Hi There,
This will be my first post on the forum so let me introduce myself quickly:
My name is Csaba and I'm from Transilvania. I'm a full time Programmer and have a few hobbies like having fun with musical instruments (Guitar and Piano). Electronics was and is my biggest hobby since I learned to solder when I was around 3 years old. I'm a big fan of you Dave's and following you on Youtube for years now.
I used SETI@Home 10 years ago for a few months but realized that I can't do any "useful" contribution, because under normal workload I couldn't afford to share resources of my workstation and had to let it run overnight to finish the tasks prior deadline. After this my opinion was that is always cheaper to build/adopt a custom solution that can be much more efficient to solve this kind of tasks then using ordinary PC idle times. Not mentioning that it will keep the CPU 100% all the time that may hurt on the electricity bill and accelerate hardware wear (ordinary PC's just aren't built to run 24/7 on 100% load like servers do).
After I saw Dave's "Find Aliens With Your Raspberry Pi" video I quickly grabbed my abandoned Pi B that was sitting in the bottom of a drawer since I upgraded to V2 and setup SETI@home on it. I was running the Pi at stock speed and quickly assembled a small script that may log the CPU temperature every minute so I can check if the continuous 100% load may cause overheating.
http://pastebin.com/f7M60fVjTurned out that the CPU temperature stabilized around 46 degree Celsius (21C ambient) at 700Mhz stock speed.
Opened up the raspi-config utility and checked the overclock ( 8 ) section, where I selected the Modest option, that may overclock the CPU to 800Mhz without overvolt. Also improvised a not so elegant but (turned out) efficient heat sink by folding a 10cm long 1cm wide metal sheet into a "triangle" and gently ziptight on top of the CPU. Now the CPU temperature is stable around 43C at 800Mhz clock. I wanted to add some thermal conductive paste between the CPU and heat sink but currently I don't have any.
The not so elegant heat sink:
After it was done I remembered about the past experience with SETI/Boinc (that just described earlier) and started to wondered "how could I make it efficient"?
Let's transform this into a serious hobby project, because I have a hardware that I don't use anymore and don't mind to allocate 100% of it's resources, but if I could power it via solar panel and battery then basically it may cost nothing to run, except the investment in those additional parts. However after a quick math those additional costs doesn't worth it more then letting to run via the mains.
I already have a small solar panel that come with the outdoor Christmas lights and it charges 3xAA rechargable batteries. The problem is that the control circuit is built to charge the batteries during daylight and turning on the output power only at night.
I also found a 2200mAh rated phone charger, in the same drawer, that I won last year at a conference from the <epam> guys. So I have another part that I've never used so I can make it useful. After disassembled it found a TP4213 ASIC in it (
http://www.datasheetcafe.com/tp4213-datasheet/ )
Bummer, there is a "but" again. The phone charger can't have both the input and output turned on in the same time (when charging the battery it will cut the output). No problem I thought, I can wire the solar panel directly to the battery with a single diode in series to avoid feeding back the solar panel from the battery. ( I'm not at home with solar panels, so correct me if I'm wrong and the panel can be connected directly to the battery. )
Another bummer. After taking a closer look on the TP4213 specs found out that the input voltage should be between 4.3V ~ 5.5V so wiring the solar panel directly to the battery may not only cook the battery but also the boost converter (I measured the solar panel output by holding it close to a 20W desk lamp and measured 8V output). The magic drawer didn't helped me out this time. I found an XL6009 based adjustable DC-DC converter, but unfortunately is implemented only in boost configuration so it's a no go.
Also I'm aware that this solution may be far from optimal, because it has no control over the battery and over the solar panel load.
As I mentioned earlier I have no experience with solar panels, but I remember that a few years ago a colleague of mine, who has done and does experiment with solar panels, told me that you need a control circuit that optimizes the load to get out the max from solar panels, because if I drain more then it can provide it will "collapse" and supply only a fraction of it's real capacity. Maybe I misunderstood or not remembering correctly, but feel free to correct me please.
The backup plan is to return to the original control board of the solar panel and figure out how can I bypass the output switching to be always on. Another plus is that the original board may have the panel and battery management implemented (or not considering that it's an outdoor unit for Christmas lights, that is used for ~2 weeks per year in winter time when there is not so much sunlight and most likely freezing cold) and also can keep the 3xAA 2800mAh batteries to the additional 2200mAh cellphone charger. An this is where the XL6009 based DC-DC boost converter come handy, because the panel output with it's original board will most likely (need to confirm by measuring it as soon I solder it back) supply less then 5V, because it's designed to drive a few LED's wired in parallel (just hope it will not go below 3V).
The original solar panel management board:
In time I was composing this log entry I did a test to see how much the Pi with Boinc will go with the phone charger battery pack, and the result turned out to be roughly around 2 hours and 40 minutes that resulted 1.27% progress on a SETI task
Before starting on the battery:
After the battery was drained:
BTW: if you want me to describe how I setup the Boinc Manager to remotely connect to the Boinc client running on Pi from a PC just let me know and I'll make a quick tutorial about it.
That is it for today, more progress to follow soon, just stay tooned.