Thanks Mike for the input... I'm not sure how much current the Homework board draws.. I'd be reading from the clock, keypad and data logger... it would constantly be scanning for input. Is there a certain size resistor I could put inline with the 12V positive that would help reduce the danger? I was burning through 9V's in 3 to 4 days before while I was testing. I may have another week or so of testing before I'm ready to move it all to the Super Carrier board which does say on it 6-12V.
Also.. on the Super Carrier board.. can someone tell me what size the coaxial DC power plug is? Size M? I've looked through its files but it does not mention the size, neither does the power supply you can buy from parallax.
The Homework Board itself draws very little current. It's what's attached to it that drives up the current usage. Best solution is to make up a voltage regulator using something like an LM7808 or LM7809 voltage regulator which will drop the 12V to 8V or 9V (respectively). Download the datasheet (www.fairchildsemi.com/ds/LM/LM7808.pdf) and follow their recommendations (Figure 10) for input and output filter capacitors.
I believe the power plug is a 2.1mm center diameter / 5.5mm outer diameter.
Mike Green said...
It's possible to power the datalogger through a PNP switching transistor and turn it on only when needed. Unfortunately, the datalogger takes quite a while to initialize itself and that may be prohibitive for your application.
Hi Mike,
My program is testing out well. I think it may be possible to power up and down the logger as needed. You suggested in the referenced post earlier that I use a PNP.. the examples in the "What's a Microcontroller?" book are all for NPN. Will any PNP from Radio Shack work? I want to control flow through the one wire which connects to 5V+ at Vdd for turning it on and off. How would the PNP be oriented... connected?
EDIT - From my research I would guess the following for an answer:
Emitter of the PNP goes to Vdd
Base goes through a resistor.. 100 Ohms? to the Stamp Pin
Collector goes to the logger.
Stamp pin has to be initialized high... and sent low to turn the logger on... and back high to shut it off.
The PNP switching transistors should work (www.radioshack.com/product/index.jsp?productId=2062585). You've got the idea in terms of connections. The Homework Board already has 220 Ohm resistors in series with the I/O pins, so you don't need an additional 100 Ohms. If you're using some other Stamp board, you will need a 220 Ohm or 330 Ohm series resistor.
Update: Waiting till I get to town to pick up the PNP.. for now things are testing out well so far. The buttons can be pressed quickly and the wait for the latch operation is down to half a second. The logger is working without any problems. The program is taking about 78% and I have plenty of RAM left also.
I have another power supply question. I noticed on some of the commercial electronic door programs that they just have a transformer going to a bridge rectifier. I would think that would make for a fairly "dirty" DC signal. Do stamps handle that kind of thing ok? Or do they require a lot of external filtering first in addition to the regulators on the the carrier board for example? The reason I ask, I have used the cheap plug in types from Radio shack and elsewhere which seem to get hot and not last more than a year on average when they are powering small video cameras. Long ago I got tired of buying new ones and started using old PC power supplies... of which, I have not had to replace one yet even though they were no longer testing good for powering PC's, they have worked great to keep the cameras going. The almost a full dozen little wall converters I've had to throw out over the years has left me with little faith in them for 24 hour, many years operation. My spare pc power supply is working fine for this build, just considering the options for the next one. Some of the 12V transformers may also have a center tap for 6V to run the stamp on. I am wondering if the the lower voltage makes the "dirty" signal less of a problem.
Comments
Also.. on the Super Carrier board.. can someone tell me what size the coaxial DC power plug is? Size M? I've looked through its files but it does not mention the size, neither does the power supply you can buy from parallax.
Thank you,
Daniel
I believe the power plug is a 2.1mm center diameter / 5.5mm outer diameter.
Have a good weekend all.. will update again next week.
Thank you,
Daniel
Hi Mike,
My program is testing out well. I think it may be possible to power up and down the logger as needed. You suggested in the referenced post earlier that I use a PNP.. the examples in the "What's a Microcontroller?" book are all for NPN. Will any PNP from Radio Shack work? I want to control flow through the one wire which connects to 5V+ at Vdd for turning it on and off. How would the PNP be oriented... connected?
EDIT - From my research I would guess the following for an answer:
Emitter of the PNP goes to Vdd
Base goes through a resistor.. 100 Ohms? to the Stamp Pin
Collector goes to the logger.
Stamp pin has to be initialized high... and sent low to turn the logger on... and back high to shut it off.
Is this correct?
Thank you,
Daniel
Post Edited (qxotic) : 9/14/2009 4:05:31 PM GMT
Update: Waiting till I get to town to pick up the PNP.. for now things are testing out well so far. The buttons can be pressed quickly and the wait for the latch operation is down to half a second. The logger is working without any problems. The program is taking about 78% and I have plenty of RAM left also.
I have another power supply question. I noticed on some of the commercial electronic door programs that they just have a transformer going to a bridge rectifier. I would think that would make for a fairly "dirty" DC signal. Do stamps handle that kind of thing ok? Or do they require a lot of external filtering first in addition to the regulators on the the carrier board for example? The reason I ask, I have used the cheap plug in types from Radio shack and elsewhere which seem to get hot and not last more than a year on average when they are powering small video cameras. Long ago I got tired of buying new ones and started using old PC power supplies... of which, I have not had to replace one yet even though they were no longer testing good for powering PC's, they have worked great to keep the cameras going. The almost a full dozen little wall converters I've had to throw out over the years has left me with little faith in them for 24 hour, many years operation. My spare pc power supply is working fine for this build, just considering the options for the next one. Some of the 12V transformers may also have a center tap for 6V to run the stamp on. I am wondering if the the lower voltage makes the "dirty" signal less of a problem.
A picture is attached of the working rough up.
Thank you all,
Daniel