MP3 player module: adding power switch & analog volume control - Page 1
Forum section: electronics / beginners

author: xeezawaki
author_rank: Newbie
timestamp: January 31, 2017, 10:11:02 pm
content: Hi!Here I am on my first post in Beginners sectionI recently purchased few cheap mp3 player module boards (about 2.5€ each, jeez... and think many years ago I paid 20 times this price for a Zen...), in order to experiment with them.The project I'm facing, it's to transform a fancy good'ol AM reciever from the 70s I bought in a flea market into a portable mp3 player. Nothing new here, as there're thousands of those kind of refurbishments on the Internets.The thing is that this module (You can find pics of it here in this link:http://imgur.com/a/363Kh), is perhaps too 'digitally-sh' to make a fancy hybrid with the old reciever and the player. It has buttons, and oddly placed for my purpose. I have no major issue with the just 'click' buttons, as I can bridge another microswitch and plug in on both ends of the actual buttons, or even desolder them and place them anywhere else.But in order to set volume up and down, I have to push & hold the 'next' and 'prev' buttons of the mp3 player module, and I'd like to alter this functionality by doing it via the old reciever's potentiometer (you can see pictures of it in this linkhttp://imgur.com/a/CGouX, which also acts as a power on/off switch. I've measured the resistance across this potentiometer (betwen the first rightmost contact and the third rightmost contact), and when the switch is in 'on' position, resistance is almost 0 ohm. If I turn the wheel to its limit, resistance is about 5K ohm.Now focus on the potentiometer use to regulate volume output, I'll make my enquiries about the on/off functionality later. My first easy thought was to add the potentiometer at the output of the mp3 player. But after a bit of research, that's a NO-NO thing to do, because if I understood correctly, the potentiometer would take all the power output from the amplifier and get fried, or worse, fry the amplifier itself.An alternative I read on stackexchange, would be to attach the potentiometer between the input of the amplifier, but I'm not sure how to do the connections, despite I did my research:Unfortunately, despite its popularity, I found no specific schematics for this mp3 module. The packaging label reads: "Shenzhen Landa Tianrui Electronics CO LTD - Model: LDTR-WG0039". But as I could expect, I found nothing. Seems that there're tons of electronics companies in Shenzhen which produce this module, but they have not even a website. The closest thing I've found that resembles the possible schematics of this board are here:http://www.chipkingdom.com/upfile/201531917474080714.pdf, but this has an extra FM reciever, not found in the board I have. Not a major issue, as they seem to share almost same components despite the differences.I think though, the circuit is apparently simple enough to reverse engineering it and draw its schematics, but I'm too rookie on the field in order to understand correctly where the PCB tracks go. I can't even understand clearly where the common GND is! (I think this is a double sided board, as have tracks on both sides?, sheesh, no idea)The good thing is that I've found both of the IC components of the board. The amplifier itself is labeled 8002A, which brings me to a double Op amplifier, which datasheet can be found easily by just googling (here is a link for the lazyhttps://www.openimpulse.com/blog/document-viewer/?pdf_file=HXJ8002-Miniature-Audio-Amplifier-Datasheet.pdf. The other IC, which has no more possibility to be the decoder itself, is labeled GPD2856C-009A. A google search for this, brings up one of the schematics I linked above (again, link for the lazyhttp://www.chipkingdom.com/upfile/201562714582989910.pdf). No in-detail specs for it as the component itself, though.So, I've did my research, but now I have no idea about what to do next... At least to figure out how that thing really works and where to put my desired pot.The other inquiry, is about the on/off functionality. I think (or would like to think) this is the easiest part. Just wire up the rightmost and the leftmost part of the usb input / batt with it and it's done?Well, my apologizes for the extension of this post, but I'm pretty motivated to get my hands on and learn about it! So any ideas, and of course, corrections and explanations are more than welcome. I also appreciate quite much links to videos, books or text explanations

author: danadak
author_rank: Super Contributor
timestamp: February 01, 2017, 12:36:28 pm
content: Problem with module, does it "remember" volume setting with switches ?If so a workable design would be to raise volume to max, connect one end of potto ground, the other end becomes input signal, and wiper contact of pot goes toinput of module. Like the way pot is connected here, attached.If module does not remember setting, each time you power up you will have toraise volume to max on module so that pot will work effectively.Regards, Dana.

author: xeezawaki
author_rank: Newbie
timestamp: February 01, 2017, 05:57:58 pm
content: I'll check out if the mp3 module does remember the volume setting. Hope so!Many thanks for your reply!

author: xeezawaki
author_rank: Newbie
timestamp: February 01, 2017, 06:59:36 pm
content: Well... checked out and no.When you turn it off and on again, the volume is set at 50%. I'm quite unsure if this behavior can be altered.When you say:Quoteyou mean wiper contact of pot should go to the input of the 8002 op amp, like in the attachment, isn't it?~~~~~~~~~~~~~~~~~~~~~~~~~EDIT: Well I investigated a bit more, and seems that I can work out something to set up the volume up to the 100% at the start. Would be a terrible idea to build up a 2 stage time delay circuit attached to a transistor iin order to emulate the long press of the volume button?

author: Relaxe
author_rank: Regular Contributor
timestamp: May 23, 2018, 03:24:46 am
content: Hey there,I stumbled upon this, trying to do the same exact thing as OP did: I modified an old computer speaker to transform into a "three button" MP3 player for my 4 years old kid.I have tried to figure out the schematic of the little thing, so I started removing parts, measuring them and placing them back.Attached is the schematic: Original drawing edited for clarity. Also added the switch and volume potentiometer in red.Also, a picture of the completed device.