BS2p40 questions

Loki008

I am sure that questions very close to this have been asked several times, however after doing some searching i think i have a general idea, but want to confirm. I appoligize for the newbie questions.

I am planning on working with the BS2p40, and looking at the schematic i have a vew questions. My project will be running off of a 6v source (4x AA batteries) and my understanding is that this is within the voltage regulator on the module and shouldnt pose an issue. The second concern i have is that i am noticing that Pin4 and 39 are both VSS. Am i supposed to connect both to the negitive terminal? I am assuming that p4 is for serial data, and that p39 is what is connected to the negitive power source. The other question is with VDD / VIN. I may be wrong, but VIN is for regulated input, and since i am doing 6v i will be connecting to VDD and leaving VIN disconnected?

I appreciate any advice to help clarify these questions that i have.


[noparse][[/noparse]edit] Also the other question i have, is should i be doing anything to add protection to the VDD and VSS pins?

Post Edited (Loki008) : 6/30/2009 5:54:23 PM GMT

Mike Green

Vin is the un-regulated power supply input. You connect your 6V supply here.

Vdd is the regulated +5V power supply input/output. In your case, this is the output of the regulator. You can leave Vin unconnected and provide a source of regulated +5V to Vdd. This is sometimes done when a large external regulator is used to supply both the Stamp and significant amounts of external logic. Sometimes two regulators are used. The one on the Stamp is used to power the Stamp while the external one powers the external logic. In this case, Vin is used and Vdd is left unconnected.

The two Vss pins are connected together on the module, but I'd also connect them externally. If you use a printed circuit board, run a trace between the two pins. If you're using a breadboard, connect them both to the same ground point on the breadboard.

If you plan to use NiMH rechargable AA cells, plan to use 5. NiMH cells have a terminal voltage of only 1.2V and 4 of them would provide less than 5V.

Loki008

Actually, i was mistake, i am likely going to use 8x AA batteries becasue i am using a 12v part, would i have an issue using 12v as described earlier. The homework board has been handling it without any issues, however i dont know if the BS2p40 has the same flexabilty in terms of input voltage.

http://forums.parallax.com/forums/default.aspx?f=5&m=250265&g=250372#m250372

Basically i have been using a homework board but am designing a PCB that i want to get made up for a more finished result. I am also going to the p40 because i am planning on adding a few extras to the project such as the EMIC text to speach module and a few other goodies.

Here is a flickr photostream of the current version
http://www.flickr.com/photos/8834427@N04/sets/72157604260366787/



This is the project i am working on (this will be revision 3 of the project) yourself and several others have been quite helpfull in making sure that i dont fry everything.
Thanks

Mike Green

12V won't damage the regulator or other parts on the Stamp, but all of the extra voltage is dissipated as heat (12V - 5V x <whatever current is used> = heat). The voltage regulator on the Stamp module has no heatsink other than the module substrate, so it can hot very quickly and may shutdown if it gets too hot. Don't try to run anything like the EMic or other "goodies" off the BS2p40's regulator. The LT1121 that is used is only rated for 150mA. With a 12V input, that's over 1W power dissipation. The regulator will reach temperatures near the boiling point of water if it doesn't shut down first.

Tracy Allen

I don't recommend reliance on 4 alkaline AAs. They do start at about 1.5 * 4=6 Volts, but they get down to 1.25 Volts * 4 = 5 volts awfully fast, and that is below the comfortable operating voltage for Vin on the Stamp. The service life and Ah capacity of the AAs is usually stated down to a level of 0.8 * 4 = 3.2 volts. See the graphs on the Energizer technical pages.

So, if you want to do want to use AAs, the options as I see it are,
4 * L91 cells, (Energizer LiFeS2 primary cells, 1.5 volts, flat discharge curve)
5 * E91 alkalines
5 * NiMH or NiCd (flat discharge curve)

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Tracy Allen
www.emesystems.com

Loki008

Thank you for pointing that out to me, I should be able to get away with a "+5V Fixed-Voltage Regulator 7805" to power the EMIC and the stamp at the same time? or should i just use the voltage regulator for the EMIC and use the onboard for the stamp?

http://www.radioshack.com/product/index.jsp?productId=2062599&tab=summary


Post Edited (Loki008) : 6/30/2009 7:25:42 PM GMT

Loki008

Tracy Allen said...
I don't recommend reliance on 4 alkaline AAs. They do start at about 1.5 * 4=6 Volts, but they get down to 1.25 Volts * 4 = 5 volts awfully fast, and that is below the comfortable operating voltage for Vin on the Stamp. The service life and Ah capacity of the AAs is usually stated down to a level of 0.8 * 4 = 3.2 volts. See the graphs on the Energizer technical pages.

So, if you want to do want to use AAs, the options as I see it are,
4 * L91 cells, (Energizer LiFeS2 primary cells, 1.5 volts, flat discharge curve)
5 * E91 alkalines
5 * NiMH or NiCd (flat discharge curve)

Thanks, i am probably going to go with 8x NiMH rechargables.

Mike Green

An LM7805 would work fine with an 8x NiMH cells pack. That would give you around 9.6V and the LM7805 requires a minimum of around 7V.