASTCH output) causes the LED to flash. Other cell counts require different zener voltages (Table 1).

During a fast charge, the LED should glow steadily, without flicker that would occur if the timer were operating. This continuous-glow condition is assured if the FASTCH voltage remains low: IC₂'s data sheet guarantees 0.4V max when FASTCH sinks 2 mA. Higher currents produce a higher voltage, which may result in flicker. In that case, you can cure the problem by adding a resistor in the emitter of Q₃. (DI #1705)