I made this thing that converts ordinary headphones into Bluetooth headphones. My laptop and phone both have Bluetooth so this will work great with them.
Roving Networks released a new Bluetooth module, the RN-52, that used the A2DP audio profile and outputted audio directly, without any configuration. I thought this was pretty cool. I’d much rather build my own Bluetooth headphones instead of buying them. So I decided to design a circuit to use the RN-52.
Like I said before, this module will output audio without any configuration steps, this means no programming or microcontroller required! This makes the circuit really simple. The circuit only has 3 sections: the RN-52, a power supply, and a audio amplifier.
The power comes from a 18650 rechargable lithium polymer battery. I picked the 18650 style because the charger is common (so I don’t have to build a craddle or dock or incorporate any recharge management circuitry), and the battery holder is also easily obtainable. I decided to design the PCB to be exactly as big as the battery holder, and add holes so I can either wear it like an lanyard or zip-tie it to my headphones.
The battery is regulated down to 3.3V with a LDO voltage regulator, which saves a lot of space and cost in comparison to a DC/DC step-up converter. There’s a Schottky diode (low forward voltage drop with good continuous forward current rating) to prevent damage if I insert the battery wrong, low voltage cutoff so I can’t over-discharge the battery. The battery itself also has protection internally for the stuff that might cause damage (warning, this is because I got a good high quality battery, not all batteries have internal protection!).
The 18650 battery is a beast and will last a few days, especially the high quality ones I have. I’ve actually done discharge tests with these and their capacity is truly what they claim them to be. Unlike cheap no-name brands who advertise 6000 mAH but only actually provide 1800 mAH. These ones claim to be 3400 mAH and actually deliver all of that.
The audio amplifier is centered around the TPA6112A2 operational amplifier. The circuitry forms a basic inverting amplifier with about 2X gain, and it can drive about 150 mW. The circuit mimics the recommended circuitry that is shown in the RN-52 datasheet and user manual…
…so this part of the project was easy to design. (protip: Don’t know what you are doing? Just copy the datasheet/user guide. This tip applies to all circuits!)
Add some status and power indicator LEDs, power switch, and a headphone jack, and the project is pretty much done. For those of you who are out of project ideas, see how easy it is to make an useful everyday product at home? Now I can walk around with my headphones without being tied to my computer by the cord. I know I can buy something like this for around $10, but that’s NOT FUN. I rather enjoy and take pride in making my own gadgets.
The schematic and PCB files are in EAGLE 6 format. Download them here: rn52headphones20130626.zip
Footnotes: It’s not exactly easy to solder this circuit because the opamp chip and the RN-52 have bottom pads that can only be soldered using reflow soldering techniques (hot air, oven, skillet, infrared, etc). I also designed it to support microphones, but that part of the circuit doesn’t seem to work, so I didn’t mention it as being a feature. (I probably made a mistake, meh, no big deal)
Another footnote: This is the 2nd prototype, the first prototype had volume control buttons and fancy stuff like that. But I couldn’t get those working under Windows because of a missing driver issue that I can’t resolve, Roving Networks technical support pretty much ignored me. So this 2nd prototype do not have those features.
Protip for you guys: use a blue, BLUE, emphasis on BLUE, LED for power indicators when you use a lithium battery and 3.3V LDO V-reg setup like I did. The blue LED will act as a low battery indicator, and become dimmer and dimmer as the battery is about to die. This is because blue coloured LEDs are made with a silicon junction that has a higher forward voltage than other colours. This higher forward voltage is typically around 2.8V, which is right around when a lithium battery is about to die, and a blue LED will become dim or stop glowing at all. If you used a green or red LED, their forward voltage is around 2V or lower, so even if your battery is about to die, the LEDs will still be brightly lit.
I may have given you a PCB. First check the revision number. The very first revision has the number 201306267. As of now, this is the only revision I’ve given out.
The parts you need to get:
|Sch Ref||Name||Value||Pkg||Ratings||Possible Part|
|audio jack||3.5mm stereo||SJ-4351X-SMT from CUI
such as SJ-43514-SMT
|BATT1||battery holder||for 18650 batteries||BH-18650-PC or BH-18650-PC2 from MPD|
|RN-52||Bluetooth module||RN-52 from Roving Networks|
|SW||switch||EG1390 from E-Switch|
|U1||amplifier||MSOP-10||TPA6112A2 from Texas Instruments|
|IC1||voltage regulator||3.3V||SOT-23-5||500mA output, 5.5V input, 0.2V drop||MIC5319-3.3YD5 from Micrel|
|IC2||voltage supervisor||2.7V||SOT-23-3||TC54VN2702ECB713 from Microchip|
|D1||diode, schottky||SOD-123||20V reverse voltage||MBR0520-TP from Micro Commercial Co, can be substituted|
|LED1||LED||blue highly recommended||0603||generic|
|LED2||LED||green or red||0603||generic|
|C1||capacitor, tantalum||10uF||1206||6V or more||generic|
|C3, C4||capacitor, tantalum||47uF||1206||6V or more||generic|
|C2||capacitor, ceramic||470pF||0603||6V or more||generic|
|C7, C8, C9, C10, C11||capacitor, ceramic||1uF||0603||6V or more||generic|
|C12||capacitor, ceramic||100nF||0603||6V or more||generic|
|C13, C14, C15, C16||capacitor, ceramic||100u||1206||6V or more||generic|
|R1, R2||resistor, chip||1KΩ||0603||0.1W power, 5% tol||generic|
|R3||resistor, chip||100KΩ||0603||0.1W power, 5% tol||generic|
|R5, R6, R9, R10||resistor, chip||47KΩ||0603||0.1W power, 5% tol||generic|
|R7, R8, R11, R12||resistor, chip||22KΩ||0603||0.1W power, 5% tol||generic|