Ken Shirriff Reverse Engineers a Low-Power LED Flasher From 1975: the 1.5V LM3909
Built in 1975 by National Semiconductor, the LM3909 could run a flashing LED for a year or more from a single 1.5V flashlight battery.
Noted reverse engineer Ken Shirriff has turned his attentions to the LM3909, a vintage chip designed to flash an LED — determining its operation from an image of the silicon die captured by Zeptobars.
"How do you make an LED blink? A vintage way is the LM3909, a chip from 1975 that can flash an LED for a year from a single flashlight battery," Shirriff writes by way of introduction. "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."
"The underlying idea of the IC is that the external capacitor slowly charges through the internal timing resistor. When the capacitor reaches about 1 volt, the LED is energized and lights up briefly. This discharges the capacitor and the cycle repeats. Because the LED is only powered in brief pulses, the circuit's power consumption is low, averaging about 550 µA."
Using a knowledge of the core approach and the high-quality die photography from Zeptobars, Shirriff sets about identifying individual components: A selection of resistors and transistors, the latter including a surprise transistor which cannot be found on the circuit schematic and which turns out to be a junction FET (JFET) which likely provides start-up current.
"This integrated circuit has just a few, large components, so their wiring can be traced out on the die," Shirriff explains. "The designers came up with a clever circuit that uses just a few transistors but can flash an LED without consuming much power. One interesting feature is the use of a charge pump, so a single 1.5V battery can light an LED that requires more than 1.5V. Another nice feature is that the current-limiting resistor and the timing resistors are part of the IC, so external resistors are not required. On-chip capacitors are very limited, so the chip requires an external capacitor, though."
Shirriff's full analysis can be fuond over on the Righto.com website.
