Reverse-engineering the LM3909 low-power LED flasher chip @kenshirriff

Ken Shirriff does an in-depth analysis of the LM3909, a chip from 1975 that can flash an LED for a year from a single flashlight battery. This chip has some surprising features, such as a charge pump that lets you power a 2-volt LED from a 1.5-volt battery. This IC was designed for simplicity, using just an LED, external capacitor, and battery.

When the capacitor charges to about 1 volt, Q1 turns on, turning on Q2 and then power transistor Q3. This causes the circuit to discharge through the LED, as shown below. The tricky part is that the positive side of the capacitor is now connected to ground. Since the capacitor was charged to 1 volt, the negative side of the capacitor is now at -1 volt. (In other words, the capacitor acts as a charge pump.) The result is that the LED sees an extra volt; with a 1.5-volt battery, it receives about 2.5 volts.

Read more on Ken Shirriff’s Blog