Discrete LM3909: Blue LED Waveforms

The circuitry and instrumentation is essentially the same discrete LM3909 as before:

With a few minor tweaks:

• Blue LED, forward voltage 2.56 to 2.97 V
• 24 Ω R1
• One Q2 current mirror transistor driving Q3

With a pair of fresh AA alkaline cells producing 3.1 V (not the NiMH Duracells you see in the picture), the blue LED blinks brightly.

The 610 mV peak voltage across R1 shows the LED starts at 25.4 mA:

The capacitor reaches 1 V, then goes about 150 mV into reverse charge during the flash (note the different horizontal scales):

The Darlington version of Q1 seems to do a decent job of keeping the cap out of reverse charge. A Shottky diode would add a few hundred mV, but I doubt there’s anything nasty going on inside the cap as it stands.

The blue LED has a forward drop of 2.97 V at 20 mA, so I’m surprised the voltage across it hits 3.1 V at 25 mA:

Very little of the voltage appears across Q3, the driver transistor:

With a pair of nearly dead alkaline cells for a 2.0 V supply, the LED current peak drops to 4.6 mA:

The LED lights brightly, then fades away exactly like you’d expect from that waveform.

The cap still charges to about 1 V and stays well above 0 V during the (much longer) flash:

The voltage across the LED now reaches only 2.7 V, which is substantially higher than the 2.0 V battery supply and exactly why the LM3909 existed:

Q3 continues to saturate, although you can see the effect of the decreased base drive during the flash:

The blue LED won’t light at 1.3 V, but still gives out a weak flash at 1.7 V, so I’d say the tweaked LM3909 circuitry works reasonably well.