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Tags: hardware diy led power-efficiency extreme
Clipped on: 2017-07-25

V2.2 Release

A project log for TritiLED

Multi-year always-on LED replacements for gaseous tritium light sources

Image (Asset 1/13) alt=

UPDATE 20170716: fixed library and PCB to produce correct solder stencil

UPDATE 20170713: added more info about the battery holder and inductor substitutions, with links to the original parts, and info about the case screws.

UPDATE 20170712: added assembly instructions

UPDATE 20170712: added the 3D printed case design



I should have done this months ago - V2.2 fixed the reversed MOSFET of V2.1, and is basically a complete design, so here it is.

I was going to re-do this in KiCAD, but I got frustrated with it again (third or fourth time), and just paid the $100 for a years worth of Eagle. The hardware and software design is in the GitHub repo.

You can order TritiLED V2.2 boards and V2.x programmer boards from OSH Park. If you have an SOIC-8 programming socket, an SOIC-8 test clip, or don't mind temporarily soldering wires to the ICSP pads on the boards for programming, you don't need the programmer boards - they just let you connect the ICSP lines with pogo pins.

The V2.2 circuit adds a 100-ohm resistor in the battery supply before the reverse-protection MOSFET as discussed in a previous log:

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I need to add a BOM in the GitHub repo, but here are the components listed out

The inductor and battery retainer are substitutions from the original parts I used. The battery retainer is a slight upgrade, being gold instead of nickel. The original nickel part is also available:

(1) Linx BAT-HLD-001-TR CR2032 battery holder. DigKey part # BAT-HLD-001-TRCT-ND $0.73 each.

I have not received any of the gold battery holders yet; from the datasheets they appear to be exactly the same except for finish material.

The inductor is simply more expensive, but with the same size and specs: the old one is not stocked at DigiKey anymore. They are in-stock at Mouser:

(1) Bourns SRR6028-102Y 1mH inductor Mouser part #652-SRR6028-102Y $0.68 each

Again, I haven't tried the Wurth inductor on boards yet, but they appear to be identical parts from the datasheets.

This adds up to $8.35 in materials for single-unit quantities (battery not included: add $0.29 for that). If you build 100 of them, you can get it down to around $6.27 (batteries included :) The older inductor was only $0.68, but DigiKey has 0 in stock with a 22-week lead time - that would save almost $1. The MOSFETs could probably be substituted with cheaper parts, too.

You could save some money by removing the P-channel MOSFET. Reversing the battery might harm the PIC then, but wouldn't cause anything to overheat because of the 100R resistor. You could substitute a shunt diode as a reverse-battery protector, too. After the resistor, it would conduct with a reversed battery to spare the rest of the circuit. You just need a diode with low reverse leakage - even the 1N4148 has a 50uA maximum leakage! The B-E junction of BJTs is supposed to make a better diode for this kind of thing. Still, maybe you shave 10% off the parts costs, not that impressive.

Assembly Instructions

Here's where all the parts go on the board. There are only a few you can screw up: the two MOSFETs, and the 1206 R and C, so watch out for those.

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Programming

You can check out this other log for ways to program the PIC. You need a programmer (PICkit3's are available cheap on ebay), and some way to attach to the IC or board.

Experimental Case

I put together an experimental case design in OpenSCAD. A case is important to protect users and the environment from the battery. Without a case, the battery might become loose and be a hazard to children or pets, or the contacts could short on nearby conductors. You must not leave these devices anywhere as markers if the battery is not secured in an inaccessible manner. The case uses screws to keep the battery where it belongs.

Here is what it looks like in OpenSCAD. You can see the details better in the rendering, because I usually print them in black, which doesn't photograph well:

Image (Asset 4/13) alt=

Moving Forward

The V2.2 design works well, and I think 5 years on a CR2032 is probably "good enough." What I'd like to see is smaller and/or cheaper designs. I can imagine forking the design to move in both directions: a cost-is-no-object tiny version, and a cost-reduced, but maybe bulkier one.

Moving back to a chip-on-board LED would help with the size, although the Luxeons I like (Z and C) are $2.74 each in single quantities.

Using a smaller inductor would reduce size and cost and increase efficiency. The problem is that the PIC, in its lower-power 500 kHz oscillator mode, isn't fast enough to produce the short current pulses required of low-value inductors.

New Architecture

What I'd like to try next is using the PIC running at 31kHz, which is the lowest-power clock it can do, still waking from sleep periodically using the WDT. This should dramatically reduce the PIC overhead current drain. To shape the short pulses for the MOSFET drive, I'll try using the 74AUP1G14/differentiator trick from the previous log. That part of the circuit had negligible current drain even with a 3V supply. Moving to an external pulse shaper will allow shorter current pulses than the PIC can do at low clock speeds. Shorter current pulses will, in turn, allow lower valued inductors which are cheaper, more efficient, and physically smaller. In this way, you still retain the flexibility of a programmable part but may be able to reduce the current overhead.

I think I feel a V3.0 coming.

Previous Log

CMOS Relaxation Oscillator Fail

07/10/2017 at 03:13 23 comments

Discussions

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Image (Asset 5/13) alt=I was looking for external nanopower WDT to solve the issue of the WDT oscillator consumption. It's already MADE to toggle a MOSFET at specified intervals, the pulse duration is programmable, and other than the startup resistance read, consumption is 35 na.

Importantly, singles are under a buck at digikey.


EDIT: I realize the 100 ms minimum time is an issue with the flicker fusion rate. I'm wondering if at 35na one could run either two or a signal delay with another FET to the inductor.

 

 

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Yup. What I'm hoping is that it's designed for enough speed to get short-width pulses out of it. The power usage can be mitigated quite a bit by disabling the comparator for almost all the time.

The PIC12LF1571 datasheet spec's 4.2uA typical @ 3V for the comparator in its low-power mode, and 21 in normal (faster) mode. If you enable it for (say) 100us every 16ms, that's only an additional ~26 or ~131 nA current draw for the peripheral (depending on mode).

> Reducing frequency will reduce its power consumption

While awake. But most of the time is spent asleep, with the osc stopped, waiting for the WDT to wake the PIC. When awake, we only run for a few (dozens?) clock cycles anyway. The issue I raised is that it takes 100x as long to awaken using the LF osc compared to the MF osc, which might offset the ~20x power reduction.

 

 

 

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The longevity of the push button. Thoughts on adding ONE more component, but dynamically adjust the light level to compensate for ambient. You automatically shutdown in brightness, too. I'm just thinking on the elimination of moving parts.
Capacitance switch?
Photo-interrupt, with the LED as the light source?
A hole to a phototransistor, AND with the back pulse to the LED, if the hole is covered during a flash, counts it as a button press. Timeout for a "debounce" of sorts.

I've had too many expensive electronics (like night vision devices) break because the entire thing was tanklike and built to last, EXCEPT the damn switch.

I have one monocular that had its IR illuminator switch replaced by the abort button from a clean agent system we decommissioned. That sucker had been in service for 30 years, and with a good cleaning, should last 30 more.

 

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Image (Asset 13/13) alt=I again forget the human element. Damn, imperfect humans!

 

Michael Stoiko wrote 07/12/2017 at 21:50

The other option that wouldn't require any changes at all would be a hall effect switch. We use those for the test button on industrial smoke detectors, along with a hefty delay, to prevent accidental use, and to cut down on mechanical failure on a device that needs to be in harsh environments, last a long time, AND be tested multiple times a year. 

 

Ted Yapo wrote 07/12/2017 at 22:29

Oh, in case you didn't see it buried in one of the older logs - you can substitute an IR LED on these boards.  I have tried 730nm, 850nm, and 940nm.  The 730 can be seen as a dull red glow by the naked eye, while the 850 and 940 are not visible.  The 850 looks very bright in a gen 1 intensifier - it's a very nice IR-only marker.  The 940 is much less bright.

 

Michael Stoiko wrote 07/12/2017 at 23:29

Sounds good.

Now i'm wondering, were I to make a an IR beacon, and if we are
ALREADY conceding to a whole microcontroller, why not do the pleasing
slow pulse rather than regular blinking? Modify the timer each time. The only thing is that could be a LOT of red/write cycles internally, unless you use a matrix of times and move the pointer around or something. I still think that cool lighthouse effect is an interesting visual to draw attention.

I'm still waiting on inductors, but I have a few board designs. I'm realizing if I really want to get into the weeds I need to build an automated LED testing rig to use my huge pile of LEDs effectively. So I'm waiting on digital pots, too. Starting from your excellent writeup.
Sorry to spam on here with vicarious building. 

I'm wondering if a second factor could also be explored;
measuring the current+voltage delivered in the circuit versus the
frequency. Optimize the power consumption by the LED, inductor, and MCU.
There has to be a point where parasitic losses drop due to resonance.



I have a few LED backlights that really put out the lumens in a distributed fashion, I'm trying to work out a diffuser so I could remove the directionality from the testing. I have a hunch that some of these backlights already are being run very efficiently, and with a larger visual area might make a very good marker for larger items.



I've got about 5 different (eagle) iterations of boards at this point, due to my hodgepodge of parts in my garage. I will need to do the proverbial "write drunk, edit sober" culling of these, though, as many were attempts to get a fleeting thought out.

I still can't build ANYTHING to test, waiting on parts AND I need to fix my fluke, which suddenly and unexpectedly died.

I'm also still digging through what nanopower devices I can find, and trying to see if I can construct a low frequency oscillator with the lowest possible power consumption.

It might also be useful for boosting voltage in my other solar projects, as those energy harvesting chips are still way too expensive to use in low cost applications.

 

Ted Yapo wrote 07/13/2017 at 02:01

@Michael Stoiko Sure, you could make it pulse - you could probably just unroll your whole loop since most of the flash is unused currently: just repeat the code for  (pulse, re-write WDT timeout, sleep) over and over.  This might be tedious to write, but more efficient than some sort of loop and lookup table.

If you just need to characterize a few LEDs, you can do it manually with a few meters and a light sensor.  It's a bit tedious, but it works.

It sounds like you have some good ideas to try - it will be interesting to see what you come up with...

 

Simon Merrett wrote 07/12/2017 at 07:23

Crikey Ted, I didn't see that ending coming! 

Well done on reaching this milestone - there has been a fair amount of new people interested in #TritiLED and this will probably be the stimulus for several of them to dive in and develop it further. 

I feel your frustration with KiCAD but I'm determined to get to grips with it now that Eagle is tightening up. Everything takes longer so I'm just being more picky about what I want to commit to FR4. 

Now we just wait for TritiLEDs to appear ready-made on Aliexpress...

 

Michael Stoiko wrote 07/12/2017 at 20:16

Then we can hack them into all sorts of other useful timing circuits, and exploit the price break!

 

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