LM3909 IC 1.5V LED flasher circuit. A circuit that uses the now obsolete LM3909 IC to flash an LED from a single 1.5V cell. This IC and circuit is now a piece of history. I had one of these circuits running on a PCB for years, the circuit finally failed. My attempts to repair the circuit were unsuccessful. It appears that the IC finally failed. Read on for a look at some electronics history.
LM3909 1.5V LED Flasher Circuit Diagram
Below is the circuit diagram of an LM3909 LED flasher taken from an out of print electronics magazine. I built this circuit on a tiny PCB many years ago. The circuit operated from a single 1.5V cell, but could also operate from a single 1.2V rechargeable cell.
Flashing an LED from a Single Cell
I remember the LM3909 being expensive, costing many more times than a 555 IC. The problem with using a 555 is that it could not be used to flash an LED from a single 1.5V cell, but had to operate from a higher voltage. A 555 also drains a lot of current from a battery because of its internal voltage divider resistors.
When the LM3909 became available it was popular with hobbyists because an LED can not be lit up from a single 1.5V cell. Here was an IC that would flash an LED from a 1.5 or 1.2 volt cell, pretty impressive. The cell would last for a long time too.
LM3909 now Obsolete
Unfortunately the LM3909 is not available anymore. It was made obsolete several years ago and has no equivalent or replacement part. There are some transistor circuits available that will flash an LED from a single cell.
Attempting to Revive my LM3909 Circuit
The only LM3909 IC that I have was used in a LED flasher circuit built on a PCB. Fortunately I had used an 8-pin IC socket on the board, so could remove the IC to test it on breadboard. The PCB and breadboard test circuit are shown below. Notice that only two additional components are needed in the circuit – a capacitor and LED.
After many years of service, the LM3909 finally failed. After testing the PCB circuit with a new battery and then building the circuit on breadboard and testing it, the LM3909 was finally declared dead.