Attiny25/45/85 FW Development Thread

Problem spotted!!!

I have OTC cap and R1 turned 90º, connecting B+ and ground with 19k resistor, and PIN 2 and 7 with OTC cap :person_facepalming:

I can’t test repositioning them now as i am with family celebrating new year, but it is pretty evident that it is the problem.

This morning, measuring resistance as Rick told me, i got really odd data, but i thought in bad joints; but now i’ve read Tom having the same problem in tow boards with only the desing in common (and the same board as me), i’ve decided to visualy examine the board and bingo!.

Thanks all of you for the help and happy new year!!!

Well, 2 of us will have a Happy New Year Laughing.


From post #582 above, the reason for the low amps is a broken bond wire - it's hard to see it because of the silicone around it, but pretty sure it's broken. Got a clean dedomed U4 1C, so probably go with that.


Measuring resistance is always a good thing to do. It will help pin-point the problems quicker as it did for you. Kiss that DMM happy new year to it too, it earned it.

Consider a loupe if you soldering often. My loupe is a tool I use very often. I have two, a 10x one (plastic) and a 30X glass one.

It is satisfying when hard work pays off!

Congrats and happy new year!


I haven't been able to keep up with you all as I haven't had any mod time for few months now. I see Tom E mentioned doubling the main cap. I believe it is common for most smd caps to lose like 82% of their capacity as voltage approaches rated capacity. I've been using 50v X7R 10uF's and haven't had any of the issues noted above yet. I'm not saying the caps I use are the reason. It could because I haven't had a chance to try out the latest FW versions. Sorry if this doesn't help. I feel way out of the loop these days.

Hhmm, I gotta check - pretty sure I got mine at Digikey or Mouser - whatever was recommended from day one.

TK posted in the firmware thread she seemed to have the same problem, same fix. Not sure - are those 50v rated ones better than what comfy recommended wayyy early on? Pretty sure we always used X7R's though... Hhmm, I'll check it all out.

Update: looks like I bought them from DigiKey and they are 16v X7R's. I shouldn't be going over 4.25v in these builds, so I'd think they are safe?


Wish I knew the answer. I think CC recommended 16v rated caps and it does seem they would be adequate. I only use 50v caps because I regularly build 4S lights.

Perhaps 10uF is barely enough and then the combination of heat and some voltage (4v) is enough to bring capacitance lower than is needed.

That depends upon the dielectric type and the physical size of the cap. Smaller caps (for the same capacitance value) use dielectric materials that rend to be more susceptible to the effect… i.e. an 0805 X7R cap usually performs worse than a 1206 cap.


Good to know. Thanks texaspyro. :)

Also, we know there's a spike in this design, think from comfy's tests on a scope and think Richard confirmed, and the 13A can tolerate it, but we've been suspecting the 25/45/85 is more sensitive to it. On the 85's w/Narsil, the 0.1 uF cap across gnd and Vcc would fix it, but I thought that didn't seem to work on the 25 w/OTC firmware - I had it on and took it off. Ohh - I can still program the MCU's btw with the SOIC-8 clip with that cap jumper on the MCU.

In Spain it’s a tradition to get presents from the “Three Magi” on 6th of January, so here is mine for you.

It is designed so that, in case it is needed, C2 could be easily stacked over diode and C1.

If there is something you think could be improved, let me know.

I’m far from beeing an expert at this circuit/electronics stuff, but may I make some suggestions:

-in my mind this thread deserves getting sticky’d, I think it’s important atm (the more people see it the more people could contribute)

-would be nice if everyone that wants to share his experiences would give detailled informations on:

  • if there are issues, if so, what kind of issues
  • the board that’s been used
  • MCU that’s been used
  • the FET that’s been used
  • what LED and how many
  • what firmware and which version
  • fuses that’s been used (start up time could have some influence on the pre-flash issue for example)
  • what you have tried to fix the issues you’ve experienced - any changes?

EDIT: by the way: thanks Tom for sharing your experiences and coming up with solutions, great work! May I ask what start-up time you had set, 64ms?

I've built a ton of well working 85's and a couple of 25's now, so I could certainly do this.

We are eager to know about your experiences :slight_smile:

I’ve yet to run into problems, but I’ve only built one driver.

wights FET+1 Board
1x XM-L2
bistro rev. 165
4ms start-up delay

Only thing that’s not working for me is the moon mode, it just doesn’t light up. Will try reflashing tomorrow to see if it’s just too low of a PWM-value.

Tom, did you manage to build a fully functional triple with a 25/85 driver yet? From your experience with that one issue not showing using a weak cell, I’d think the more amps we push through the FET the more likely we’ll see weird behavior… Also, did you always use SIR800DP as FET? Again, thanks for all the insights.

Hhmm, everything I got and used has been posted here, maybe not all summed up in one spot though.

The fuses are documented in the beginning of the source code, and also in the t85fuses.bat file, also shared on the google doc link (fuse values tell you the start-up time being used).

Google Drive share for Narsil

If that doesn't work, please let me know. Atmel fuse calculator here:

I'm @work now, so don't have access to all my resources though, and don't have the time here either.

I use the SIR800DP FET exclusively now.

For a triple? Yes - in a SupFire M6 that I upgraded. Also had enough problems with a 219C and a high amp single LED, and solved the problems.

For your moon mode problem, are you using a 380 mAh 7135? If so, you need a higher value PWM. You need maybe a min value of 4-5 for 380's, for 350's, a PWM value of 3 always worked for me, using PHASE mode of course, not FAST.

I ensure now I use only 350 mAh 7135's in all FET+1 drivers.

Thank you very much Tom. I’ve been reading through all different types of threads concerning driving a FET properly, but having major problems understanding everything and not confusing stuff. Even the “old” threads with great inputs from comfychair, PilotPTK, wight, JackCY and many more. What a pity most of these guys left and comfy’s pics are all gone… I’m glad that you’re still here!

ATM, I only have tiny25’s here, so my plan was to illuminate the bistro with a triple XP-L HI. :smiley:

But with those issues reported especially with big amps, I thought I wouldn’t be able to get it running properly. Heck, I’ll just give it a try.

EDIT: yes, i was using 380mA AMC’s. Thanks for the advice

Hhmm, Mitko led me to believe the FET+1 wight drivers were maybe not performing so well, so my last driver mod/build was using a 25 on a single FET driver, and found the "fix" was to stack a 2nd 10 uF cap on the regular C1 cap (doubled up).

Didn't use Bistro, but believe I tried it and had the same problem before the fix, so I think the double up cap would work fine for Bistro. I'm thinking now, Mitko might have run across an issue with using firmware not supporting FET+1's well, where you don't get the max amps you can from the FET, so think it was a firmware issue, not hardware issue of the driver.

So it's weird, but, I got two fixes I've been using - a 0.01 uF cap across the grnd and VCC pins of the MCU (on FET+1's using Narsil), or stacked C1 caps (straight FET driver running my own firmware for a power switch light) or the equivalent with a 22 uF cap instead of the 10 uF used for C1.

Also the 12K resistor across the FET gate worked great to 100% eliminate the flicker going from hi to moon on a power switch setup using a Tiny25 with FET only driver.

Thanks again for taking your time and sharing very useful information Tom.

I played with my driver today, swapped the 380mA AMC for a 350mA one, still no moon. Had to adjust the values. I wonder why this driver doesn’t show any of the issues that’s been talked about. Maybe I’ll encounter some when I hook it up to a triple. It’s still driving a single XM-L2 now. I am also going to test a longer start up time fuse setting to see if that’ll make any difference.