UPS for MP3 player

I have a cheap MP3 player that runs on 1 AA cell. I'd like to power it off a wall wort 3 volt AC to DC converter I have. I'd also like to have a battery in the circuit so that if house power is lost, the player will continue uninterrupted off the battery. The player is going to provide background music and music on hold through a small telephone system. The problem with just running off a converter is that after any type of power interruption, the "play" button on the MP3 player has to be pressed to get it working again. The circuit should be designed in such a way as to keep the battery charged and power the player off the converter except when the power goes off.
Any ideas?

I think I'd tackle that problem in a different way. Instead of using a battery for those rare occasions when you experience a power-cut, why don't you just use a "power-on reset" type circuit to activate the play function of the device? It would involve some minor hacking of the MP3 player, but it would be a reliable solution.

What do you think? Or are you heart-set on using battery backup?

Brian

That would be perfectly fine. The problem is I have no information on the MP3 player and wouldn't know where to begin to "hack" into it. Do you have any ideas on a circuit that would momentarily close a set of dry contacts when first powered up? I could then wire it in parallel to the play button. Is that what you had in mind?
Also should I post this question on the circuits forum?

sorry for my diagram, here's my jpeg version

Can you not use a 1.5VDC wallwart to charge a 1.2V Nicad battery that is always there running the MP3 player? AC goes and the battery runs it?

mramos1- Wouldn't I need some type of limiting circuit to prevent overcharging of the battery? Since the wallwart I have is 3 volt, can I use a resistor to reduce the output to 1.5 volts? How do I determine the proper resistor value?

jumides- I'm confused by your diagram. Is there only 1 diode between the battery and the MP3 player positive? Does this circuit prevent the battery from being discharged over time even if the converter is running the player the entire time?

holabr said:

That would be perfectly fine. The problem is I have no information on the MP3 player and wouldn't know where to begin to "hack" into it. Do you have any ideas on a circuit that would momentarily close a set of dry contacts when first powered up? I could then wire it in parallel to the play button. Is that what you had in mind?
Also should I post this question on the circuits forum?

Click to expand...

Well, you'll probably find that the play button momentarily switches 5V through. If that's the case (take the device apart and use a DVM to see) then all you need is a power-on reset circuit that momentarily applies 5V to the same place the play button does, after a power-cut. A power-on reset circuit can be as simple as a resistor in series with a capacitor connected across the 5V rail. At the junction of the resistor / capacitor, connect a NAND gate that has both it's input pins tied together (A Logic Inverter). When the circuit is first powered up the capacitor will be charged via your resistor and the output from the NAND gate will be a logic 1. This will activate the play function of your MP3 player. After a short time the capacitor will have charged up such that it is above the threshold of the NAND gate and the output will return back to 0. It will stay 0 until the power cycles again.

I'd draw you a circuit but I'm in work at the moment. Does my explanation make sense?

Brian