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196  Forum 2005-2010 (read only) / Development / Re: Using diodes to drop voltage on: January 23, 2009, 07:08:20 pm
Yeah, I've had to buy power resistors before (you'd think that industrial workers would be able to understand the concept of a 30% duty cycle on a welder but NOOOOOOO) and I don't really want to do that for this application. For right now I'll hook up an LM317 to get the prototype up and running and in the future I will almost certainly use some form of buck converter to get the voltage down to the proper level.
197  Forum 2005-2010 (read only) / Development / Re: Using diodes to drop voltage on: January 23, 2009, 03:03:39 pm
The TO-220 form factor regulator is from TI. And that was the $15 part I was referring to that Digikey had.

I did order up one of those convertors from Sure electronics. We'll see how I like it.

Thanks everyone.
198  Forum 2005-2010 (read only) / Development / Re: Using diodes to drop voltage on: January 23, 2009, 01:45:23 pm
Yeah, I do think that in the long run a DC-DC convertor will be the best option. I plan to make more than one of these units so I'll likely go with some sort of buck switching reg for the rest of them. I'll probably just use the LM317 for the initial prototype though, just to get something working. China is a long ways away and if I wait I'll be sitting here for two-three weeks.
199  Forum 2005-2010 (read only) / Development / Re: Using diodes to drop voltage on: January 23, 2009, 12:49:46 pm
In small quantities the DC-DC convertor is actually around $15 from digikey but I could search around for a better price. I've already got LM317's, heat sinks, and thermal grease laying around so that's the more expidient option. However, it also wastes about 57% of used energy as heat. The DC-DC convertor is about 90% efficient instead of 43% efficient. Like you said, the LM317 and heatsink is cheap. I probably really don't have more than $2-3 into the linear parts to do one unit. Choices, choices...
200  Forum 2005-2010 (read only) / Development / Re: Using diodes to drop voltage on: January 23, 2009, 12:28:32 pm
Yeah, I know about DC-DC convertors. I could do that, it's just a little more money. Maybe it's worth it though.

I'm planning on drawing maybe 0.5A. This is still way more than 1W heat dissipation (at 13V the drop to 5.6 is 7.4V times a half amp is 3.7W heat dissipation)

It makes sense what you said about heat dissipation in diodes. I'm still in the learning phase and I just couldn't get my head around it. But what you said makes sense.

So the first diode (in my original idea) would see 13V at one end and 12.4V at the other for a total drop of 0.6V. 0.6V * 0.5A is 0.3W. So the best diodes could do is to spread the heat dissipation out among more components. And thus all I could hope to do is increase the cost but not the efficiency. This makes sense now that I really think about it.

I don't mind using a heatsink. Now I just have to determine whether it's worth it to use a linear reg with a heatsink or spring for a DC-DC conversion circuit to get higher efficiency.
201  Forum 2005-2010 (read only) / Development / Using diodes to drop voltage on: January 23, 2009, 10:55:38 am
I know that diodes drop a specific amount of voltage across themselves. What I'm trying to do is run an LM317 regulator from a 12V battery. I plan to output 5.6V from the regulator. Batteries tend to never actually run at their rated voltage, it could be more (upwards of 13.8V) or less (down to maybe 10.5). I'm thinking of putting a couple of 1N4001 diodes in the path to step the voltage down a little bit before it gets to the LM317 reg. This would cause less heat dissipation at the regulator. I could maybe chain several diodes up and output around 8.1 to 11.4V to the reg. Am I wrong in thinking that diodes, despite their voltage drop, do not really dissipate much energy and thus could be used to keep general system efficiency and heat loss in check?
202  Forum 2005-2010 (read only) / Development / Re: Voltage tolerance of arduino? on: January 20, 2009, 09:04:43 am
Ahhh, skipped over that before. 16Mhz isn't explicitly listed but it appears as if the lower limit is about 4.3V or so at 16Mhz. It certainly appears that 4.5V is safe, even at 20Mhz.
203  Forum 2005-2010 (read only) / Development / Re: Voltage tolerance of arduino? on: January 19, 2009, 09:08:53 pm
Thanks, that's pretty much what I figured. I have a couple of other things connected to the arduino, the rest of which are 10% tolerant. But, it's still probably best that I get as close as possible to 5V to prevent that as an opportunity for screwiness.
204  Forum 2005-2010 (read only) / Development / Voltage tolerance of arduino? on: January 19, 2009, 08:48:49 pm
I swear I've seen this somewhere before but now I tried looking around the website and also at the Atmel AVR168 docs and I don't really see clear enough an answer. I'd like to know what sort of tolerance the input voltage to the arduino has. I mean, if I bypass the regulator (well, it's likely I'm going to produce my own board...) and supply a voltage what is the tolerance? 10% (0.5V +-) The avr168 docs seem to indicate that the chip can run at at low as 3.3V and still maintain 16MHz but I did see somewhere that supplying less than 5V to the arduino could create shaky performance. So what's the real answer? I'd like to try to stay in the 4.7V to 5.2V range but if I end up with 4.5V should I expect trouble?
205  Forum 2005-2010 (read only) / Development / Re: Battery Charging? on: January 14, 2009, 01:57:28 pm
I'll be using standard NiMH rechargable AA batteries.

So it it simply that I connect up a power supply of say 10% of the battery rating, i.e. for a 1000mAh 1.5v battery I put in 1.5v at 100mA and charge for 10 hours?

No, at least, never ever count on that. You don't take the capacity of a battery and take 10% to find the trickle charge. The trickle charge is 10% of the high charge rate. In other words, if the high charge for a battery is 100ma then the trickle is 10ma. I'm building a 12V lead acid charger that is 400ma normally and 40ma trickle charge. I really do not think that you want to charge little 1.5V AA batteries at 100ma constantly. Also, you lose power due to resistance (which is dissipated as heat) so you cannot just take the charge rate and multiply.

So, be sure to find out what the proper charging rate for the battery you are charging is.
206  Forum 2005-2010 (read only) / Bugs & Suggestions / Re: Library build error (HELP ME) on: March 04, 2009, 04:48:04 pm
I think you might want to look at the source files in the output directory. These are what the compiler really sees after the arduino environment does it's magic. Likely there is something screwed up with the way one of those files looks.
207  Forum 2005-2010 (read only) / Interfacing / Re: Arduino + SM-630 Fingerprint Verification Module!! on: January 16, 2009, 11:25:42 am
Thanks AdderD i've figured out the working now after having read it 3 times. All i need is some help in terms of how to send the data packets? there seem to be Hex codes for all commands so how do i serial print them? Serial.print(0x4D); - is this right?

When they show you a hex code they mean to send that actual value as a byte. If you just put Serial.print(0x4D); then the actual number will be output as a string. In this cause you'll print 77. Printing 77 requires two characters. (7, then 7.) But you want to print the byte correspoding to ascii code 77. You do that like this: Serial.print(0x4d, BYTE);

and how do i read them returns an integer while the manual talks of Hex values? Finally, should i read each byte as i receive it or should i store all the bytes till i reach the last packet and then compare the whole set of instructions?

This is a snippet of the code I'm using in my project:

   for(d=0;d<MAXTRIES;d++) {
      if (Serial.available() > 0) { //anything waiting?
         in_byte =;
         if (in_byte == 0x7E) break; //if it's the right character then drop through

What that does is use Serial.available() to see if any bytes are waiting to be read from the serial port. If not delay a bit and check again. You'll also see that you can directly equate hex numbers and decimal numbers. In this case I read a byte with and then see if that byte equals 0x7E (which for my fingerprint scanner is always the first byte sent back from the unit).

Finally, if you know that the reply should be, say, 20 bytes then use Serial.available to see if there is 20 or more bytes waiting. Then grab them all at once. You can do this so long as the reply you are expecting is less than the buffer length of the chipset. The serial buffer is 128 bytes.
208  Forum 2005-2010 (read only) / Interfacing / Re: Arduino + SM-630 Fingerprint Verification Module!! on: January 16, 2009, 10:02:10 am
I'll try to look at this later for you. I'm actually building something with a fingerprint scanner so I've already gone through this recently.
209  Forum 2005-2010 (read only) / Interfacing / Re: Best GPS For Speedometer? on: October 02, 2009, 08:44:59 pm
I know that you wanted GPS recommendations but have you considered just interfacing with the existing speedo (if it's electronic) or using a hall effect sensor or something? Do all of the electronics have to be in the helmet? Obviously this would require some sort of wireless link between the vehicle and your helmet.

Do you plan to support multiple vehicles? If you are using just one ATV it might be more economical and reliable to go with a non-gps speed sensor. GPS is decent but it's not always terribly reliable. I've had a lot of trouble with some GPS sensors not wanting to get a signal in a variety of situations and that's annoying. I just don't think that you'll be able to count on the GPS to always be up and reporting your speed.

Depending on your requirements the GPS might be the way to go or it might be a continuous thorn in your side.
210  Forum 2005-2010 (read only) / Interfacing / Re: Arduino and tachometer from 12V car battery gen on: January 27, 2009, 05:01:30 pm
You could rectify it and then use a voltage divider. Since you won't be drawing hardly any current this should work. Then just count the pulses. You can set the arduino to interrupt on a pin when it reaches a certain threshold. Just make the interrupt handler increment a counter.
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