Led Flasher - Power Budgets

Here we have a real change of focus. No RF to be found!

I was fishing around in my junk box recently and stumbled across a LM3909 Led flasher I built some 35 years ago in high school. It got me thinking about how one would approach this today. 

There are lots of interesting links if you search for them. It certainly got me thinking.

A 1Farad capacitor proved to be 0.87F after measuring the voltage change while charging through a 8.2k resistor. The capacitor was then charged to 1.5V and the led flasher connected(circuit below). After 10 minutes the voltage had dropped to 1.027V giving an average current draw of 700uA. 

 

 

The fact that a slower flashing version drew slightly more current surprised me. In comparison the LM3909 has a typical current drain of 500uA flashing at 1Hz. 

My suspicion is I can do a lot better than this. There are some claims out there for current draw so low that I question the basis for them. The circuit above can easily be seen 50m away at night. Flashing a LED that can only be seen when it's resting on your eyeball is just nonsense. Rant over.

One of the interesting concepts I saw was using a micro to flash the led. A credible site with useful information on how to do this can be found here. František's approach to calculating the power required agree with mine.

I expect that a micro pulsing a led would consume around 44uA at 3V. 

 

Time On	0.004	secs
Flash Rate	2	secs
Time Off	1.996
Led Current	20	mA
Average Load	0.000040
Micro Sleep current	0.000004
3V current	0.000044

 

With the overhead of a 1.5V to 3V boost circuit the total current draw from a AA battery could be around 140uA.

The Energizer datasheet shows at this low drain around 3000mAh of capacity can be obtained down to 0.8 Volts. Since the boost convertor I plan to use can keep running below 0.8V the life might be slightly higher. Perhaps 2.5 years is possible?

The heat of summer has not started so I'll see if I can test this concept in coming weeks.