Wiring Help Desperately needed

Ok, here is a problem which has me completely befuddled. I'm really hoping one of you with lots of electronics experience can help me out here.

First of all, here's the diagram (switch and speaker removed for simplicity); saber is running off of four AAA's.

http://i169.photobucket.com/albums/u...n/Untitled.png

I am using the MOSFET rather than the typical TIP42 transistor because the "source" to "drain" resistance is so much less; with the TIP42 I was getting a voltage drop of ~2V; plus the transistor was getting quite hot. The resisters are necessary for it to run properly. This part of the circuit I was helped with by a friend at work (as I didn't even know what a MOSFET is), but he is unavailable to help me for a while.

The problem I'm having is runtime; you can audibly hear the saber running down. It seldom lasts more than a few minutes, and often less; then the sound gets so garbled it's awful.

This makes no sense to me, since the batteries I'm using are rechargeables with 850 mAh; to try to solve the issue, I brought down the current being fed to the LED by the driver to ~720 mA, so the saber should last around an hour... but it doesn't.

Does anyone have any ideas?


Thanks so much



Luke

we talked about this in the chat window and why tapping a battery pack in different places is bad. if you need any more help, please let us know love. ^_^

First of all you should go with a tried and true method. I use the TIP42 all the time with a SINGLE 3.7V Li-Ion and don't have any problems with it and I get a good bit of runtime and don't get the voltage drop that you are getting. If you are using 4 NiMH rechargable batteries (which I have used lots of times with this set up) you don't need to tap the sound board in the middle of the batt pack. The economy board will handle the 4.8V just fine. (it is designed for 4.5V) I have even used 6volts on those boards with no problems. If I were you I would wire it up like our wiring diagram suggest, take readings again and show us pics of how you have it wired up. Be sure the emitter of the TIP42 transistor goes to the Batt + and the collector goes to the LED + and the base goes to the econo board LED - output. I have wired up over 40 econo boards and at least 15 of those with the transistor set up with NiMH, Li-Ion and akaline battery packs and have not had any problems.

I did forget to mention, it's not the normal economy board, it's from the original "build your own lightsaber" kit from back in the day, which, from what I've read, is a bit more touchy on voltage. (It ran off of two "C" batteries)
It would be much easier to just use a 2010 hasbro board wired normally, but my customer wants this board installed "if at all possible." Again, it would be simpler with the board I have to use a 3.7v cell, but the added cost of internal battery/charger setup is outside my customer's budget - so I am stuck with 4 AAA's.

Fortunately, the source of the problem was discovered, so I now have an avenue to move forward with.

And if all we ever did was a "tried and true method," how would we ever discover anything new? :D

I understand completely being restricted by what a person wants in their saber and budget constraints. You could have gotten a 3V voltage regulator pretty cheap too, but as long as it works don't fix it. LOL What did you find was the problem?

I'm also curious of your wiring setup before you used the MOSFET.
From the great deal I've learned about the TIP42 after experimenting with them, with enough current to the base it should give out the battery voltage directly, if wired correct of course.

The problem, as Azmaria noted, was tapping off the middle of the battery pack. I didn't foresee any problem, but I was told this results in a "self-draining circuit."
The solution I'm heading for is, in fact, a 3v regulator, which I ordered last night. I didn't even know they existed before then, as the only ones I had seen were the 12v and 5v ones.

Skott - the wiring setup was the same as above, if you replace the resistors and MOSFET with the TIP42. Perhaps the board didn't give enough current? I didn't understand all the reasons my friend gave as to why the MOSFET would be better - something about voltage switching vs. current switching, and greater efficiency. Anyway, that part of the circuit seems to work just fine.