need more power
softcon
Posts: 217
Can someone tell me what to use to get more power to run external devices from the usb homework board?
I'm trying to get the pmb648 to talk using the emic2 board, and not having much luck.
I can either read gps values, or I can speak results, it seems both is too much for the bs2 to handle.
Anyone know how I can remedy this situation?
I'm not opposed to plugging in external power boards, but the final product needs to be able to run from batteries, as I plan to use it outside away from power outlets.
I was wondering if adding a capacitor or two would be enough to allow he pmb648 to continue receiving data, (since it doesn't need to remain in pulling state constantly), so it could pull power from the capacitor(s) to do it's draw once every 5-10 seconds, while the emic2 can continue to run from the 5v provided on the board.
Alternatively, would it work to plug the pmb648 into the vin port? The emic2 can't do this, (docs specifically warn against such behavior) but I don't see any such notes in the pmb648 docs, so wondering if that would solve the problem. Don't want to fry anything (like I did last time) so wanted to ask here first.
If the vin thing won't work, I know there's secondary power boards that can be used, but if I get one of them, would it be something simple to install/use? Remember, I don't (currently) have access to someone who can solder, so unless it can be done with jumper wires/leads of some sort, I can't use it for testing purposes.
Just for reference, this is the homework board (usb) that runs off of a 9-volt battery. I do have 2 propeller quickstart boards, but they too use 9-volt battery power, and only give 3.3 volts, which isn't going to be enough power for both of these devices together either.
Any suggestions on what to try to keep it portable would be appreciated.
I'm trying to get the pmb648 to talk using the emic2 board, and not having much luck.
I can either read gps values, or I can speak results, it seems both is too much for the bs2 to handle.
Anyone know how I can remedy this situation?
I'm not opposed to plugging in external power boards, but the final product needs to be able to run from batteries, as I plan to use it outside away from power outlets.
I was wondering if adding a capacitor or two would be enough to allow he pmb648 to continue receiving data, (since it doesn't need to remain in pulling state constantly), so it could pull power from the capacitor(s) to do it's draw once every 5-10 seconds, while the emic2 can continue to run from the 5v provided on the board.
Alternatively, would it work to plug the pmb648 into the vin port? The emic2 can't do this, (docs specifically warn against such behavior) but I don't see any such notes in the pmb648 docs, so wondering if that would solve the problem. Don't want to fry anything (like I did last time) so wanted to ask here first.
If the vin thing won't work, I know there's secondary power boards that can be used, but if I get one of them, would it be something simple to install/use? Remember, I don't (currently) have access to someone who can solder, so unless it can be done with jumper wires/leads of some sort, I can't use it for testing purposes.
Just for reference, this is the homework board (usb) that runs off of a 9-volt battery. I do have 2 propeller quickstart boards, but they too use 9-volt battery power, and only give 3.3 volts, which isn't going to be enough power for both of these devices together either.
Any suggestions on what to try to keep it portable would be appreciated.
Comments
First, verify that your total max current draw (BS2, Emic, GPS, plus any other loads) is under an amp.
Then try 5x AA (or bigger) cells or a 6V gel cell or a 7.4V LiPo instead of your 9V.
The LM2940 has reverse polarity protection, but you might have other connections that are not protected.
I suspect it may be easiest to just share the ground and provide other regulated power to the other devices. But it is certainly nice to run the board on a wall wart at times.
If I hook one of the 9-volt battery connectors to the 9-volt connectors on the board, then on the wire ends, connect another battery pack (I have some 7.2V 1a battery packs with the flat square batteries that came with chargers, so they would be easy to charge). Would something like that work, or should I just hook that battery pack to the ground/vin of the hw board? Assuming I get it right, and don't blow anything up
It might be a bit tricky getting it positioned just right, since the battery has terminals on each side, and keeping the wires still while the battery is in place would be a bit of a task, but I'm sure I could strap it down with wire ties or something.
Come to think of it, (if I can find it) I also have another 7.2 1a battery pack, but it's two batteries (looks like overgrown C cell batteries) that's about 8 inches long, and has two wires going into it, that may be easier to wire up, since it's connectors are inside a moulded plastic plug and might lend themselves to manipulation easier.
So, anyway, suggestions, questions, gotchas, what not to do?
The reason I mention that polarity is important is that I found the dongle's red wire should go to minus and the black wire should go to plug as this is a 'battery replacer' and that seems to reverse the normal use as a battery connector.
The HW board was engineered for beginners and their rather big mistakes. So reverses of polarity are protected by the LM2940 and since this is a low dropout regulator, it works fine at 7.2 volts.
I do have one 6x AA cell pack that has the 9V battery snaps built in. That will work on the HW board directly, but it is a bit heavy and hangs over the reset button.
You can then solder wires to back side of the removed connector and add a barrel jack as Loopy suggests. I usually trim the plastic material that made up the top of the 9V battery to make the plug a smaller size.
As others have warned, double check your polarity.
So, that's my progress at the moment, I'll update again when/if something changes.
An alkaline 9V PP3 battery is likely to be rated about 500mAh, but designed for loads of 50mA or less. Try to discharge it at 500mA and it'll drop well below 9V and perform miserably.
A NiMH or NiCd pack from a RC model car might be rated at 1000mAh but produce several amps happily...
Typically LiPo cells often are able to delivery all their charge in 5 minutes or less without significant drop in performance - many tens of amps.
Choose the right type of battery chemistry and the right model of battery for your application. Capacity and current requirements both need to be met.
Your Homework module is powered by external source, via a regulator capable to supply 500mA maximum.
The BS chip generally needs no more than 50mA and can drive max 20mA current thru ALL I/O pins combined.
I am just guessing some values, but in general they should support the following argument.
You cannot drive more I/O current than what the BS is designed for, period.
Your on board regulator will limit max current (500ma) no matter how "big" (Ah) your battery is, and eventually thermo limit / shut down - or if abused - smoke!
If the EXTERANL device you are controlling takes more than BS I/O current capability you need to have a suitable interface.
The regulator input voltage should stay close to desired output voltage 9 V is the best, anything HIGER that 9V will be dissipated into heat limiting the current and accelerating the thermo shut down of the regulator.
Basically you need a "relay type interface " - small input current controlling large output current interface.
The relay interface can be a plain mechanical relay or other transistor based interface.
If the device you are interfacing with DOES NOT require more than 500 mA you can still run it using your Homework board power.
1. bs2 usb hw board http://www.parallax.com/Store/Microcontrollers/AllMicrocontrollers/tabid/758/CategoryID/1/List/0/SortField/0/catpageindex/4/Level/a/ProductID/781/Default.aspx
2. Emic2 text-to-speech board http://www.parallax.com/Store/Accessories/Sound/tabid/164/CategoryID/38/List/0/SortField/0/Level/a/ProductID/105/Default.aspx
3. gps module http://www.parallax.com/Store/Sensors/CompassGPS/tabid/173/CategoryID/48/List/0/SortField/0/Level/a/ProductID/644/Default.aspx
I got it all to work yesterday, so apparently it was an issue with wires not being tight enough first time around.
However, even though this setup works, it doesn't work for long (10-15 minutes tops) while using a brand-new 9-volt battery.
Thus, the search for "more power"
If I offload the powering of the gps device to this flat square battery I have, then the bs2/emic2 can last longer off the 9-volt battery, and the gps would also last considerably longer as well.
I'm still in the early stages of writing code, at the moment, the gps simply feeds back the coordinates, and I speak them in raw form (37.0029,-78.0060 type of output).
When this is done, it will allow labeling of coordinates, to allow my friend to choose a destination (home, school, bank, dollar store, ...) whereupon it will pull the coordinates for said place, compare them against current location, then indicate which direction to walk to get to the required destination.
Obviously, 10-15 minutes is not nearly enough time to arrive, so something needs done to extend the amount of time the bs2/emic2/gps will run together.
Thus, my inquires into how to get more power for the bs2.
I myself will be using a similar setup, so that when I'm out and about riding bicycles, walking, taking son/daughter to boy/girl scouts, or going to the skateboard park, I'll know where I am and how to return home at all times.
It's obviously not going to be turn-by-turn directions, since the bs2 doesn't have enough memory for me to input coordinates of all streets in town, so I can give the detailed directions required for turn-by-turn navigation, but it will be sufficient to get us where we would like to go and back again, and that is my goal.
Radio Shack still sells the 7.2 volt battery, though I'm not sure the sales person was correct when reading off the specs, claiming 3360 ma for battery output, but nonetheless, I feel confident that with either this battery, or a combination of this flat square one, and the 9-volt used to power the bs2 itself, I can provide power long enough to accomplish the task. Using the 7.2 volt 3K+ battery might be better, since it would power everything for many hours, and it's rechargable, which would certainly make for usage considerably more hands off, having to replace only a single power source instead of two (flat square rechargable battery and 9-volt one as well).
In any case, the devices do work as is, I just need something to provide a *lot* longer uptime for all parts.
I'm sure one of these two approaches will work just fine, it's just a matter of securing wiring so that minimal intervention is required when changing/charging batteries. I'd rather take the batteries out of the device to charge them, removing any need to solder boards and such, so that will be my design unless something convinces me otherwise.
If anyone has thoughts/questions/experience with this sort of thing, feel free to jump in, this is my first major project embarked on alone from design to completion, so I'm always willing to listen to those who know more about these sorts of things.
I'm fully confident in my abilities to make it work, I just don't know enough about power requirements/sources to do this without some experimenting/breakage, and obviously, I'd like to avoid the breakage.
max 50 mA BS2 ( actually less )
max 200 mA voice module ( depending on usage)
max 64 mA GPS
Your Homework regulator will handle that and MAY
work with 7.2V rechagable battery if it is a low drop regulator - better talk to Parallax about that for sure. They generally "recomend" fresh 9V source.
Your friendly RS salesman need to tell you the CAPACITY and MAX current load of the battery.
For example - in theory - 1AH (AmpereHour) battery of max load of 500 mA should last you 1 hour.
Keep in mind that "standard" 9V alkaline battery was not designed to supply more than 100mA and it is not surprising that your setup worked for short time.
http://support.radioshack.com/support_electronics/doc69/69958.htm
I would still check with Parallax, 5V regulators require MINIMUM input.
I would strongly recommend to use "idiot diode" ( serially connected to your supply battery either terminal, plus preferred ) to prevent accidental reversal of you supply.
It would further reduce the input voltage, so you really need to talk to Parallax about it.
Any 1N400x type diode would work - 1N4001 or equivalent.
As far as capacity - my best guess , from the supplied specification, this battery should last you over 2 hours under full voice output ( uses most power). It will work longer under casual usage.
Keep in mind that NiCd batteries discharge voltage holds pretty steady, but goes bellow usable level rather quickly at the end. Your system may work OK for hours than suddenly go dead.
I am not sure an 'idiot diode' is worthwhile as the LM2940 or LM2937 already has reverse polarity correction and a 1n400x usually drops 1.1 volts. That has to be used carefully or you are back into the low-dropout mode that drains a lot of extra power.
When all else fails, check the regulator chip for its correct identity and read the PDF very carefully. It think it is the LM2937, but have provided both PDFs.
http://www.cedt.iisc.ernet.in/tfcindia2011/Datasheet/LM2940.pdf
http://www.ti.com/lit/ds/snvs100d/snvs100d.pdf
At first, I was in love with low-dropout regulators as they seemed to allow you to use lower voltage batteries. But then I learned to read the curves on the PDFs and see that to do so, they waste quite a bit of extra power below a certain voltage. There is not much to be gained over a convention LM7805 that requires 7V for a regulated 5V.
What they really do is they allow you to squeeze an extra bit of power out of a near dead battery. One must stay above the low-dropout region to get really efficient use. And that is why adding a protective diode may just make matters worse.
Quescent current of the regulator in the low-dropout region may be as high as 60ma, where as it is normally at 15ma with higher voltages (about 7 volts or above for 5 volt regulator).
A 6V 150ma battery is just going to get murdered by the regulator, only 3-5 hours use at best.