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196  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?
197  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.
198  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.
199  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?
200  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.
201  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.
202  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.
203  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.
204  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.
205  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.
206  Forum 2005-2010 (read only) / Interfacing / Re: Ball and beam c code help? on: May 11, 2010, 07:06:27 am
That code is CCS format code for PIC chips. You really should go to: and ask there

I'm a member over there (hence how I know what compiler was used for your code) so maybe I'll reply over there.
207  Forum 2005-2010 (read only) / Interfacing / Re: Where do I find an electic door lock? on: July 21, 2008, 11:00:50 am
Yes, in the US at least you have to have the capacity for a door to be opened even in the case of a power outage (for precisely the reason you stated.) Of course, if everyone who could sue you were inside during the fire then.... nevermind. ;-)

The bottom line is that a lock can and should fail secure on the OUTSIDE. But you'd better have the capacity to still open the door from the inside.
208  Forum 2005-2010 (read only) / Interfacing / Re: IR photodiode transmitter/receiver on: November 02, 2009, 12:18:47 pm
What is visible to you and visible to your camera could be two very different things. If you are doing digital photography then the IR light could easily show up. Make no mistake, CCDs can see IR just fine! Normal film may not be as sensitive but is still probably at least marginally reactive to IR.

You do want to use a resistor with an IR LED. It won't necessarily be the same one you'd use on a normal red LED. You have to calculate the current flow given the LED's voltage drop and max amperage. IR LEDs can usually take 100ma so if the drop is 1.8 and you are feeding with 5V then the proper resistor would be 32 ohms. I think a 33 ohm would be closest reasonably common size. Also, to get better range you want a finer viewing angle. Sending the IR energy over a 30 degree angle is not good if all you want is a trip beam. Find the smallest angle possible.

You probably aren't doing anything wrong with the photodiode but maybe it's not really what you want. You might need the higher gain of a phototransistor. It's also possible to actually use a photodiode backwards of the way you normally would and get more gain and a lot more noise. Rather than do that I'd use a phototransistor with lots of gain.
209  Forum 2005-2010 (read only) / Interfacing / Re: Car diagnostic (speed etc.) on: October 21, 2009, 01:46:11 pm
First off, yes if you have OBD1 or 2 then that would probably be easiest. Maybe... It's not that bad to get the speed if your speedo is getting pulses.

You can't easily check the speedo signal with a multimeter. The problem is that the line level is rapidly switching between 0 and full voltage. What is your multimeter to think? Depending on how fast the wheels are spinning you might be able to see the voltage swing back and forth or it might just totally confuse the meter. However, if you have no o-scope then you still could use a multimeter to find the maximum voltage and step it down if it's too high. Then provide the signal as an input to the arduino and you can watch the signal transitions within a sketch. But it's more fun to use a frequency to voltage converter. You can even make one yourself out of discrete components. Google is your friend.
210  Forum 2005-2010 (read only) / Interfacing / Re: Car diagnostic (speed etc.) on: October 21, 2009, 06:31:52 am
It's certainly *possible* one way or the other. Does that car have electronic gauges or is there a speedo cable from the transmission to the speedometer? If you have electronic gauges then it'll be more straight forward. What you do is find the signal wire for the speedo and hook an oscilloscope to it (don't have an oscilloscope? Get one, I'll wait...) Now, you should see that the signal wire is likely a digital signal going from 0V to likely +12V. The trick is to figure out how many volts it is (because you cannot directly hook up +14V to an arduino pin!) and how many pulses per mile are being output. There is a remote possibility that this info is available somewhere. If not you'll have to sort of guess then calibrate your guess later. It could be in the ballpark of 16,000 to 64,000 pulses per mile. From there you should be able to integrate for time to come up with the current speed. Frequency to voltage chips are also available. Using one would make your job a *lot* easier. Then you could just hook the F2V chip output to an analog pin on the arduino and read that. You'd still need to figure out which voltage corresponds to which speed. That's easy enough. You guess then drive 60MPH on your gauge while someone else follows you. They tell you how fast you really went and you divide from there. Note that you should really be careful doing this as 120MPH and 60MPH are not the same to an officer of the law. smiley-wink If you are obviously off by a lot then fix it before you go out racing around.

If you've got a mechanical speedo then you would probably have to put a hall effect sensor somewhere (and don't forget the magnet to go with it) and read the output of that to determine speed.
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