Turning on one device from another

Thread Starter

peecee69

Joined Sep 2, 2020
8
I have a seemingly simple problem that I can't figure out how to resolve. I've repurposed a few bakelite radio cabinets with upgraded internals. I'm attempting to build one with a little class D amp with a volume knob and a Bluetooth module. Both devices are powered independently by the same 12v source (a 26650 5-battery bank). In order to use it, I have to turn both the amp and the Bluetooth module on. When done, I need to turn both off. I'd like to be able to trigger the Bluetooth module on and off when the amp is turned on. Is there a relay of some sort I could use to accomplish this? The amp has a 12v input, line level input and stereo speaker output, and the Bluetooth module has 12v input and stereo line level out. I was hoping I could solder a lead wire onto one of the exposed posts for a trigger (like a remote amp turn on for a car stereo) that goes through some sort of logic board for 12v to send a signal to the Bluetooth module to power up. I just don't know where to start with this, other than asking here.
 

Tonyr1084

Joined Sep 24, 2015
5,091
Welcome to AAC. We're going to need more information on the units in question. To be sure, you've given a lot of detail, which is a good starting point. However, there's a few more points of information needed. Model numbers will be of help as well as a possible diagram or schematic of how you hope to achieve the solution. It's probably very easy. But we (or me) are going to need more information.
 

Thread Starter

peecee69

Joined Sep 2, 2020
8
Thank you for the quick reply. The devices in question are from parts-express:

Part 320-699 - TPA3116D2 2x50W Class D Stereo Amplifier Board with Volume Control
The amp board does not have any tech specs or guide posted on their site. I haven't been able to find one on the internet. Did a bit of searching and found boards that are close, but not this exact one. I've asked any they say they don't have one because it's so simple. Doesn't matter, but I'm not going to argue with them.

Part 320-348 - 12 VDC Bluetooth 4.0 FM Radio MP3 WAV FLAC Audio Preamp Board with Function Cables and Remote

Bluetooth Module Manual
 

Tonyr1084

Joined Sep 24, 2015
5,091
If the board is so simple then perhaps we (someone here) can decipher a photograph of the component side of the board and of the solder side of the board. Until then I can't begin to tell you where to hook up a jumper wire.
 

Tonyr1084

Joined Sep 24, 2015
5,091
OK, that's easy. Just connect the BT 12V lead to the cap C6 Positive lead. You can either connect the negative wire to the GND terminal.

1599060587068.png
 

sghioto

Joined Dec 31, 2017
1,473
I'd like to be able to trigger the Bluetooth module on and off when the amp is turned on.
I think it would be better to turn on the Bluetooth first, if it's possible to get inside the unit and find a switched 12 volt connection. This could power a module or relay that powers up the amp and it's all controlled by the remote.
SG
 

Thread Starter

peecee69

Joined Sep 2, 2020
8
Where on the BT are you going to wire in the 12V? Is the BT switched too? That'll be important as well.
BT turns on as soon as 12v is applied to the unit. I can turn it on and off with a remote, but it will be enclosed in a cabinet and never fully utilized. It's sort of a waste for what I want to do, but I couldn't find anything cheaper. I have an old FM Motorola portable radio that was my grandparents, but it hasn't worked for years. I'm going to shoehorn the amp (using the volume knob on the module for on/off as the original knob did), the BT, a 26650 battery pack, and a new set of speakers, into the cabinet. I won't have access to the BT module unless I cut a hole in the back panel for it, and I'd rather not do that. I'd rather just mount it all inside there and not worry about the extra BT module features (like SD cards, USB, etc.). I plan on using it while at car shows and vintage camper rallies (the color perfectly matches an old camper I restored).
 

Thread Starter

peecee69

Joined Sep 2, 2020
8
Do you mean as far as using the remote?
They make an IR repeater that would allow using the BT in the cabinet.
SG
No. I mean for the physical interfaces of USB and the SD Cards. There's also an FM tuner that I won't be using either. I have used IR repeaters for many things in the past, but this won't need it. The remote works fine through the grille of the radio (if I'd ever need it, which I doubt). I literally just need the BT part, and didn't want or care for any of the other functionality for this project. If I can find a BT only module, I may replace this with that and use this in something else.
 

Audioguru again

Joined Oct 21, 2019
1,934
Wait a minute, your numbers are backwards. You said you have "a 26650 5-battery bank". You said the Bluetooth module needs 12V. But the battery will be 21V when fully charged, not 12V.

1) Will the amplifier and speakers survive 21V? The amplifier will produce 25W per channel into 8 ohms or 45W per channel into 4 ohms. Walmart and Parts Express say that a maximum voltage of 19V is recommended for the amplifier, maybe because its cooling limits it.
2) The Bluetooth module will explode if you do not add a 12V voltage regulator.
3) How will you charge the 21V lithium battery?
4) How will you limit the battery voltage from discharging below about 15V which will kill it?
 

Thread Starter

peecee69

Joined Sep 2, 2020
8
Wait a minute, your numbers are backwards. You said you have "a 26650 5-battery bank". You said the Bluetooth module needs 12V. But the battery will be 21V when fully charged, not 12V.

1) Will the amplifier and speakers survive 21V? The amplifier will produce 25W per channel into 8 ohms or 45W per channel into 4 ohms. Walmart and Parts Express say that a maximum voltage of 19V is recommended for the amplifier, maybe because its cooling limits it.
2) The Bluetooth module will explode if you do not add a 12V voltage regulator.
3) How will you charge the 21V lithium battery?
4) How will you limit the battery voltage from discharging below about 15V which will kill it?
The battery bank supplies up to 21V based on what port I use to power the amp (I'm using the 12v out). According to the tech people at parts-express, the amp can accept 5v-24v. I have the exact same setup (without the BT) that has been in use for about 3 months without issue. I just bought the same parts, plus the BT, to modify the radio.

1. I think they will. Your math is spot on (you knew that already - lol), and I came up with the same numbers.
2. That is exactly my concern. Hopeful the amp puts out a clean 12v signal from the post on the board as noted above. But, since I haven't done it yet, there's always room to improve.
3. The batteries are being charged with a AC-DC adapter that was purchased in the kit that had the batteries as well, so I'm going out on a limb thinking they're matched correctly.
4. The battery board says it has circuitry to prevent that.

More good feedback. Thank you!
 

Audioguru again

Joined Oct 21, 2019
1,934
Are you using the 21V battery to supply 12.6V from only 3 of its 5 cells? Then 2 cells are wasted. The charger might have trouble "balancing" the charges.
 

ThePanMan

Joined Mar 13, 2020
41
Since it appears the TS might not understand the batteries all that well - perhaps we need to focus on how the TS is planning on charging them. Li-Ion and LiPo's have to be charged in very specific profiles.
 

Thread Starter

peecee69

Joined Sep 2, 2020
8
I understand perfectly that the available 21V will only discharge at 12V because that's the port I'm using on the battery board. This same setup has been working fine in another box, charging and discharging perfectly. Let's stay focused on the original question - how to power on/off the BT module from the amp. I think I have enough information to at least provide feedback in this thread once I get the chance to continue working on this project (hopefully this weekend). I appreciate all the feedback so far.
 
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