# Thread: Standlight circuits (Will this circuit work?)

1. ## Re: Will this circuit work?

Thanks for taking so much time to explain things to a newbie. Here's a couple more possible numpty questions.
Would it be possible to dump all the circuitry going into Stdn and put the switch between the battery and ground instead?
What made you choose a capacitor over a battery?
Re the switch; instead of a 10 minute timer, would it be possible to use a very low power transistor to switch it off when Vcc went below 0.8v. It would probably need some sort of logic where it went off if the dynamo was off and the standlight was off, then it goes off, and only when the dynamo starts, does it comes back to life. (not really thought this through yet). Might there still be a tiny drain on the battery?

Nick

2. ## Re: Will this circuit work?

Originally Posted by nicknoxx
Thanks for taking so much time to explain things to a newbie. Here's a couple more possible numpty questions.
Would it be possible to dump all the circuitry going into Stdn and put the switch between the battery and ground instead?
It would be possible, but I'm not sure that it'll work out well. You'll be draining the battery at the same time that you're charging it. If you have a lot of stops, such as in city riding, it might not be possible to keep charge in the battery. Also, if it's cold, it's harder to charge the battery. I commute in temperatures down to about 2F, so this is important to me.

Originally Posted by nicknoxx
What made you choose a capacitor over a battery?
you mean a battery over a cap? The rationale is in the thread on standlights, but the short story is that it gave me a much longer run time, which I liked. I tried a moderately sized supercap, and it was just awful by comparison. The supercap was a bit larger than the AA nicad.

Originally Posted by nicknoxx
Re the switch; instead of a 10 minute timer, would it be possible to use a very low power transistor to switch it off when Vcc went below 0.8v. It would probably need some sort of logic where it went off if the dynamo was off and the standlight was off, then it goes off, and only when the dynamo starts, does it comes back to life. (not really thought this through yet). Might there still be a tiny drain on the battery?

Nick
Everything uses power. It's just a matter of how much! Even the Zetex is using some power in shutdown mode, but since it's CMOS circuitry, it's only a few uA. It's a lot less than the nicad's self discharge current, so it's not a problem. The other trick is making a circuit that works down to 0.8v. You can't buy a discrete mosfet that turns on with a Vgs of 0.8v or less. You could use BJT's, but they'll use a lot of current. Maybe use a CMOS comparator?

I think I'd rather try the timer circuit scheme. It turns out that it's Linear Technology that has the family of handy timers.
http://www.linear.com/products/timerblox
You might be able to build a low power 10 minute timer with this stuff. The LTC6991 oscillator can be set up with a period of 9 hours, so it can do a 10 minute pulse with no problem. Unfortunately, it needs 2.25v to run. Maybe charge a large ceramic cap with the forward voltage from one of the LEDs, and use that to power it? Lots of possibilities....

regards,

Steve K.

3. ## Re: Standlight circuits (Will this circuit work?)

Steve mentions his Zetex based circuit which continues to look intriguing. When he first described it I wrote to Zetex (or whatever the company is called here in Australia) and they kindly sent me a few of the tiny things to try out. I still haven't done anything with them - I think their small size and my fumbly fingers deters me!

But for those wanting a relatively simple but effective light, I'd like to put in another plug for FrontRanger's design. I am so pleased with it! I have now built 3 twin-LED lights using cheap DX parts and this circuit and they all work well. Using a 150ohm resistor and a 1F 'button' supercap the circuit keeps the 2nd LED running at a decent brightness for 3-4 minutes at least. If you stick to cheap DX parts and a simple build you can get a great light for very little money. I just use simple circuit board with straight line copper strips.

I'm about to build my 4th light, another twin but this time using 40mm 'U' channel and the Ledil Rocket smooth spot lenses. My aim is for the whole lense column on the standlight side to be illuminated and visible from the sides. The electronics will be housed inside a bit of sealed celluloid 'chip holder' tube that most recycling places have plenty of and will be sheltered form the weather by the light's simple handle-bar bracket which is made from light alloy angle. I'll post some pictures if someone can explain to me how to do it - I've tried a couple of times without success!

Last week my Mr-11 triple dyno light developed a leak in the rain and started working intermittently. So I figured it was time to bite the bullet and install an e-delux that I'd had as a 'spare' on the shelf for a while. I'd bought it quite cheaply as although it was new it had a very short cord and no one seemed to want it. I'd forgotten what a bright beam it has! There's a darkish tunnel I ride through on the way to work. While the MR11 triple is pretty bright at night, it's a fairly diffuse beam and hardly shows in this tunnel where there's still ambient light from each end. The e-delux by comparison put a bright spot on the ground as soon as I was in the entrance. Which is handy as people leave all sorts of rubbish lying around inside!

Sam P.

4. ## Re: Standlight circuits (Will this circuit work?)

I've been looking further into the ZXSC310. To calculate the value of the sense resistor you divide the sense voltage by your chosen peak current. The trouble is that while the typical sense voltage is 19mV the tolerance allows it to be anywhere between 14 and 24mV which basically means you have to built it, measure it and adjust the resistor depending on the tolerances in your particular circuit. If you've built and tested a circuit and wanted to re-make it with surface mount components you'd have to start over.

5. ## Re: Standlight circuits (Will this circuit work?)

Steve, I read the old battery charging thread with some fascination. Thinking this might be for my next dyno build.

From the Flickr Circuit diagram: "I thought it was all self-explanatory?? Other than the "missing pulse detector" part, of course. I have to sketch out the waveforms to keep track of what's supposed to happen. Fortunately, it works, so I can forget all about it again.
Maybe I can add some comment blocks here on Flickr to explain the functional blocks?"

I vote 'Yes' Steve. The block info would be good to bridge for us with a lack of electronics background. (For those of us who have rudimentary knowledge of component function and can solder bits, but don't understand circuit design and are oscilloscope-less.) A lot to learn here or relearn. No need in both places. Just post here when/if you do it with the link repeated. Thanks.

6. ## Re: Standlight circuits (Will this circuit work?)

the schematic on my Flickr page has been updated with comments about the various functional blocks. The comments on the "missing pulse detector" block are more or less adequate, but it's not a bad idea to dig out the datasheet for the dual on-shot IC and make sure that it all makes sense.

Everything else seems pretty straight forward (to me).

The comment about the 'scope-less folks does remind me that a scope can be essential when trying to figure out why things aren't working quite right. As the various versions of my schematic indicate, I was having some trouble with the fairly simple task of shorting out two LEDs at low speeds. The scope let me see that there was some weird oscillation going on that was messing up the operation at some speeds. I never did understand the oscillation, but decided to just simplify the mosfet driver.

The scope is also very useful in making you realize that, despite how much you may think that the circuit ought to behave a certain way, the circuit doesn't care what you think! A scope is a great way to discover what is really going on in a circuit.

regards,

Steve K.

7. ## Re: Standlight circuits (Will this circuit work?)

regarding tolerances in the ZXSC310: the tolerance issue isn't unique to this little switching converter. An essential part of good engineering is to keep track of all tolerances (and there are a lot!) and know what the effect is on the circuit's operation. Honestly, if you look at the ZXSC310 datasheet, you'll see that the output varies quite a bit as the input voltage changes. It's not much of a regulator at all, but it's still pretty handy for a standlight. The eye isn't good at detecting small changes in light, so it shouldn't make much difference to the perceived brightness.

regards,

Steve K.

8. ## Re: Standlight circuits (Will this circuit work?)

Thanks for the info, Steve.

Looks like I should have bought and built that Heathkit 'scope decades ago! If you can measure it, then it is science and engineering; if you are conjecturing it, then it is wishful thinking. I guess with interest compounded, the money I saved would buy many better ones, now. Some tools are essential to some tasks so you either ante up or fold. Plus a SON hub for the Mercian. Have enough on the bike light plate at the moment, but it helps me plan a strategy for the future.

BTW I think you said AA 700 mAhr NiMH. I have AAA's of 700 and 1000 mAhr. So why a AA not a AAA if they are the same capacity?

9. ## Re: Standlight circuits (Will this circuit work?)

Steve, using the component values you have, what speed do the extra two LEDs kick in?

10. ## Re: Standlight circuits (Will this circuit work?)

I'm using a portable Tektronix scope that probably dates from the ... mid to late 80's? It's a good, basic analog scope with 2 channels. Bought it used 7 or 8 years ago for \$200. Not a bad investment, although I don't use it all that much.

The battery is a AA nicad, not NiMH. The nicad is more tolerant of abuse and temperature extremes than NiMH. An AAA nicad would probably be fine too, since the AA keeps the standlight running for 30 minutes or more. It might even be interesting to try a cell from a 9v nicad, although those are getting much harder to find nowadays.

Steve K.

11. ## Re: Standlight circuits (Will this circuit work?)

umm... I don't know what speed it is that the two extra LEDs are switched in <insert sheepish face here>.
I've just been manually spinning my front wheel in a trueing stand to test the circuit, so I couldn't tell the speed.

I think I did try the circuit with the bike in the workstand.. but I don't know what the speedometer said then. Besides, it spins down so fast that any speedometer reading would be suspect.

My goal is to have the threshold around 12mph or so, just so I can avoid blinding people on the trails and stuff. I also picked the current values such that I didn't see the LEDs dim when the extra pair was switched in.

My plan is to retrofit the circuit into the light starting around May or so, figuring that that'll be when I quit needing the lights on my commute. Hopefully I'll have it done before the days start getting short again.

Steve K.

12. ## Re: Standlight circuits (Will this circuit work?)

You can get 280mAh NiCd 1.2V button shape cells on ebay for peanuts or tiny ones (15mm x 6mm) with 70mAh.

13. ## Re: Standlight circuits (Will this circuit work?)

Not quite a standlight question but... Tonight, having been banished to the bike shed, I was rummaging around in the parts box(es) and discovered an old Niterider 19-led tail light from a 12V halogen (I think) system from a few years ago. I never possessed the headlights or battery and only have the tail light because I found it on sale for a few dollars in a dusty bike shop in Adelaide. I think I liked the sealed case it was in and thought I might be able to turn it into something a dynamo could run. I recall taking it apart with this in mind but gave up on discovering it was all tiny surface mount stuff with lots of tiny resistors etc. I presumed at the time that the, given the plethora of resistors, all of the leds were running in parallel.
Anyway I had my SON dynamo wheel set up in a fork on the bench and thought 'why not'... Connected it up and gave the wheel a spin and presto, very bright tail light!
So apart from the fact that it's probably only running on half-wave, is there any real reason why I shouldn't connect it up permanently on one of my 'dynamo powered' bikes? Would things be appreciably brighter if I used a proper full wave rectifier? Anyone ever figure out how these leds are arranged?
thanks,
Sam P.

14. ## Re: Standlight circuits (Will this circuit work?)

is there any way that you can apply a limited DC current to the light and measure the voltage across the light? That should tell you (roughly) how it is wired.

If the taillight was designed to work with a 12v light, then it's unlikely to be a good match for a dynamo system. If you can figure out how the board is wired, you *might* be able to cut a few traces and install a few jumpers in order to make it work with the dynamo. The trick will be making it do this while not impacting the current that is delivered to the headlight.

My personal solution to this issue was to wire the taillight in series with the headlight, and build the taillight as a number of red LEDs wired in parallel. As long as you don't mind the loss of some slow speed performance, it works quite well. It's especially well suited for use with an LED headlight that provides good light at low speeds.

regards,

Steve K.

15. ## Re: Will this circuit work?

I was thinking that you could simplify this circuit by replacing LM317 with an LM7805 5 volt linear regulator. The capacitor could only charge to 5 v, but it eliminates the issue with matching resistors, or even having the reference resistors at all, and avoids mixing and matching clamping diodes. Any reason why this wouldn't work? Could it be spimplified further by removing the transistor and just having the 150 Ohm resistor (like the first circuit suggestion)?

It's been a pleasure reading this forum, thanks for all the work that you've all shared.

16. ## Re: Will this circuit work?

I don't see any reason that a 7805 couldn't be used instead of the LM317.
I don't recall the issue of clamping diodes, or the mixing and matching thereof, at least with regards to this circuit.

The transistor switch could be removed too, with some reduction of the run time of the standlight... or is it just that the supercap will take longer to charge? Maybe the latter.

Looking at the circuit again, I'm wondering about the headroom voltage required by the LM317 (or 7805) and the Vf of the LEDs. I normally assume a Vf of 3.5v, which allows 2v of headroom for a 5v regulator. No problem there. However, I was looking at the datasheet for a Cree XP-G, which is around 3.1v at 500mA, and this would only create 6.2v at the 7805 input. That might not be enough to produce 5v at the output. I'm guessing that it might be close enough, since the current passing through the regulator as the supercap reaches full charge will be small (13mA, with the 150 ohms between the supercap and LED).

All in all, it seems like it should be okay. While I have a preference for somewhat complicated circuits, there's a lot to be said for simplicity, especially when the person assembling it has limited experience. Every additional component is another opportunity for a bad solder joint, wiring failure, etc.

regards,

Steve K.

17. ## Re: Will this circuit work?

Originally Posted by Steve K
The transistor switch could be removed too, with some reduction of the run time of the standlight... or is it just that the supercap will take longer to charge? Maybe the latter.
I'm thinking I should keep the transistor now, and the main reason for having it in my mind is the increased efficiency from cutting off the current path through the regulator and resistor, which allows the use of a lower current limiting resistor and a brighter standlight. Here's my new question: Can we elimminate this resistor altogether and just rely on the transistor's current gain to control current through the LED? If there's a gain of around 30, you could let 3 mA through the base with a 1.7 kΩ resistor and (theoretically) get 100mA through the emitter, tailing off from there. Am I making incorrect assumptions because I'm new to this?

Originally Posted by Steve K
Looking at the circuit again, I'm wondering about the headroom voltage required by the LM317 (or 7805) and the Vf of the LEDs. I normally assume a Vf of 3.5v, which allows 2v of headroom for a 5v regulator. No problem there. However, I was looking at the datasheet for a Cree XP-G, which is around 3.1v at 500mA, and this would only create 6.2v at the 7805 input. That might not be enough to produce 5v at the output. I'm guessing that it might be close enough, since the current passing through the regulator as the supercap reaches full charge will be small (13mA, with the 150 ohms between the supercap and LED).
-looks like you need another LED in your headlight! I'm thinking of wiring a taillight in series with the two headlights, which should give me more than enough headroom.

Thanks for all your help and great ideas!
-Andrew

18. ## Re: Will this circuit work?

Originally Posted by eärendil
.... Here's my new question: Can we elimminate this resistor altogether and just rely on the transistor's current gain to control current through the LED? If there's a gain of around 30, you could let 3 mA through the base with a 1.7 kΩ resistor and (theoretically) get 100mA through the emitter, tailing off from there. Am I making incorrect assumptions because I'm new to this?
-Andrew
Using the transistor in the linear mode might be adequate. I think the downsides would be that the gain might vary over temperature, and the base current will change as the supercap discharges. The resistor should provide more predictable behavior, but there's no harm in experimenting with it. I don't think there's any way that you could damage anything (he said optimistically. )

regards,
Steve K.

19. ## Re: Will this circuit work?

Not that I'm trying to revive anything here but I thought this little addition to this thread might be worthwhile. It's the schematic for the B&M Standlight Plus. I guess the relevance is that this circuit is probably the most widely used rear standlight in existence and is probably used repeatedly across many ubiquitous brandnames in Europe. I cribbed it from a German website a few years ago. I've often wondered how I might adapt it to run, say 8-10 rear leds with maybe 2 as the standlight. The B&M is designed to run in parallel with a front light, each light having its own bridge rectifier. Anyway, here's the circuit and a parts list:

R1 220 Ohm R2 100 Ohm
C1 1 F, 5.5V
D1, D2, D3 1N4001 etc

Rectifier B80 1500R etc

LED1, LED2, LED3 superbright LED, 3000 mcd, 5 mm, ca. 1.6v, 20 mA

Any ideas about developing this further into, say, a 'Super Toplight' will be read avidly!

Sam.

20. ## Re: Will this circuit work?

I've got something I sketched out quick... I'll post the image later, but it's pretty simple. I just replaced R1 with a simple two-transistor current regulator and another LED. This improves the amount of light produced with no increase in power consumed.
http://www.flickr.com/photos/kurtsj0...in/photostream

What sort of improvement were you looking for, and what are you willing to give up in order to get it?

regards,
Steve K.

21. ## Re: Will this circuit work?

Originally Posted by Steve K
What sort of improvement were you looking for, and what are you willing to give up in order to get it?
Hi Steve,

To the limited extent to which I have thought about it, I would be seeking something that:
- could be connected in parallel with the front light without robbing it of power (there are counter arguments to this parallel arrangement of course)
- was much brighter - both on the main tail light and the stand light - than the stock B&M affair which I find pretty minimal.

I have built both 6 led and 10 led rear lights (parallel led strings in series with the front and with a reverse direction diode, much as you use) and have been impressed with the results. I'm now just trying to figure out how that performance could be married with a standlight. The front light will be a simple double or triple and I anticipate that there will be plenty of current to spare from the dynamo.

I know that you prefer to use an active circuit of some sort and I appreciate the increased efficiencies gained by that approach. However with a 2 X led front light I have had very acceptable results with Frontranger's (or was it Bandgap's) simple diode clamping feed to a supercap with a bleed resistor feeding the standlight led. It seems to me that in the Toplight circuit above diodes D1-3 perform this same clamping function, the major difference from Frontranger's circuit appearing to be that the two LED 'sets' are in parallel rather than in series.

I have some high powered red superflux leds, supposedly rated at 100ma each (I wonder if this is for real). I have though of substituting these for the Toplight circuit, trying 2 in parallel for the main rear light and another 2 in parallel for the stand light. In parallel strings I am assuming they would draw 100ma for each side of the circuit. Does this mean there would be 300ma or more left for the front light? Or would the front and rear lights need to be in series for this 'balanced' sharing of current? You can see I'm grappling with some fairly basic concepts here...

This notwithstanding, if you feel there's a better way of doing this I'm interested! As you pointed out a couple of months ago, it's getting colder down here, winter evenings bring on creative thoughts etc...

Sam.

22. ## Re: Standlight circuits (Will this circuit work?)

The basic limitation of a parallel taillight is that any power that you allocate for the taillight is power that you take away from the headlight. You can compensate by using a headlight that has a high impedance, so it draws less than 0.5A and produces a voltage over 6v at its terminals. Properly designed, a buck regulator could be used to power the headlight LEDs to produce this behavior.

Otherwise.... you are limited to basically 6v and whatever current you want to steal from the headlight. The classic taillight is budgeted to get 100mA, so that could be a guideline if you want to make a commercial product.

If you are just trying to reduce losses in the B&M design, you'll want to find ways to get rid of the resistors and those rectifier diodes.
- you could use a mosfet bridge to replace the diode bridge rectifier.
- use a more sophisticated current regulator instead of just putting a 220ohm resistor in series with the primary LED. My two transistor design is pretty basic, but is an improvement. I've got a slightly more complicated design that would allow another series LED... just need to find the schematic in my files.
...here it is...
http://www.flickr.com/photos/kurtsj0...in/photostream
- use the little Zetex boost converter to drive the standlight LEDs. This makes better (more complete) use of the energy stored in the supercap.
- use better, more efficient LEDs.

those are the basics. It's up to you to decide how complex you are willing to make the circuit in order to get improvements. You'll be hard pressed to get as much light from this sort of design as you can get with the series taillight that I prefer.

regards,
Steve K.

23. ## Re: Standlight circuits (Will this circuit work?)

Thanks Steve for that explanation. Given that the eDelux and other Grman dynamo lights all seem to operate satisfactorily with the Toplight in parallel, I assume that they are all high-impedance as you outline. I understand that the eDelux at least (and I think the preceding halogen E6) has some form of regulation circuit.
Incidentally I should have properly referenced the original source of the Toplight schematic. I can't edit the post above so I'll clarify here - it came from Stefan Brix's site at: http://www.brix.de/elektrik/toplight-umbau.html

Both of the circuits you show appear to be significant improvements on the existing Toplight, which I presume was designed to either a budget or to a fairly basic level of conspicuity. Actually, I was checking mine out last night as it's getting dark early here now and the 2 little leds in the existing standlight are quite bright. I think my main concern is that the main running light is just one 5mm led. I guess the Toplight was designed for the Eu utilitarian cycling situation and some years ago at that. Our expectations of a rear light have grown somewhat, especially if you need to ride on main roads.

Comparing your two circuits to the original Toplight I'm starting to understand that improving it cannot simply be done by fiddling with resistors etc. The main issues seem to revolve around:
- optimal capacitor charging and...
- optimal regulation of discharge.

Unfortunately I think the opportunity to master active circuitry has passed me by although the 2 transistor design you've posted adds a second led to the main light and looks simple enough for me (and it may fit inside the Toplight case too which was my original objective) so I'll give that a go. Actually, the more I think about it, the more it seems that the whole issue of having a standlight may be more significant from the front light than the rear, especially given the crash stats posted on the Niteflux thread. Which lends weight to the simple idea of simply having a rear multi-led (or single power-led) series-driven light with maybe a supplementary flasher when stationary. Time I think for me to get the breadboard out and test some of these ideas out.

Thanks again,

Sam.

24. ## Re: Standlight circuits (Will this circuit work?)

Here's a doodle based on something posted by Znomit a while ago. All I've done is omit the capacitor boost and draw in an extra led string in parallel. As per the sub text of this thread, I'd like to know "will this work?"

Sam.

25. ## Re: Standlight circuits (Will this circuit work?)

I'm not sure what specific red LEDs you plan to use, but if they have a lower Vf than the white LEDs, you'll significantly reduce the brightness of the white LEDs. I think you'll need something to limit the current to the red LEDs, either a simple resistor or a current regulator circuit.

what's the function of the diode labeled "voltage clamp"? It looks familiar, but I can't recall exactly what it is for. It would provide quick charging of the supercaps and prevent.... well, I was going to say that it would prevent discharging the cap through the two white LEDs, but it won't be charged enough to put much current through the two white LEDs in series for long. It doesn't clamp the charge voltage of the supercap either.... unless you get lucky and the Vf of the white LEDs is only 3v or so (I'm assuming that the supercap is rated for 5.5v). Maybe 2 or 3 diodes in series are required?

Steve K.

26. ## Re: Standlight circuits (Will this circuit work?)

Thanks Steve,

Apologies for not remembering the 'lowest Vf in parallel' issue. You have pointed this out to me before. I wondered why this seemed too-straightforward-a-solution to having both front and rear standlights! This is why putting different leds in series solves such problems eh!

I understand that the diode marked 'voltage clamp' is there to reduce charging voltage to something the 1f capacitor can tolerate. Actually I was lazy and drew only one - Znomit and others actually used more than one, even a combination of Si and Schottky to get the drop right so charging is optimal. I think the resistor varies feed to the 2nd led to optimise brightness and run time.

Anyway, as you note it is the likelihood of differing Vf between the red and white strings that is at fault here.
What I thought was a flash of brilliance has faded... thanks for the reality check ;-(

Sam

27. ## Re: Standlight circuits (Will this circuit work?)

For a couple of reasons you'll need a resistor under the word "white" to limit the current to the red end.
It'll keep the white LEDs running.
It'll limit the current to the red LEDs. 500mA through two red LEDs is crazy bright.
You might want another red LED in there too for efficiency as 3 red ~ 2 white.

For simplicity you could just put a single red LED and resistor in parallel to the bottom white LED.

28. ## Re: Standlight circuits (Will this circuit work?)

Hi I have been studying this thread with some interest, I am also wanting to build a dyno light with significant stand time, as much as 5 minutes for a time consuming street crossing on my commute. My bike is also used for touring and having good light even if I am crawling or walking up a hill is needed. I am leaning more towards charging NiMH AAA since I have a quantity of cheap cells on hand. Below I have drawn Martin's circuit #7 with a battery and resistor in parallel with my LEDs. I was thinking that 7 AAAs would be appropriate to match the 9v drop of the LEDs, the batteries would approach 1.3 V fully charged, which is below the 1.4 I see hot off the charger. A 2ohm resistor in series with the battery would limit the stand light current to a value that should still provide good visibility. Is there any reason this would not work, other than possibly shortening the life of the cells. If it matters I will be using a Sanyo dyno hub.

Thanks

From Jul 1, 2011">

29. ## Re: Standlight circuits (Will this circuit work?)

I'd like to claim 5 minutes, and probably longer, with my front and rear set up. I don't use batteries or crazy high power LEDs, but what I lose in ultimate performance, I make up for in cost and simplicity to build.

The relevant photostream is here, plus new photos of front lights working
http://www.flickr.com/photos/jdp298/...7626884852454/

With plenty of words of description.

It's been so successful I did the same build on mother's old bike, which has an old and inefficient Sturmey-Archery half-hub dynamo. Results are just as good, bright lights and long lasting when you stop.

30. ## Re: Standlight circuits (Will this circuit work?)

Originally Posted by bfromcolo
... I am also wanting to build a dyno light with significant stand time, as much as 5 minutes for a time consuming street crossing on my commute. My bike is also used for touring and having good light even if I am crawling or walking up a hill is needed. I am leaning more towards charging NiMH AAA since I have a quantity of cheap cells on hand. Below I have drawn Martin's circuit #7 with a battery and resistor in parallel with my LEDs. I was thinking that 7 AAAs would be appropriate to match the 9v drop of the LEDs, the batteries would approach 1.3 V fully charged, which is below the 1.4 I see hot off the charger. A 2ohm resistor in series with the battery would limit the stand light current to a value that should still provide good visibility. Is there any reason this would not work, other than possibly shortening the life of the cells. ...
I'd feel better about that circuit if there wasn't a switch between the battery and the LEDs. If that switch is open while the dynamo is running, then there's nothing to limit the charging voltage, which would kill the batteries before too long. Presumably, the switch is just there to allow the light to be turned off while the bike is stopped. Many years ago, I had a simple design that used 5 nicad AA's in series that would charge while the light was off, and provide light when the light was switched on. It "worked", but the constant charging at 0.5A killed the nicads in less than a year.

My standlight design uses a nicad AA that is charged from the voltage drop across a diode, which is the same principle as yours. I favor the NiCad because it can tolerate continued trickle charging better than NiMH. My NiCad has survived many years like this, so I guess it's not too bad of a method.
I would use something a bit lower than 1.3v, just to be conservative. My AA nicad will keep my standlight running for more than 30 minutes (it's not all that bright), so even though it's not fully charged, it is sufficient.

regards,
Steve K.

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