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Status and ordering PCB and parts for solar power for low current projects... or... (several other circuits!)

Includes buffer 'battery' to carry you thought the night, of cloudy spells, and watchdog timer to fix brownout lock ups (filename: PCB269-SolarWBackup-StatusAndOrder.htm)

Please note: This page will not always be 100% up to date.

At the time of writing (02 Dec 17), an assembled, "working" instance of PCB 269 exits! And two further copies of that version of the PCB are on hand.

At the time of writing, I also have three copies of a second version of the PCB on order, which should be available to me from 14 December 17. I also have small stocks of most of the components for the board, including a "ready to use" ATtiny85 for one instances of PCB 269. (The ATtiny85 provides the circuit's watchdog feature... and if you don't need that feature, yet, the board works fine without the ATtiny.)

PCB 269 is an implementation of a circuit by Nick Gammon. It is for providing power to low demand Arduino (or other) systems. There's a "battery" (supercap... avoids the evil nature of lipos) to carry the system over times with no sun. There's a watchdog to break lockups which can arise from slowly rising voltages. The PCB 269 main page gives you all the details.

Great news? Not expensive! Probably about $10 for "bits and pieces, including the PCB. To this, you need to add a supercap... $5?... and a solar panel or two...$5?. Plus of course whatever it was you wanted 24x7 power for.

So much for the good news!

Assembled Solar power supply unit with watchdog

I'm just a hobbyist. I don't look to make money out of this, nor am I geared up to be a fully professional distributor. I do hope to help fellow hobbyists, but this isn't "my day job". And the whole thing is a bit "chicken and egg". I'm not going to order a dozen copies of the PCB to have a good stock on hand, when I've had, so far, no indication that one person wants one, let alone a dozen.

Bottom line: SERIOUSLY interested in BUYING bits... write me! I'll give you a personal and up to date update. Major suppliers or contributors to forums known to me, who could test, evaluate, etc... I would supply you a free copy of the board, and what parts I happen to have, on a promise of evaluation work, and return or purchase of the board and parts on conclusion of the work.

See PCB 269's main page for the "definitive" "word" on most issues. This page is meant (and working towards!) being just about stock levels and ordering.

Project status

A unit has been assembled, and tested. Works as expected. Exhaustively tested, under all scenarios? No, of course not. But basic "stuff" seems to work as advertised. There was a flaw in the silkscreen for the first version of the board, "ver 17b06"... it told you to insert Q3 "backwards" (180 degrees rotated from what is right), but otherwise the board was "good".

The biggest "problem" with this lovely solution: It won't power very much "all through the night". In fact, so far, I've not got it to power very much over an interruption of Vin of more than about 10 minutes. But there are several avenues which can be explored to see if this can be "fixed". Or maybe my hopes are too high for this simple, elegant, and ROBUST "answer" to getting power with a public power supply is not available. I only designed the PCB. The circuit came from Nick Gammon, whom all Arduino enthusiasts should learn about, if they don't know of him already. He had the circuit driving an optimized Arduino, taking weather sensor readings, and sending them to a base station over a radio link. PCB 269 implements only the power supply section of his work, but the page cited a moment ago goes into the details of the rest, too.

Also remember that if you "just" want a little circuit to see low demand systems across short dropouts arising when a public power supply fails and a standby generator kicks in, then PCB 269 is working well enough already!

Versions of the board

(This merely the highlights, but comprehensive as to existing versions. See main page for full details.

Ver 17b06: Three copies. Will probably never be more. "Ok", except for the silkscreen for Q3 being "backwards". (For an ST LE33CZ-TR)

v 17b28: Three copies on order. As yet untested, but this was just a minor "baby step" forward. Tried to fix Q3 problem. (I say "tried" only because the new board is, 02 Dec 17, still untested.) Added pads to make it even easier (never hard!) to fit the Gammon-specified MCP1700 as Q3. Moved "stuff" around a bit. Added a few links for disabling parts of the circuit when they're not needed. Added LED and resistor to indicate Vin present.

Bits and pieces

None of the things that go in this board are terribly expensive, but don't you hate having to place an order with a supplier for one resistor?

Suppliers, of course, are not charities. It is only reasonable if they put a p&p charge on small orders. If there is sufficient interest in PCB 269, i will be quite happy to make up and supply kits of parts. I can already help my first few "customers". Eventually, if the interest is there, I will work up something more satisfactory. For the moment- email me, tell me your wants, and a bit about how much experience you have (how much hand holding you will need) and I'll tell you what I can do at the time you email.

The programmed and fused ATtiny85 for the watchdog feature is likely to be the biggest problem for both of us. If you can "deal with" sourcing and programming and fusing your own, it will be a big help. I do, present time, have "one and a half(!)" available, though.


I would be, by a wide margin, most happy with payment by Paypal, "gift to friend", from a bank account. Yes- it leaves you (and me!) bereft of some protections. We're not talking a lot of money here, people! (I'd consider other variations on the Paypal scheme, especially if YOU paid the charges, if that's an option.) Or a check/ cheque, US dollars or British pounds. Not so good, but possible. Again, I am particularly happy to consider variations on my proposals for people "know to me" from their presence in forums, especially the main Arduino forum. In any case... you are the "customer"... in a limited way. Tell me what "works" for you?


When I went to order parts, to keep my life simple, I made some substitutions. I can't promise that they were valid substitutions! As I mention elsewhere: you are the final inspector.

1) For D1, a 1N752 was specified. I used a BZX55C5v6. (You can get the spec sheet for this, and many other things, from RapidOnline.com's website. For part numbers, see "Parts list" section (which is in a separate page, which opens in a new window or tab, if use the link. Just close that to return here.)

2) For C1, the supercap, I used a 1F, 5v5 Panasonic EECF5R5U105. Note that the pads on the PCB have been done to facilitate adaptation for whatever supercap you decide to use. I'm not sure that the voltage rating of the one I used is as high as it should be for this circuit.

This next section became recursive. You can skip my manic ponderings, and come back to them later, if you wish?...

Very late in the day, I discovered a disturbing little line in the datasheet for the supercap I had purchased. It recommended "1 mA or less" as a typical discharge current for the supercap. I couldn't find a "max safe" rating. "I read on the internet" that supercaps can typically endure "high" charge/ discharge currents... so I'm hoping that my supercap is actually more capable that the rather short datasheet would suggest. If you go supercap shopping, I hope you can find one with a stated greater current limit. Or that the circuits you want to power use very little current!

If you read the fine print in the rather limited Panasonic datasheet, you find: "Recommended discharge current: 1mA or less"... but that qualified with a footnote saying "The recommended discharge current is a reference value. Please design your equipment (circuit) in consideration of IR drop."... the IR given in the datasheet as "less than 30 ohms at 1 kHz." I think they are saying this thing will provide high currents unless there is resistance in the external circuits. Fine. But without a max power dissipation rating, we're somewhat in the dark as to how low we can let the external resistance be before problems arise. Panasonic see the supercap as a good source of voltage to back up memory circuits... fine! And THEY would need little current. But what about our wants? I hope you have better luck in your supercap shopping!

Further work on supercap selection: I have never been so frustrated in searching for a datasheet!

"Supercap" is a bit of a nickname. "Electric double layer capacitors" may turn up things that supercap won't.

I finally found one for a supercap that "came clean" on permissible discharge, thank you Farnell! Their VEC6R0 504QG - Supercapacitor, 0.5 F, 6 V from Vinatech costs £5.99 (Nov 17, plus p&p, etc) and the 6v rating helps make it "good" for PCB 269, even if the "mere" 0.5F is a little disappointing. (It should be noted that supercaps, unlike, say, lipos, discharge in a linear manner... they don't maintain their voltages for large parts of the discharge cycle, and then drop suddenly.) The datasheet says that this supercap may be discharged at a rate of 1A, and suggests that would bring the voltage on the cap down to 3 volts in one second. Not an operating scenario we are looking for! But reassurance that our much more modest discharge current wants are acceptable to the device!

MORE on supercap shopping!... Hurrah... the Vishay datasheet for their 196 HVC ENYCAP supercaps gave me "max discharge current"s (MDC) of 25mA to 3A for their range, along with recommended charge currents. The MDCs varied with the capacitance of the supercap in question. They come in 5v6 and 7v0 spec'd versions. As discussed elsewhere, 5v6 may be sailing a bit close to the wind, but may be okay. 7v0 will be more expensive... but you can get a whopping 15F, 7v0 Vishay MAL219691215E3 from RS Components for £11! If I read the datasheet correctly, the max charge current for that is 50mA, max discharge 70mA... but with this, we encounter a new issue: "Lowest discharge voltage". "Permanent operation below [4v0] is not permitted"! What happens if you DO discharge further? Could another zener diode somewhere "fix" things? Sigh! (There are multiple ideas here. But if you "disconnect" the supercap unless the voltage it provides is at least 4v0, there may be implications for how the watchdog does its thing. (I haven't thought this one through yet, but it would need thinking through. Or maybe I just worry too much?)

3) For C4, I used a slightly larger capacitor, 490uF instead of the specified 440uF.

4) My "most serious" "mistake", as far as I know: For Q3 (the regulator supplying 3V3), I used an STLE33CZ-TR... because it was "the one I could get"... instead of the specified MCP1700. Someone who knows about these things reviewed my choice, and, no promises, thought it might be okay, though a little less efficient. I.e. the MBP1700 wastes less electricity, might make a circuit slightly better at making it though a "no sun" period than the circuit using the regulator I used. See further comments in "Specific Board Notes". (That's part of a separate page, and so will open a new tab or window. Just close it to come back here.)

Parts List

Here, in one tidy place, a list of what you need. With part numbers, if you choose to order from RapidOnline.com's website, which happens to be in the UK. If you compile a similar list for a major supplier for customers elsewhere, it would be much appreciated if you were to email it to me, for inclusion here.

I am hoping, eventually, to offer kits of parts.

You need....

1) The PCB. Eventually, likely to be available from OSH Park. For now, email me if you are willing to suffer along with other early adopters. If you have a reputation at forum.arduino.cc, or similar- so much the better. Tell me your forum name in your email. (I am "tkbyd", there.)

2) A programmed (including some fuses) ATtiny. I am using ATtiny85s. The board has a place for an 8 pin DIL package. I don't know of a source of programmed ATtinys at this time, but programming your own, and setting the fuses, is not, it seems... though I've yet to try it!... hard. The devices cost very little... less than $3.

3) Hardware:

a) A socket for an 8 pin DIL.
b) A 5-way block of screw terminals for attachment to a PCB-
RapidOnline part number 21-4478 (65p... all prices excl p&p, and good at 11/17)
(Called "P1" in first version of board, P5 thereafter.)
c) (From the boards "v17b28") Optional: 2 pairs of 0.1" separated pins to make two selectable links, L1, L2.
d) The solar panels! (If you are using solar as the ultimate source of power. PCB 269 can also be used as a "bridge" to carry circuits across switch-overs from mains power to generator backup.) I can't give you as much advice as I would like on this front. 6v panels would seem to be "right". Which is about as helpful as asking a bookseller for a "red" book! (More on this later, I hope! For now, buy something cheap via eBay or similar, promising 6v!) The panels used with the original of this are no longer available, but I'm sure replacements abound. They cost $25, and were described as follows...

    Power Output: ~6V @ 1W (~160 mA)
    Output Type: DC Voltage
    Dimensions: 100mm / 3.94" x 198.43mm / 7.81" x 0.77mm / 0.03"
    Operating temp range: +32 to +158°F (0 to +70°C)
    Weight: 27.93g

4) And the rest....

D1: Zener diode (See substitutions)
Spec'd: 1N752. I used BZX55C5V6
RapidOnline part number 47-3016 (6p)(< (price, in pence, i.e. hundredths of GBP (£))

D2, D3: 1N4148 diode
RapidOnline part number 47-3309 (1p ea)
(Discussion at https://jeelabs.org/2012/05/14/forward-voltage-drop-on-a-diode/ suggests that a BAT43 might be a better choice next time. Thoughts welcome! Note that this would mean voltage applied to supercap would be very near the maximum rating, for the "5v5" supercap at one time spec'd elsewhere, here

(Optional) D4: small LED
(From v17b28). To indicate power being available from the solar panels, or whatever primary source you are using.

C1: Supercap (See substitutions)
Spec'd: 0.47uF. I used 1 F, 5V5.
RapidOnline part number 11-2170 (190 (i.e. £1.90))
(See discussion here of D2, D3) See also notes in the "substitution" section. The Rapid Online 11-2170 may be "special" in not being suited to "high" discharge currents. (The datasheet recommends "1mA or less" as typical discharge usage. Remember: C1 is going to be supplying the current flowing through the (nominally) "5v" output of PCB 269, and powering the ATtiny, and supplying the voltage to be regulated down to 3v3.)

C2, C3: 1uF caps
RapidOnline part number 11-3425 (4p each)

C4: Capacitor (electrolytic)(See substitutions)
Spec'd: 440uF. I used 490uF.
RapidOnline part number 11-3513 (6p ea.)

C5: 0.1uF cap
RapidOnline part number 51-8686 (20p)

R1: 220R resistor. The first batch of boards was done with a footprint for a 0.125W resistor here. That was too small. A 0.25W resistor should be used.)

RapidOnline part number (TBD)

R2: 150R resistor, 0.125W
RapidOnline part number 62-0350 (1p)

R3, R4: 10k resistor, 0.125W
RapidOnline part number 64-0098 (1p each)

(Optional) R5: resistor, 0.125W
(From v17b28)- Something suitable for LED D4. Simply indicates power coming into the board. Will "waste" a little power while power is coming in, but draws no power when the system is running from the supercap.

Q1: Mosfet transistor 2N7000
RapidOnline part number 47-0180 (10p)

Q2: ATtiny85 microprocessor

You will need to program it, and set fuses. An ATtiny85 would be "perfect"... others may suit... if in 8 pin DIL package. RapidOnline part number TBD... I bought my first ATtiny's from Hobbytronics.co.uk

Q3: 3V3 voltage regulator (See substitutions)
Spec'd: MCP1700. I used ST LE33CZ-TR
RapidOnline part number 82-3010 (67)
Board v17b28: Beware error in silkscreen- see "Specific Board Notes". From board v17b28, alternate pads exist, to allow easy use of originally specified MCP1700. (It has different pinout.)


Eventually, the idea is to make the boards "freely" available. I'm not going to try to "make money" on this. I might initially release it via a Kickstarter project, or in partnership with, say, Sparkfun, ModernDevice, Hobbytronics.co.uk (if any of them would take it on!!) or similar... but only in hopes of bringing the board to a wider audience. Nick released the circuit without asking for any reward. My board merely implements his circuit, so it should be "free", too. Eventually, I expect that you will be able to order copies of the board directly from OSH Park, at cost. But I hope also to get in place an easy way for you to obtain a "kit of parts"... including, optionally, a programmed ATtiny, for those who want the optional watchdog timer reset part of the design. I say "free"... but that doesn't mean that I won't make a charge to cover my expenses, and give me some compensation for the overheads of keeping the parts on hand, my time "picking parts" to assemble the kits, and postage. Duh. Sorry!

The rest is scraps of "stuff" to be refined, when time allows. The offers are "good"... it is just the way they are presented that needs work!

Getting one: Even if I have no PCBs on hand, *IF* you don't mind supplying your own "bits and pieces" (although I MAY be able to help with some), and are willing to risk being "stuck" with TWO PCBs... though if I get a second order in time, I'll let you buy just one) I'd be willing to supply the PCB at this stage. Cost including p&p to UK or US address, airmail: $14 for two boards, $10 for one. Maybe a little less, if I'm able to avoid international mail in your case. If you are interested, contact me, so I can tell you current situation. (Parts avail, shipping times, etc. Include where you want them sent (at least what country) and how you intend to pay when you enquire... I will immediately order boards if we can agree terms, send off when I get them. Between ordering and receiving, I usually wait less than 2 weeks. I MIGHT, of course, have some on hand.)

It has to be said that the supercap solution will not, easily, give you a lot of battery backup. (For more on this, see my "Testing PCB 269" page)

The PCB discussed here, based on the Gammon circuit, gives you a power supply suitable for charging by a solar panel (though that's not the only option). (This voltage is called "Vin" in this discussion.) It has a super-cap to get you through the night, and a watchdog circuit to ensure smooth re-starts after instances of too many hours of too many clouds. There is an optional 3v3 output, regulated, and a second output to run a "5v" Arduino, not regulated. (This voltage is called "Vout" in this discussion.) (The Atmega 328 will run on voltages from 1.8 to 5.5) The limitations? The circuits you power must not draw much current... unless you can solve the issues relating to "big" supercaps, and unless you buy big solar panels. (But you can start with a $5 "wonder" from eBay, though!)

Another use of this circuit is to provide a "bridge" to get a system across that tiresome "gap" which some people with unreliable public electricity and emergency standby generators. When the public supply fails, there is often a brief moment of no power before the generator takes over. And another dropout when public power is restored. PCB 269 would be good for bridging the gap... you would just use an ordinary USB charger, powered by the public/ generator supplied volts, to supply PCB 269's Vin. For such short periods without power on Vin, you could power a more current hungry system without having Vout from PCB269 fall too low.

The circuit in question was originally designed to power, 24x7, without human assistance, a small data collecting system, which sent the data collected over a radio link to a "base station". Adding the radio-equipped Arduino would add about $15 to the project, at a rough guess. (All the details of that, too supplied at Nick Gammon's page.)

Nota Bene: I don't know much about some important issues relevant to this circuit, so please be particularly aware of my usual disclaimers. In particular, I have almost no experience of supercaps, and one is at the heart of this circuit. The good news is that the circuit was designed by someone who knows a lot. All I did was implement the circuit in a PCB. But along the way, I've made some substitutions, perhaps poor ones. (You can reverse my substitutions easily enough, if you can source the parts! Or if I buy some in, as I hope to.)

Be sure to read the "I read it on the internet" bit on PCB 269's main page about things you need to remember when working with supercaps.


Specific Board Notes

A word about "version" IDs...

I indicate version with bit of text in the silkscreen on the board. It is a date string in the format yymdd, where "m" is 1-9 for Jan-Sept, or a,b,c for Oct, Nov, Dec. The date string is prefixed with "vers", "ver" or "v".

The first batch of boards was "ver 17b06" (i.e. the version of 6 Nov 2017). Only three had been produced at time of writing; these are unlikely to be in circulation. The next batch was "v17b28".

Notes on version differences...

Ver 17b06 Remarkably "okay" for a first attempt! Thank you Nick Gammon, who designed the circuit) And KiCad, schematic and PCB design tool. (Good checking tools... but you have to use them!) Lacked the optional links to allow disabling things, lacked the "power present" LED and associated resistor (D4, R5)... and had a flaw in the silkscreen: You had to insert Q3 "the other way around", i.e. it's flat face needed to face away from the supercap (C1), contrary to what the silkscreen says. (The fact that by mistake a fitted a mosfet at Q3 was another "little glitch" to contend with in the first instance of the circuit! It did no harm, other than "failing" as a voltage regulator!). In this version, the 5 way strip of screw terminals was called P1. (P5, subsequently)

Second version of the board: v17b28 Many components were rotated 180 degrees, a few migrated great distances, in particular C5 now below Q2. But no fundamental changes to circuit.

Pads added so that at least two different voltage regulators can be used to generate the optional 3v3 supply. The silkscreen "D" marked Q3a should suit the part I mention in the substitutions section of this. The three pads marked less obviously marked "Q3b" should suit the MCP1700 specified in the original circuit design by Nick Gammon. I'm told that it would waste slightly(?) less of the charge you have in the supercap when the circuit is running from the power stored there. Please be careful here... I'm pretty sure that most things in this page are "right", but which legs of which voltage regulator go where is one area where I've already made a mistake once; do extra double checking here. (And let me know what you discover?)

Continuing the specific notes for v17b28 of the board...
The name of the 5 way strip of screw terminals became P5. (P1, previously.) P1 to P4 are pads which you can use if you add features to the program in Q2, the ATtiny85 providing the watchdog feature. I doubt you'll want to, but now the option is there.

Two pairs of pads, linked, as the board comes to you, can be used for pins, or for soldering in a small wire loop, to "turn on or off" two segments of the circuit. (The pins would be for a shorting link, for reconnecting the pads, if, after cutting the link in the copper track, you decided you wanted to re-enable the circuit element.)


The first link, labeled "L1", lower right, near Q3, the voltage regulator, lets you isolate the LED which can indicate the presence of Vin. I've picked out the relevant elements in a rather brash reddy/pink, in the illustration to the right. (This image reflects the PCB in v17b28... please pester me if the current version is significantly different.)

The second link, labeled "L2", extreme lower right, lets you take the voltage regulator providing 3v3 out of the circuit. I've picked out the relevant elements in lime green, in the illustration to the right. (This image reflects the PCB in v17b28... please pester me if the current version is significantly different.)

Pads for ground wire connector

Also near extreme lower right there are two pads on the ground track with no apparent function. I've picked them out in cyan in the illustration. If you wish, solder a small loop of bare wire into them to create a convenient place to hook the "ground" connection for your voltmeter to.

You are the final inspector

You are responsible for any consequences of using what is on any of my pages!

Please get in touch if you discover any flaws in the board, or any ways to go wrong. How are using it would also be of interest.

I would welcome news of any use you put the PCB to... especially if it comes with a photo, and permission to mention here. By all means give me with that any website, blog, etc, you want publicity for.

If you found this of interest, please mention in forums, give it a Facebook "like", Google "Plus", or whatever. I've almost given up writing these pages, because it seems they are seldom read, and of course not every reader will use them... so... is there any point? If you want more of this stuff, help!?

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