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Comments for “Led lamp for kid's hat”

Neat trick on the on/off switch.  Thanks for sharing!

— Bill on 25.10.2021 at 22:34 (UTC)

Interesting. You might get more battery life by running 2 leds in series with one load sharing resistor if you can stretch the boost convertor a bit. I haven't done the maths so it's just a hunch based on loosely similar stuff I've been doing with LEDs. eg Flashing a led for 7 years with a micro, boost chip and AA battery.

Richard VK6TT

— Richard on 19.01.2023 at 10:45 (UTC)

@Richard Yeah, currently about 10% of energy is lost on the series resistors. Another way to go would be to use a current mode LED driver chip and much smaller series resistors, assuming the LEDs are closely matched to each other.

— Petteri Aimonen on 19.01.2023 at 10:56 (UTC)

There must be some really obvious answer to this so I apologise for my ignorance but to make the output lower why not just increase the value of the resistors in series with the LEDs?
Kind Regards

— Mike Avison on 19.01.2023 at 15:59 (UTC)

@Mike Yeah, that would work fine - or lowering the voltage a bit. But it was more about the typical LED problem that they are tiny bright spots - sanding the case a bit blurred it out a bit, so that they are not so annoying while being visible from far away.

In practice the lamp was a bit too heavy though, the hat fell off all the time :D

— Petteri Aimonen on 19.01.2023 at 16:32 (UTC)

As a minimum you should pwm the LEDs to save power. Look up the titled on hackaday, he dimly lights a single led for 10 to 20 years on a tiny single coin cell, a cr2032, it has 225mah vers a aaa has 1100mah, that's 5 times less.

— --Oz-- on 24.01.2023 at 00:33 (UTC)

@Oz I don't think PWM would save power compared to just reducing LED current. That I could certainly do, to some extent - but when visibility is needed, very dim is not enough.

When comparing battery energy, you need to take into account the voltage also. AAAs are 1-2 Wh depending on quality, CR2032s about 0.6 Wh.

— Petteri Aimonen on 24.01.2023 at 05:55 (UTC)

Pwm is really the way, why, because your eye has persistentce of vision. A very quick blink of the led "looks" like it is on much longer than it is to your eye. Depending on application around 60Hz is a good starting place. If the led moves around quickly and you don't want to see it flickering (probably not needed in your application), then raise the frequency. To make it easy for you to test, by a square wave generator on Ali for $5, or just build a 555 with adjustable on/off times, but seriously the Ali generator is much more intuitive.
US $2.66  20%OFF | PWM Pulse Signal Generator Module DC 3.3V-30V 1Hz-150KHz 1CH Adjustable Frequency Duty Cycle Square Wave Reverse Protection LCD

— --Oz-- on 07.02.2023 at 07:04 (UTC)

@Oz In all studies I have seen, constant current driving is slightly more efficient than PWM driving. Consider e.g. http://pe.org.pl/articles/2013/11/68.pdf

The total efficiency depends also on other parts of the system. PWM driving from a 3V lithium battery can be more efficient than boost driving from 1.5V alkaline battery. But lithium batteries have safety concerns which is why I didn't want to use them for this project, and two alkalines is too heavy.

— Petteri Aimonen on 07.02.2023 at 07:35 (UTC)