DC to DC Converter

Andrew and Morgaine,

It is kind of easy, but it all depends on how much electronics you're into. The trickiest part is if you want to be able to charge the batteries while using the R-Pi.

So, let's drill into it! Powering the R-Pi:

(1) 12 volt sealed cell battery.

This is the most easy one: a linear regulator (LM7805) with a heat dissipator and 2 bypass capacitors should do the trick.

(2) 2 to 4 cell lithium packs

It depends... with the packs in series  (2S or 2P2S) you have between 5.6V and 7.3V. In that case, the voltage is too low for a 7805.

The circuit is almost the same, but using a LP2954-5V or a LM2940CT-5.0, the last one being more common. A 2S2P pack would drain in around 1-2 hours.

(3) perhaps even a set of AA batteries.

I would go for 4 AA batteries and a LM2940CT-5.0. The problem is that they will discharge very quickly and the system will shutdown.

The most beautiful solution I can think of is a 2AA pack running thru a LT1302 boost converter (see the Minty Boost http://www.adafruit.com/products/14). The main problem is that they would drain in around 1 hour...

Thank you Rafael.  It looks like the AA solution is generally "out", unless it's "just for fun".  But I will check out the MintyBoost.  It sounds like fun.

I do have four packs of 2150mAh, 20C, 3 cell, 11.1v lipo packs.  Would one (or more in parallel) of these work with the LM7805ACT.LM7805ACT.?  Is there a cut off voltage?  I don't really want to kill the lipos on the first discharge.

For the 12volt sealed, it is a small one with 1.4 Ah.  However, I may graduate to something bigger.

I would like to charge the batteries while running the RPi.  I have a small solar unit that I was hoping to use to get me started.  (1.8 watts/ 125mAmps Max, current 125mAmps at 15v, I see 16 to 25 volts on a meter in full sunlight.)

I was going to ask about the solar charger in a separate post, but it seems the overall project should be considered.  For my first project, I want to run an RPi outdoors, with a web cam and motion detection software.  I would use IR at night.  I'd like to run it off battery at night and solar/battery during the day.  Later, I'd like to network it (WiFi, Bluetooth, or modem) with another RPi unit for image storage on a common drive. 

I don't have a clue how to calculate the overall drain.  While I used to be able to read a schematic, calculate loads, voltage drops over cabling, etc. that was MANY years ago.  Trust me, I've forgotten most everything except how to follow instructions :-)

If you will lend some guidance, it will be greatly appreciated.

Thanks in advance,

Andrew

Andrew,

Check Roger's solution! This is a better solution, as switched regulators are much more efficient. I didn't point it out, because designing one is not easy, but since we have them ready at ebay...

This converter is a step-down converter, so you must have an input voltage well above the 5V. Somewhere above 6V should work, as Roger pointed out.

As for the uninterruptable charging, the solar minty boost might shed some light. It uses only one cell and a step-up converter, so you might have to change it.

As for the endurance, let me explain how you calculate it:

1) The R-Pi uses 5V at 700mA, leading to a 3.5W power consumption

2) All your batteries offer 11.1V, with 6450mAh total charge (in parallel), which goes to 11.1*0.645=71.595Wh available energy

3) Dividing the available energy by the power consumption: 71.595/3.5=20.45h. But this implies in 100% efficience.

4) The converter has a minimum efficience of 85%, so the final endurance must be: 20.45*85% = 17.39h or 17h 23m 15s.

I think it might work for your application...

Your calculations miss that the 'pi uses only 440mA on average. So there is only 2.2W of power used by the 'pi. (on the other hand, a "worst case" calculation is also worth doing).

The 700mA minimum is including the 2x 100mA for the downstream USB ports. If you're not using anything there, the maximum drops to about 500mA. You'll be a bit closer to that than my 440mA measurement if you are running full-HD video decoding on the GPU..... which you probably won't as I don't see you powering an HDMI screen from that battery......

On the charging front, the currently suggested solar panels are too small. They would provide almost enough to power the 'pi in ideal conditions. But no margin to charge the batteries during daytime so that you can survive the night. You need about 3x more solar power to survive in summer and 5-10x (wild guess) more in winter.

I usually do the worst scenario, since I don't want to estimate 27h (using 440mA) or 24h (using 500mA) and be surprised by a low battery after 17h of usage, because a wifi adapter and a webcam went nuts!

Anything beyond 17h is profit! ;-)

BTW, I think that, if Andrew is using it with a webcam and a wireless link, he will need a powered hub! Which lead us to even more consumption...

Morgaine,

   Sometimes engineers can be very effective at marketting because

they are believed even when they should be challenged, just like a discount

store can be very profitable because shoppers tend to mistakenly assume that

all their prices are equally discounted.

   Eben is asked what the greatest challenge currently is, and says its ramping

up volume.  This may be true, but if unchallenged, tends to lead one to believe

that he thinks the design is solid and all that's necessary is more production. 

But I think this theory ought to be challenged.

Wow.  Thank you guys.

Okay, I was just going at this project laissez-faire, but with all this help and to be fair to you then I really should spec out the requirements and at least the high-level use cases.  That is not something I have done.  I've got a big project now at work and it may be several days before I can even find some free time at home to do this.  But let's keep the dialog going, as knowing what is reasonably possible allows me to write requirements that don't have to be revised for workarounds.  Anyway, let me know if I'm on track here...

• Solar Panel (will most likely need to increase the capacity).
• Minty Boost to charge LiPo batteries.
• LiPo batteries
• Battery Eliminator Circuit to prevent over discharging the LiPo batteries.
• L2596 module to control the voltage to the Circuits 1 & 2.
• Circuit 1 - PiPower to power the RPi (later wireless something).
• Circuit 2 - Other peripherals (at least the web cam  and probably also some IR LEDs for night).

Perhaps we need to consider a method that will allow the entire system to restart in the case where the batteries have discharged to the point of shutdown but then after time the solar has recharged the batteries enough to support operation again.  (I'll put this in the requirements.)

Roger, yes, I'll take you up on the beta PiPower.  But I'm thinking I should wait until we have the full strategy defined and parts identified before I start ordering.  What do you think?

Thanks again guys.  Aside from me needing to properly define the project and write the requirements, what are the next steps?

Regards,

Andrew

High level goals of the project -> strategy (what components). -> box diagram.

What are you planning for "power strategy"? Are you going to be able to conserve power by shutting down the 'pi for some lenghts of time?

Farnell's Q1 financial results are out (covering Feb-Apr)

http://www.premierfarnell.com/content/results-first-quarter-financial-year-ending-03-february-2013

RPi is mentioned under "Strategic Highlights" as bringing in new customers, but no mention of

bringing in profits.

• "Continued progress as a digital enterprise, with 2.7m visits in the quarter to our element14 community as it played an important role in the global launch of Raspberry Pi which will add in excess of 60,000 new customers for the second quarter."

Roger Wolff wrote: What are you planning for "power strategy"? Are you going to be able to conserve power by shutting down the 'pi for some lenghts of time?

Power is certainly something that will need to be considered.  The objective is to create yet a another motion monitor and recorder.  Rather than using a PIR, I was hoping that with the low power consumption of the RPi that I could use a web cam.  (The web cam and appropriate software allows for a much finer level of motion filtering than a PIR.)  Ultimately I want the unit networked with other similar nearby units and I want the 'primary' unit to be able to either receive calls or make cellular calls to transmit the images (or maybe with a cheaper cell phone at least indicate that images have been recorded).  However, I'm finding that I may indeed need to go with a PIR strategy that would let me turn off the RPi for extended periods.

Thought:  I could use 1 to 4 PIRs to wirelessly trigger the RPi to turn on.  That would achieve the level of control I need over what gets recorded and is more likely to only capture warm bodies (which is good).  Having somewhere around a 100 yard trigger range would be very good.  But this brings another element into the project.  I don't even know if such a wireless trigger is available at any reasonable price.  My goal is to have the cost of any individual unit around $100 (sans cell phone).

Thanks again,

Andrew

Another thought (option to above):  Instead of using a radio signal to remotely trigger the RPi to turn on and record, perhaps invisible light could be used.  It would limit the range and force line of sight, but it might keep the costs way down (and require less power).  But it might also eliminate problems with cross-signals between the RPi units.  (With radio, there would need to be a unique signal so only the designated RPi is turned on.  This is another complexity.  With invisible light, this issue is eliminated.)

Regards,

Andrew

This is only tangentially relevant to the question, but it's on topic for "DC to DC Converter" --- http://nathan.chantrell.net/20120430/the-magic-of-the-boost-converter/#more-2675 .

The Pi's full current requirements are somewhat on the heavy side for the capabilities of the Maxim MAX757, but other than that, it's well suited to being driven by a 3.7V Lithium-Polymer cell such as is charged by my Olimexino-STM32 --- http://www.olimex.com/dev/olimexino-stm32.html .

I could in principle charge the Li-Po from the Olimexino's mini-USB input or from the barrel connector (which accepts 9-30V and hence would be great in-car), and the Li-Po cell could drive the boost converter to power the Pi for mobile use.

As long as one isn't too demanding on HDMI and USB power requirements, this should work, and in effect gives the Pi a 5-30V input power range and mobile capability, while at the same time massively expanding its I/O potential through the Cortex-M3 board (which costs only 17 quid).

Morgaine.

This is only tangentially relevant to the question, but it's on topic for "DC to DC Converter" --- http://nathan.chantrell.net/20120430/the-magic-of-the-boost-converter/#more-2675 .

The Pi's full current requirements are somewhat on the heavy side for the capabilities of the Maxim MAX757, but other than that, it's well suited to being driven by a 3.7V Lithium-Polymer cell such as is charged by my Olimexino-STM32 --- http://www.olimex.com/dev/olimexino-stm32.html .

I could in principle charge the Li-Po from the Olimexino's mini-USB input or from the barrel connector (which accepts 9-30V and hence would be great in-car), and the Li-Po cell could drive the boost converter to power the Pi for mobile use.

As long as one isn't too demanding on HDMI and USB power requirements, this should work, and in effect gives the Pi a 5-30V input power range and mobile capability, while at the same time massively expanding its I/O potential through the Cortex-M3 board (which costs only 17 quid).

Morgaine.

I have the impression that boost is a bit more tricky than step-down. On the other hand charging a dual-cell is slightly more tricky than a single cell.

A model-A should be somewhat more appropriate for this. You're not going to have wired-ethernet in the field. That means the lan chip won't be powered. And the lan chip is a power-hog as it uses mostly 3.3V which is inefficiently generated from the 5V.

Which leads to the following suggestion: When battery-powering your 'pi, you could generate 3.35V using a buck converter from whatever your powersource is and deliver that to the 3.3V pin on the GPIO.

That's a good point about the Model A.

Model A would also combine well for mobile use with the Olimexino-STM32 charging its Li-Po cell, plus a Slice of Pi board and XRF radio module for occasional low-power messaging, as we were discussing in Simon Monk's Pi+Sensors thread.

Can the HDMI be turned off completely, as well as the graphics parts of the VideoCore?

Yes and no respecively: HDMI can be turned off by not connecting an HDMI compatible screen.

No the graphics part will be generating low-bandwidth composite video.....