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5056  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: why should I use a H-Bridge on: March 09, 2010, 10:39:39 pm
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So half the batteries drive it forward and half of them backwards. It doesn't make for a very even battery usage. You end up changing them all when only half are discharged.

Yeah - this was a problem with it; but it shifted the extra cost to the user, away from Milton Bradley...  ;D

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Can't see why you say that if all LEDs are on there is still a short through the transistors of the bridge.

Ok - so how do you design a "safe" h-bridge such that no matter what the inputs are, there isn't a direct short? That is the question I would like to see answered - that, or how do I ensure that the outputs of the Arduino are always in the "safe" position? Right now, my only answer to the question is to delay the power-up of the h-bridge until I am certain the Arduino has everything set properly (via some kind of "enable" pin or something), coupled with a fuse on the power supply rail feeding the h-bridge.

Seems kinda like a hack.

 smiley
5057  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: why should I use a H-Bridge on: March 08, 2010, 06:30:16 pm
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without an H bridge you can only control the speed of the motor in one direction

Actually, on the Milton Bradley Big Trak, they used a weird "half h-bridge" design with a dual ended power supply; only two transistors used per motor.

Regarding h-bridges: on another thread, someone mentioned that using only NPN transistors for such a bridge could be a bad design; it wasn't said, but I got the gist that it was better to use NPN on one half, and PNP on the other. I have designed an h-bridge using 2n3055 NPNs (TO-3 cases), but after hearing that information, I looked into other designs - I found this:

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

If I used the 2n3055 complement (MJ2955) for the other half, is this really a better design? It seems safer (ie, no possibility for SEDs/FEDs), and would allow me to implement coasting and braking (something I can't do with only NPNs?).

Am I looking at this wrong? Is there a way to use only NPNs and still get the safety from shorts? I am already using opto-couplers, and only drive one half at a time - but if both halves are high, fireworks (I imagine!) could appear...

Is an NPN only h-bridge safe, or -must- you use complementary pairs? If I have to re-design, no big deal - I am just looking for some feedback (PM me if you want).

 smiley
5058  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: store to buy sensors on: March 04, 2010, 01:20:33 am
Thanks for the links and such, digitalman2112; I guess $5k is cheap when you're looking at $75k. It also says I am in the wrong line of work.

The music video was pretty cool. Thanks again - I always like getting introduced to new music...

 smiley
5059  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: store to buy sensors on: March 03, 2010, 12:14:21 pm
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Since $5585 USD is "affordable", I wonder what their idea of "expensive" would be?

That's pretty funny; I knew these sensors were fairly expensive, but I never realized they were multi-thousand dollar devices:

http://www.hokuyo-aut.jp/02sensor/07scanner/utm_30lx.html

It doesn't say whether it uses angular measurement (triangulation with 2D cmos sensor array), or 2D time-of-flight (LIDAR); with that price I would guess the latter (basically working like an ultrasonic distance sensor, just at a much higher speed, timing the flight of the laser pulse for distance measurement), but maybe not?

I'm hoping for my robot project to implement a cheap multi-point-grid webcam-based LIDAR system (using OpenCVS most likely); maybe if somehow I get funding I can afford some of these more fancy toys!

 smiley-grin
5060  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Why does my resistor have no effect? on: March 03, 2010, 02:50:28 pm
If you need more than 1 amp, and all you have are 78XXs laying around (and are cheap and lazy like me!), you can parallel the lines together for higher amperages (I would only 2-3 like this, honestly - it would get absurd after that).

 smiley
5061  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Why does my resistor have no effect? on: March 02, 2010, 10:45:23 pm
If you want to get the full 1 amp rating for the 7805, you -must- mount it to a heat-sink (unless you like having smoke/fire-emitting devices around!)...
5062  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Multimeter on: March 02, 2010, 04:31:04 pm
Personally, I've never had a problem with the el-cheapo meters from Harbor Freight (discount chinese tool vendor); they tend to last for a fairly long time, they handle light abuse, and the batteries are typically standard 9V.

Not sure how you would be able to tell if there were an issue with one, unless you already owned a quality meter (Fluke or such), then you could probably compare readings...

 smiley

/not a professional
5063  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Is this right board for connecting sensors to a PC on: March 02, 2010, 04:36:33 pm
Also - keep in mind that depending on the tank, usage, and local/State laws (whatever that means in the context of your geographical location), modifying or even attaching such a device (without being licensed, etc) may violate laws; check into this before attempting any such modification, and if in doubt, ask a professional.
5064  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Batteries on: January 31, 2010, 05:08:29 pm
If you are planning on running on the on-board regulator (say via the external voltage jack), then you are going to need at least 6 volts (you can go all the way up to 12 volts, IIRC); or 5 1.2 volt NiMH cells wired in series (here is where going with a standard R/C car 7.2 volt NiMH pack makes sense; tons of current available, and easily within the spec for the on-board regulator).

 If you are going to bypass the regulator (as noted by koyaanisqatsi) by connecting to the 5V input pin on the Arduino board, then a 3 or 4 cell pack (3.6 or 4.8 volts, respectively) would work fine; just note that without a regulator, once the battery voltage drops below 3.6 volts, "strange things" will occur (everything from wrong readings on analog pins, spontaneous resets, to just turning off). Also if there are other loads connected to the battery, their power needs may cause the voltage to fluctuate to the Arduino, which could cause similar strange issues; the purpose of the regulator is to deliver the exact voltage needs, regardless of the level of the input voltage (within certain parameters, of course).
5065  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Batteries on: January 30, 2010, 10:02:53 pm
I would get name brand myself; you might also see if there is a Batteries Plus near you, they might be able to make you some custom packs (may not be cheap, though). Stay away from Harbor Freight (or any other discount tool seller) rechargeables; I haven't had too much luck with them lasting.

I once actually had pretty good luck with some rechargeable alkalines (yes, they do exist) made by Eveready, IIRC. I don't know if they still sell them or not, but they are available.

Something else: At one time, it was possible to get SLA (sealed lead acid) in C and D cell form (a long time ago, All Electronics carried them surplus). I don't know if they are still made or sold, but that is another option.

Lastly, realize that NiCd and NiMh (and maybe LiIon and LiPoly, but don't quote me on it) only have 1.2 volts per cell, not 1.5; so four cells will give you 4.8 volts, not 6 volts - that may not run the Arduino on the external power connector (I think the onboard 5V regulator needs at least 6-8 volts to work?). A better bet would be to use 7.2V (6 cell) or 9.6V (8 cell) R/C car batteries; some can be "quick charged", and there are really good ones out there that can take a beating - check out your local R/C car hobby shop, let them know what you need/what you are doing, and they can easily hook you up (pick up some servos while you're there!)...
5066  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: 3 axis stepper sheild for arduino on: February 26, 2010, 04:21:31 pm
Making it a separate board with an on-board Arduino processor would probably be the best idea.

There is already a user here on the forums who has made an Arduino-based CNC system, and put a small g-code interpreter on it (if you still had room to include such code, that would be a pretty amazing product!).

Also - if you could add the ability to switch on/off one or two other high-current loads, that would be perfect (so you can switch the spindle motor, blower or vaccuum motor, or coolant pump on and off).

Sounds like an awesome project!
5067  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: panning a PING: code check! on: February 25, 2010, 12:06:00 am
Something I had thought about my reply: what happens if the ping isn't received back? You essentially get stuck. What you would probably want to do is somehow implement a timer that is started when the ping is sent out, then checked periodically and if the time elapsed is greater than say what the time it would take to cover 30 feet or so (or whatever your max sense distance is), count it as "no object" and move on.

Sorry about the oversight...

 smiley
5068  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: panning a PING: code check! on: February 23, 2010, 04:47:40 pm
You kinda have to do those steps, because it takes a certain amount of time for the sensor to fire and receive a return (speed of sound); you can't move the sensor before it gets the return echo back.

Maybe you could approach this another way:

1. Fire the ping; on the return echo activate an interrupt.

2. In the interrupt, store the value, rotate the servo a little.

3. If at the "end of travel" (whatever that means to you), reverse the direction of the servo or position it back "home" (whereever that is).

4. Go back to step 1

This way you would only be moving the servo on receipt of the echo, running the system as fast as the ping sensor will go, not as fast as the servo will go.

It will still be choppy, but hopefully less so.

You might also mount the ping on a stepper, instead of a servo (but you will need some other kind of position feedback); you might be able to get better angular resolution this way.

If none of that works to your satisfaction, then you may want to look into a custom "flying spot" LIDAR (http://en.wikipedia.org/wiki/LIDAR) system (not cheap, not easy, but definitely homebrew-able!).
5069  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Need help with discontinued transistor on: February 25, 2010, 05:50:11 pm
Actually, it could be acting as a heat sensor!

I seem to recall that some transistors when hooked up in a weird manner (or even a sane manner) will change values, etc based on temperature. Theres even a way to use a transistor in such a manner to get "noise" out of it for a true random number generator (very useful for encryption).

So maybe it is? You can't really know unless you have the schematic handy (and even then it may be difficult to tell)...

 smiley
5070  Forum 2005-2010 (read only) / Frequently-Asked Questions / Re: Need help with discontinued transistor on: February 25, 2010, 05:06:05 pm
Alright - now you're cross posting, and that looks bad (nettiquette and all); you're off the hook this time...

 smiley-wink

As far as what the weird config is for, with a picture of what is going on, who knows? How are the leads connected? Is it arranged in some weird form of darlington? Maybe they were stacked to conserve space or use the same heatsink (doesn't sound like a great design, though)? Maybe they were stacked (and if the leads are in parallel) added together for slightly more current handling?

Any number of possibilities, and analog design like that is not my forte, unfortunately. You might have better luck regarding this if you post on a board for vintage (is it vintage) audio amplifiers or stereo/hifi enthusiasts...

Good luck getting it fixed!

 smiley
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