Red flashlight circuit for astronomy.

[andrejsstrumfs]

Hi! Can someone please sanity check this circuit? This is a red flashlight for astronomy, using single cell lipo and 1W red led with analog dimming. Q1 is to disconnect battery from load while charging, Q2 is to disconnect battery when discharged to 3V. Instead of driving U3 with pwm directly, I use R9, C5, R11 to inject current to feedback pin, because presumably driving leds with pwm is not good for preserving night vision.

Andrejs

[Peabody]

I haven't checked the math on the Q2 cutoff circuit, but I think you need some hysteresis there to deal with the threshold voltage and make sure it doesn't flutter.  That would be provided by a resistor from the drain of Q2 back to the ref input of the 431.  I found a video that shows this in some detail:



I don't know about the LED circuit.

[andrejsstrumfs]

Thanks! That's a great suggestion

Andrejs

[LaserSteve]

If the green led is on during normal operation I'd make it very dim or another color.

[andrejsstrumfs]

Both leds are active only when charging, but thanks anyways

Andrejs

[Peabody]

Depending on the power source used for charging, you may need to populate 5.1K resistors on the CC lines on the USB-C connector.

Can you explain how your dimming circuit works, and why you use it?  It's a constant current driver, which monitors the voltage across Rsense.  So are you just raising the voltage there to reduce the output current?  And why do you think this is better for night vision than using PWM on the EN pin?

[andrejsstrumfs]

Thanks, that's good idea, will add the resistors.
About the dimming circuit - you are right. This method is actually recommended in datasheets in many other led drivers, for example MIC3287, MIC2287 and others, so I think it should work also with this one. And for the night vision, some people say they do not feel any difference, others say they feel it. My reasoning is that pwm makes the led flash at full brightness, and even if your brain thinks the light is dim, the rhodopsin in the eye gets spent anyways. And since this is just a hobby project, then why not

[tooki]

Hi! Can someone please sanity check this circuit? This is a red flashlight for astronomy, using single cell lipo and 1W red led with analog dimming. Q1 is to disconnect battery from load while charging, Q2 is to disconnect battery when discharged to 3V. Instead of driving U3 with pwm directly, I use R9, C5, R11 to inject current to feedback pin, because presumably driving leds with pwm is not good for preserving night vision.

Why would PWM matter? The averaged light level and wavelength are what matter. (Well, and the PWM frequency, if you do PWM.) Our eyes have a minimum response time. A high PWM frequency physically cannot be detected. (3kHz and up. None of the annoying 100-1000Hz bullshit so many people think is sufficient.)

[andrejsstrumfs]

Well, more likely than not you are right, and I didn't find any sources confirming that this is a problem at all(not that I tried very hard). But as I said, this is a hobby project, I do not care for part count and price and I wanted to try if dimming will work that way, so..

[Solder_Junkie]

You are over thinking, when visiting an observatory we were all given an LED torch... it was very basic but more than adequate and comprised a silicon rubber moulding with a coin cell battery and a red LED. When you squeezed the case the wires of the LED touched the battery.

OK, that "throw away" torch was as basic as it comes, but the point I am making is that you don't need anything complicated. A 2 x AAA size battery holder in a small project box with a switch, a series resistor, a red LED and maybe a variable resistor is all you need.

[donlisms]

I've spent quite a bit of time under dark skies.  Somewhere in this thread I saw a reference to a 1W led, which is way more than you'd want under normal circumstances, but I suppose there are cases where you could use a lot of light, as long as it's not directly in your eyes.

Just because it's red doesn't mean you can have as much as you want.  My favorite red flashlight is indeed very simple, and the thing that makes it valuable is the trimmer pot that lets me keep it as low as possible.  It serves as the off switch also, so I can slowly and carefully dial from "off" to "dim"  without going too far, and ruining the dark adaption I worked on for half an hour.

In other words, the usability is the most important thing.  How you get there doesn't matter much.

The chemical reaction in your eyes, to lose dark adaption, happens very (very) quickly.  I would be nervous about PWM, unless I tested it, or someone else tested it carefully.  Nothing worse than looking back in the eyepiece and no longer seeing the nebula because of this cloud of insensitivity in your eyes covering it all up!

[andrejsstrumfs]

I also wanted focusable light, so you can illuminate larger area or only read your starmap, large dimmer and switch controls so it is usable with gloves and since it is a headlight - adjustable angle.
Dimmer and switch button is the left wheel. Right is just a screw to tighten it down when you adjust the angle.

Andrejs