Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: boyb on July 01, 2008, 11:47:17 PM
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So I just "finished" the $50 robot but it's not working and I have a few questions (Everything is complete, and the programming said it was successful).
Whenever I turn the robot on the LED flashes and I can feel the servo clicking with it. From my experience (which is pretty limited) that seems like it's a current overdraw. This led me back to one of my original confusions with this project:
-The tutorial gives you two methods of a battery pack (6V) or build your own (4.8V) + a 9V.
-It says to make a 4.8V with 4AA batteries, but isn't a AA battery 1.5V so 1.5*4=6V?
-Based on my assumption (and measuring with a voltmeter) I made my own 4AA alkaline battery pack to supply 6V to the system and continued on as though I had a 6V battery pack.
Basically what I'm asking is:
-What they mean when they say 4AA batteries will make 4.8V?
-Is it likely to draw too much current to use 4AA (6V) alkaline batteries instead of the battery pack? (I'm also using cheap no-name batteries, the better ones are powering my Wii remotes :) )
-What size resistor should you use with the LED (I know it'll only vary the brightness, but what did you use?)?
-My servos are still unmodified, but if everything else is correct then though should spin to a specific location and not a specific speed correct?
-What size capacitor and voltage rating did you use for the capacitor just after the voltage regulator (sorry, but I thought that part of the tutorial was kind of confusing).
-If the PonyProg2000 confirms that it was programmed successfully (using the less expensive 10pin header cable), can you be sure it is correct (is there some feedback to ponyprog that confirms it is complete)?
Sorry that this is a bit long winded, but I have been trying to figure this out for quite some time now and I really want to get this working so I can continue studying up on (and testing) the more technical aspects of it.
Thanks in advance!
Cheers,
Boyd
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i can answer one of you questions lol, the batery pack you need to feed in 4.5 volts you need a pack with 3 slots for a AA battery, 1.5 x 3=4.5,i know its not much,but i tried at least lol,i still havnt programmed my $50 robot,so i cant help yoo in that respect
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I haven't built the $50 robot, but...
I'm assuming that the tutorial is saying 4 AA batteries is equivalent to 4.8 V because they are using rechargeable AA batteries, many of which only carry 1.2 volts, so 1.2 * 4 = 4.8. Non rechargeable alkalines are typically 1.5 V, which is what you're using I'm assuming.
As for the resistor with the LED, like you said it varies the brightness, pick a value you think is good... or just use Ohm's Law ( V = IR ) and figure out what resistance you need to get 20 mA flowing through the LED at your given voltage. 20 mA going through an LED is a decent amount.
Your servos should in fact spin to a specified location, rather than speed, when they are unmodified. However, it sounds like they're trying to spin past the boundaries they have... try making your own program to simply send a 1.5 ms pulse to the servo every 20 ms or so. That should center the servo.
As for the capacitor, I typically use a 47 uF cap across gnd and out pins on the voltage regulator, you can also add in a 0.1 uF cap to filter out some of the high frequency spikes on your supply.
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doesn't the tutorial say 220 uF though?
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did u happen to order the same parts that admin suggest? I highly suggest you double check his list. They direct you to the website and will tell you what is needed. You can also check the color bands on the pic admin shows to find out what size resistor to use. I believe it was the 340ohm resistor but dont quote me on that as im not at home right now to check. Also, i highly suggest you modify your servos as you will have to take everything apart after to modify them anyways. And you can rule them out as a problem.
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doesn't the tutorial say 220 uF though?
If the tutorial suggests a 220 uF then go with that, I've just always used 47 uF and have never had a problem with that value.
As for the resistor, someone mentioned 340 Ohm --- 5 V / 340 Ohm = 14.71 mA, this is suitable for the LED, however, if you want increased brightness you can go down to a 220 Ohm which will provide 22.27 mA of current.
edit: Whoops, forgot the LEDs voltage drop in the above. use R = ( V_source - V_led ) / I_desires
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Thanks, I got most of my answers, but could someone still let me know if they've tried, or know what the difference between using a 6V battery pack or 4AA (1.5*4=6V) batteries as your power source? Is that likely my problem? Is it likely that the batteries can't supply the required current.
One final thing (not really electronics though): the servo modification tutorial uses a different servo than suggested, I did purchase the suggested Hitec HS-311, is there a post for modding these? I notice the insides are slightly different and although I can probably figure it out, I don't want to risk damaging them as it takes a while to ship up here to where I live.
Thanks again everyone, you've been helpful.
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Alkaline batteries have a fairly high internal resistance, therefore the current they can supply is pretty low. However, I am fairly certain they have enough power to run the servos... but in the future look into NiMH or Lion batteries as they can supply far more current than alkaline.
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I too was a little worried when I first saw that my hs-311 servos were different than the pics admin posted. but they are still the same servo, just made a little "cheaper" you might say. You may notice that the "pothead" is all plastic. you may also notice that you can only spin a small piece of plastic at the top of the "pothead" to center it. This worried me as there was not very much room to super glue the "pothead" into the centered position. What i did was fold a tiny piece of paper and shove it down inbetween the pothead and the sleeve to give the glue something to stick too. For some reason my glue would not set if there wasn't something for it to adhere to. You may not want to do this if you ever want to switch your servos back. saying that you'd also have to order a new gear set. Other than that, you should be able to follow the guide on servo modification without too much trouble. If you have any questions just pm me. Ill try to walk you though as best as i can.