Let's take a look at a little LED flashlight. A keychain flashlight from eBay although I'm sure it will be available from many other sources. Tools In this, because it isn't very big, it's quite small. It's kind of semi-fingerish type size.

So this thing has eight modes and if you switch a mode on but you don't change it for a while, when you press the button, it will go straight off. This is good. so the first mode is a focused beam of light. It's not too bad if I hold it at waist height and point at the ground as if you're walking out the street.

It does unfortunately produce a 150 millimeter to six inch rough diameter circle, but then it's got a splash light around that. But it does have a hot spot in the middle when you press it again because I pause there, it turns straight off. If I press it twice, it goes through high. Then it goes true though.

then it goes to red. LED Then it goes to flashing red. Then it goes to alternating red and blue flashing uh White which is quite handy. little work light and then flashing light and yellow.

I'm sure why, but uh, then it goes off. and in any of those modes, if you just pause for a while, it will automatically switch off. when you press the button again, it doesn't just go into the next mode, which is good. There are also two other little LEDs in here for charging because it does have a USBC charging port on it which will take the circuit board out and we'll see if it's got the required resistors.

I'm guessing that little chip. there is a Lth7 type charge chip and I'm seeing a little ubiquitous eight pin chip down here, but I'm only seeing one obvious transistor which I'm guessing is going to be switching the the hyper reality. Anyway, let's open it. So if I take that off, it reveals a fairly High parallel LED in the end there and uh, if I unplug the USB charge port which incidentally, it's got the little uh offset pivot so you can hinge out the way.

There are some that when you open them, it's very hard to get the plug in. This one doesn't have that problem. It's good and at this point in time I did try before I've not got this out yet. It comes out so far and then it stops.

I have a sneaky feeling that's the switch that's stopping it. Oh, that was easier than expected. So is it going to come out now? It slid forward now. but the switch is actually following that.

Let me push the switch down. Let's come on. Oh, that is not pushing down very far. It's definitely a switch that's stopping this sliding out.

Uh, you can see that when you slide it forward, it just hits the end. There does this Led come off? I'll turn them back off again. It will turn back on again soon. hopefully not.

Uh does this come off? Is it in a connector? Am I going to break this probably I Don't see any connector in there through the plastic I Don't know. like birthday I Think it probably. you just have to use extreme force and push that down. although plastic is very tough so I don't really see that happening too easily.

I hope that it's going. it's going I Think it's probably easier to push in and then get out. there. we go.

That's what we want. So what do I see here? Well, the first start I see a really big circuit board. Why is it so big? And they've actually slotted the other circuit board into that and it's at an angle, which should explain why the beam of light was at a strange angle there. That could be fixed because uh, I can see the solder connections here that you could reheat to actually get that straight.

A ticket. This does slide into grooves, not instantly seeing grooves. Maybe it just sits down on the Lithium cell. Okay, I tell you what.

I'll do the usual I'll take a picture of this, we can explore it and I'll reverse engineer it one moment. Please, let's explore for the second time because this is actually take two. The reason is take two is because in the first take I raised a question about this transistor and the only way to answer it was to re-disassemble the thing and do some tests. So I just did that I took it apart again, having reassembled it and put it back together.

and uh, that did the tests. Interesting stuff. It did. Answer: Answer the question.

it's okay, this is good. So what we have inside is the charge control circuitry based in the Lth7 and it's got two LEDs associated with that, the red one and a green one. and they've got a 1K resistor for them. There's the current setting resistor down there, which is the A2 key resistor.

so it's going to charge roughly 500 milliamps. Um, there's the bare minimum capacitors. It's got the one across the input from the charging that USB Supply but it's also got one directly across the processor for the decoupling, which is fine. Uh, there's a button for selecting the mode slightly scuffed because of the force that was required to actually get out of the case.

Um, and there are the LEDs which are driven directly with 120 ohm resistor in series with them all and a 1 ohm resistor being used to limit the current to the main LED but switched by this transistor which I thought was going to be our mosfet and it turned out not to be a mosfet. which is surprising because what I marked as the gate is actually the base and it's being driven directly from a pin and I thought that was going to be driving huge amounts of current into that. Turns out it's not as bad as I thought it was going to be. Uh, the USB connector.

The USBC connector does not have the resistors so it may not be recognized by some smart. Chargers You might plug this thing into charge and nothing happens, but it will work with dumb Chargers Let me bring in the schematic so we can explore that together. Have I missed anything here? No, I have not. I think I've got everything there.

Classic circuitry. very basic symbol circuitry which is good with like simple circuitry. The USB Supply comes in and there's a decoupling capacitor across it and then there's one resistor feeding the two LEDs that indicate the charge status. Let's add the Wii don't keep bits onto those.

There we go. the donkey bits. The Lth7 when it's charging, pulls its charge status input to the zero volt rail. So by using one resistor which is powered from the USB side, so it these LEDs can only light when it's charging.

the resistor limits the current and this red LED has a lower forward voltage than this green LED and by using that approach, when the unit is actually charging the Lithium cell, it pulls this to the zoo vote reel the red LED lights and it pulls the voltage here to two volts with respect to the Gear volt rail. So the green LED can't light. When it ends charging, it turns off the red LED The current goes straight through that resistor in the green LED and it lights the green one and that shows it's charged. There is a 2K resistor for the setting the current approximately 500 milliamps.

I would guess there's a Lithium cell with no protection which is slightly unfortunate. Um, because it seems to go down to the point this process cuts off round about say just about two volts. That's not great. The processor has a decoupling capacitor across it and and it's got that button pulling to the zero volt reel.

It can control each of the red, white, yellow, and blue LEDs and they've just got a single 20 Ohm resistor. The reason for that is that only one is ever lit so they can sure that resistor I can tell you the currents afterwards as well. Here is the standard Npn transistor that is turning on the main LED via this one Ohm resistor. and that's the bit that had me perplexed because I thought that if if these lines can drive these LEDs at quite High currents like the red one drives with that resistor at about 40 milliamps, what is it putting into a 0.6 volt diode? Junction It turns out that that pin because what I did I took it apart I removed the LED and I powered it up from a 4.2 volt Supply and just click the button once to turn that on.

The Crescent current draws 26 milliamps. So this may be an LED driving output that has been used to. Actually, it's like enabled as Led mode and then it puts out just twin current limited Supply in this case, 26 milliamps to the transistor, so that's not bad I Thought for a while that that might have been passing a lot of current through that. It would be interesting changing that to an E2shb mosfet.

I can't find them I've got a roll of them somewhere I've got about a thousand mosfets somewhere I don't know where they are, they're hiding. That's the worst thing about surface mount components. A current 4.2 volts is 360 milliamps, of which that part of that will be the 26 milliamps base current. Uh, at low, it's 103 milliamps rare to 37 milliamps flashing red 18 milliamps, so that's presumably just a 50 duty cycle.

The police mode alternating biops and forwards 15 milliamps weight 24 milliamps, which is quite good. That's okay for that. Led flashing white 11 milliamps and yellow is 20 milliamps. The red seems to take a lot more current at 37 milliamps because it's got that lower forward voltage down at 3.6 volt which is roughly half the battery status.

Um, it's uh, 210 milliamps at full weight 60 milliamps low weight red 26 milliamps Um. flashing red 12 milliamps please 10 milliamps weight 13 milliamps flashing weight 7 milliamps and the yellow is 11 milliamps So very quickly from a fuel cell, it drops down to below about 20 milliamps for the white. LED I Didn't bother going much lower I've just measured at what point it kind of cut out with the red on it. Cut about 2.3 volts I Thought the red was going to be the lowest Ford voltage.

Now in reality, it's going to be the main white. uh, one with that diode drop effect. the junction Drop the Base drop of 0.6 volts. So it does potentially drain the battery down below sensible levels.

But if you're using this as a flashlight, uh, torch, whatever you want to call it, uh, at that point it's going to be very dim. It's the only really an issue if you just left it on continually accidentally, which you could do and that is more or less it. It's an interesting thing. it's not too hard to get back in.

just make sure you've got this little rubber bun turned out the way Exact blocks the USB port green, but when you actually push it in, it seems to align at the back. Um, the lithium cell holds up. There are no guide reels, but it does seem to go on to pegs at the back I think and that keeps the back in position. and at the front, it's got the round LED to keep it up.

so that's what keeps it all aligned. but it does go in quite easily Clicks in and then you can just push this rubber button back in again and that's it. So uh, it's an interesting little light. It's odd I think I'd Rather have seen a mosfet there, but it makes me wonder if I'd put a mosfet there because it's not pull down resistors on it.

Um, is there a risk that, uh, if the battery went completely low to the point the process cut out is the risk that when you powered it back up again, the mosfet could be an unknown state. It could be partially turned on. It could be trying to light that Led. Uh, I Don't think that would really have an issue I Think the voltage drives fast enough.

The processor would kick in as soon as it initialized. It would Define that as an output and then pull it low. Uh, as soon as it initialized. So that could be uh, e2shb mosfet I'm pretty sure that would work quite well and would mean that the losses in the unit it would actually make it better I wonder why they didn't do that in the first place? Money probably.

But there we go. It's a neat enough little light and it does have functionality. You could change these LEDs if you wanted to different ones there. any colors you wanted, but it looks quite smart and it actually works quite well.

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