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cool baby mobile...

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mr.v.

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Hi all-
I wanted to make a "mobile" which is basically a spinning device that plays music on a crib for my new son. You can buy ones for $50 but they suck, so I thought I'd make a cool one. However, I ran into some electronic problems and was looking for some help.

Here's the plan:
1) Have a toy 12V motor power the spinning rocket ships I tend to attach to the motor. Power the 12V DC motor with a 12V AC-DC converter. This was a snap.
Here's the problem...it spins WAY too fast. So I was wondering, in order to slow it down, should I put a resistor or variable potentiometer in front of it? I know in theory this would reduce both the current and the voltage across the motor (slowing it down???). Or is there a better way to do it?

2) Have an old portable MP3 player play the music. It has a 1.5V battery. I soldered terminals onto the battery leads and attached them to a 1.5V DC current. It ran fine. The problem is I'd like the whole thing to be powered by one power source. So in parallel, I need to change the 12V from the AC-DC converter to 1.5V only across the mp3 player and NOT across the 12V motor. I discovered I needed a DC-DC converter for the second parallel circuit. The problem is, at Fry's and Radioshack the only DC-DC converters they sell are for Cars and they are enormous. Does anyone know where to buy a small-low voltage 12V-1.5V DC-DC converter or how to make one that's reasonably small and not terribly expensive?

3) play the mp3 through a small 2-1/4" 8 Ohm speaker. So I bought a mini-jack plug and soldered the terminals of the speaker onto it. I can hear faint music when I crank the MP3 player to the maximum but it needs to be louder. The trouble is that it was meant only for headphones. How do I amplify the signal from the MP3 player before it reaches the speaker...preferrably also using the same 12V DC source.?

Is any of this possible?

Any help would be appreciated including links to resources on how I can solve these problems myself. I've been searching the internet for information but I can't seem to find where I can buy some of these things such as a small-low cost DC-DC converter or small speaker amplifier or how to make them. I unfortunately majored in Biology in college which really doesn't help here =)

Thanks again for all your help.
Vik
 
okay I think I've figured out how to amplify the sound...is this about right?

I need a bi-pole transister with 2n-nodes sandwiching a p-node.

The speaker is rated at 0.2W and 8ohm impedence. I think this means that using P=(I^2)*R that the maximum current through the speaker will by 0.158 Amps. And the voltage drop across that should be 1.26V (from P=(V^2)/R. So in theory, I need the current to be at a maximum of 0.158A so applying I=V/R (assuming that the transister circuit will be 12V) I need a total resistance of 76ohms...is that right? So in theory I should have the ciruit consist of a 12V DC current, the transistor, the 8ohm speaker and a 68ohm resistor.
Am I doing the calculatons correctly?

Thanks for your help,
Vik
 
3) play the mp3 through a small 2-1/4" 8 ohm speaker. So I bought a mini-jack plug and soldered the terminals of the speaker onto it. I can hear faint music when I crank the MP3 player to the maximum but it needs to be louder. The trouble is that it was meant only for headphones. How do I amplify the signal from the MP3 player before it reaches the speaker...preferrably also using the same 12V DC source.?
it should work fine with the speaker in question..
you might have hooked it up wrong , because the speakers in a set of headphones is exactly the same as the one you described..
 
1) The 12V rating on the motor is the maximum voltage. If you used a 6V supply it would spin half as fast. If you want to stick with the 12V supply you can use PWM to slow the motor down. PMW (Pulse width modulation) justs switches the power to the motor on and off really fast. The ratio of how long the power is on vs. how long its off detirmines how much power gets to the motor. There is tons of info on PWM on this forum.

2) There is an IC called a voltage regulator that is designed to take a high voltage in and output a stable lower voltage. They arn't very efficient but they are really cheap.
 
i was @ radioshack 2day and i saw this little 1.5v motor. im sure it could spin a little baby trinket w/ ease. i just dunno how much voltage the speaker would need. i have a little radio that runs of one AAA battery and it can drive headphone speakers loud enough to hurt an eardrum, which im sure would be loud enuf. maybe you could run everything off 1.5v??
 
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