Yootoo SD2 host info and modding

This is a compact 21700 light with a tailswitch. It is inexpensive, well built, and handsome, so I made this thread as a general info thread for reference and discussion. First I’ll have some pictures and dimensions, then in the next post I’ll show some mods that I did. Most of the discussion so far has been in this thread.

Stock the light comes with an SST20. The mode currents measured with a clamp meter were 2.6A, 0.75A, and 0.18A.


I’m not sure what the intended function of the retaining ring is. It is flimsy and not secured tightly. I suppose it would loosely retain the driver if it wasn’t already glued in. It might also function to space out the battery tube a bit, effectively lengthening the battery compartment. The battery compartment length is pretty tight; an LG M50T just barely fits with the driver spring compressed all the way. To remove the driver I removed the LED MCPCB and pushed the driver out using tweezers. Does anyone know what type of driver this is? Its diameter is a large 23.5mm so it is challenging to fit different drivers in there securely and with good heat transfer.


The driver cavity depth is only ~3.5mm. The shelf is thick at 3-4mm. The thermal path is very direct, and since there is not a lot of thermal mass the light heats up relatively quickly. The shelf surface is anodized, but flat.

The reflector OD is 26.4mm. The reflecting diameter is 24.5mm. The height is 18.4mm. The emitter hole is 7mm. Unfortunately it looks like the inner few mm of the reflector is misshapen. It looks like the surface is too vertical. I have seen a similar problem in the emisar D1 reflector. The beam is still good; it just means the inner couple mm of reflecting surface is not used.


The retaining ring is left-hand threaded. The tail spring is low profile and looks to be nice and thick. I measured the resistance of the tail spring and switch and got very low values. The spring and switch each had around 4mOhms, measured at 5A, for a total of 8mOhms, which is very low. I thought this was too low a resistance for a switch, but I measured multiple times and that value seems correct.

This host has some nice characteristics which made me choose it for my night riding helmet and bar light pair: 21700 cell, compact while having a good sized reflector, and lightweight. But the unusual driver size and shallow driver cavity have made the mods challenging. See the next post for those two mods.

My night rides are typically ~50 minutes of riding on curvy rocky rooty trails in the woods. I had been using a pair of S2+s at 3A each, which were adequate, but I wanted to boost the brightness some. I wanted to keep the lights self-contained rather than having a separate battery pack just for simplicity. So my goal was to have the brightest 1hr runtime mode in a flashlight small and light enough to use comfortably. I tried some 26650 lights I had (F13 at 260g w/cell, on my helmet) and the extra weight was very noticeable on my head so I wanted to go lighter than that. So I decided to look at 21700 lights and the SD2 satisfied my requirements. It weighs 162g w/M50T cell, only 40g heavier than a loaded S2+. I modded one light with a SST40 for my helmet and one with a XHP70.2 for my handlebars.

The 5000K SST40 in the SD2 reflector keeps approximately the same beam size as the sliced SST40 in the S2+. I used a convoy configurable linear driver with a low resistance low profile spring from mtnelectronics. To fit the driver in the 23.5mm driver cavity I used the 17-to-22mm convoy brass driver adapters. I actually used the threaded retaining ring that comes with the adapter because it is slightly larger diameter than the adapter itself. With some reaming and filing of the retaining ring I got the 17mm driver to fit with its top surface flush. This was important because the battery compartment length is tight. I soldered some small spacers made from folded copper wire on the driver adapter to add even more space to the battery compartment so the cell fits without being crushed.

I run the SST40 at 4.3A and the beam intensity is 20.5kcd. The S2+ with sliced SST40 at 3.04A (8x7135) had 14kcd. The output, measured by integrating sphere, was boosted by 48%. It should be regulated at 4.3A until probably around 3Ah into the discharge (out of ~4.6Ah of the cell). So not perfect regulation, but it won’t drop very drastically for most of the last 1.5Ah. The 2hr mode at 2.3A would be regulated for most all the discharge.

For my handlebar light I decided to use a 5000K XHP70.2 P2 from kaidomain. I have found that for the off-road riding I do I don’t need a very intense beam from my handlebars; the helmet light takes care of spotting the trail far out and around corners. So I decided to maximize output with a wider beam. The driver cavity depth is only 3.5mm which is shorter than most boost drivers so I had to modify the 22mm FX35 boost driver from kaidomain. I replaced the inductor with a shorter inductor of the same inductance. I have some more details of the driver mod here. The inductor height was just right so I could thermally sink the top of the inductor to the bottom of the shelf using some thermal paste. I decreased the output current to 2.2A (6V) using a R056 sense resistor to get the right runtime.

The battery tube just barely made contact with the driver ground ring so I added more copper wire spacers both to improve the contact and to increase the battery compartment length. Again I used a low resistance spring from mtnelectronics. The bottom left picture shows the driver sitting on the battery tube which shows how those spacers are making contact with the tube. This is not ideal for heat transfer from the driver, but without a lathe it’s difficult to find/make adaptors/pieces that fit.
The reflector opening was just large enough to fit the XHP70.2 dome. I did some slight shaving of the dome around the edges so there was no binding. I removed the silicone and phosphor to the side of the dome so the reflector could sit as low as possible, but those exposed metal surfaces are connected to LED positive and negative so I had to insulate those from the reflector. I used an adhesive tough plastic insulating sticker I got from mtnelectronics. I cut the inner hole to just fit over the dome.

In my S2+ I had a sliced 3V XHP50.2. The SD2 with XHP70.2 beam is a bit wider than the S2+ with sliced XHP50.2. I’m very happy with the tint and beam. There is a slight hotspot, but the corona extends all the way out to the edge of the spill and the beam looks very smooth which is what I want for the handlebar light. Throw is about 6.5kcd on high, compared to 5.5kcd of the sliced XHP50.2 in the S2+. Output was boosted by 48%. Power consumption is about 14.2W so this will give me a bit over an hour on high.

Thanks for the pics and info. I also picked up the SD2 and for the price it’s a bargain. The runtime with a 21700 is impressive and it has enough thermal mass that it takes a while to heat up. I also like its styling. I plan to swap emitter with something warmer, high CRI and it’s definitely a nice 21700 host.

I have three of these. Two were intended for modding with rngwn’s GT40 leds, but indeed I found out that the driver cavity is way too shallow for a boost driver. Not sure what to do with them now, they are still nice hosts.

The driver looks like a basic FET driver with some current limiting resistance.

The host “thickness” seems like a “digivolution” from the Amutorch AX1, that also has little space for other drivers (taller).
As far as I perceive, Amutorch/YooToo drivers are efficient, although their high low modes are a bit too high for my preferences.

Thanks for the information on this! :+1:
I also saw recently some info (maybe size comparison only :person_facepalming: ) of the SD1 with Convoy S2+ and…it is smaller than the S2+ ! So…maybe there are som modding potential of these lights :stuck_out_tongue:

Hey djozz, check out the short inductor mod I did above to get the boost driver to fit. Maybe you can use those 12V LEDs afterall.

I missed your post somehow, so thanks! I will look into that and may try it. :+1:

I was going to take the lights on their first trail ride but the trails were unexpectedly closed. Instead I did a street ride to get an idea of the temperatures they’d get to. Ambient temperature was 80F. Had the lights on high for about 12 minutes, but the temperatures seemed to have equilibrated probably about 5 minutes in. SST40 light head temperature was between 120F and 125F, and the XHP70 light was between 110F and 115F depending on how fast I was riding and any breeze. Temperature was measured with a little IR thermometer.

80F is probably the hottest ambient temperature the lights would be used in, so they should be fine as long as I don’t stop for too long.

I swapped out the LED with SST-20 4000k FA3 bin from Intl-Outdoor. The reflector works great with this LED and makes a well defined spot and an even spill.

I agree with this. The reflector is a really good match for a domed SST20. Creates a broad (for this emitter/throw capability) and clean hotspot. I’ve swapped in a 5000K LH351D from MTN, and solder bridged the sense resistors. Running about 5.3A on a fully charged ‘Tesla’ Vapcell IIRC. I gotta say I just don’t like it as much with this emitter. Probably going to swap an SST20 back in it eventually. The girlfriend currently has this light hostage though…

Awesome host though! Well done and for the ~12 dollars that I paid for it, EASILY the best value I’ve ever gotten.

Thanks for that info. The modes still work the same?

It seems like there is some regulation going on. This comment is about the SD1 but I assume the drivers are similar. I’ve not seen any regulation/runtime graphs except for a short one in flashaholics review video that shows what looks like a timed turbo stepdown.

I have (had?) a video of the light connected to DC supply and sweeping through 2.8-4.2V or so. I don’t remember it being regulated. The driver sure is weird though. Mode spacing stayed reasonable, though did get a bit tighter visually. Meaning it’s now medium high turbo IMO. This really helped the tint of the sst20 though.

I’ll see if I can find the any notes I had from the modding.

My measurements for throw were something like 27kcd with the resistor modded sst20 and 16kcd with the lh351. I’m going to try slicing the 351 if I ever get it back lol

Edit: posted a first impressions/review a while back…

I see, thanks. So it looks like those are just current limiting resistors, as djozz suspected. Good to know you can just bridge them and at least get a decent FET driver.

Sd2 (stock) turbo voltage sweep Sd2 (stock) turbo voltage sweep - Album on Imgur

Here is that video…

very interesting light, build and use case. thanks for sharing it with us…

This light is currently on FLASHSale at BG… FYI

Could someone please point out the sense resistors to me?

They are the resistors marked R9 and R10 in the picture of the PCB shown in the OP.

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Installed a P8000 22mm Kaidomain 8A driver and a 5000K SFT40.


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Is it better than Convoy’s buck 8A driver?