Turn Any Wired Headphones into Wireless

I love listening to music while working. Last week, I found my old earphones in a drawer. These are old-generation wired ones; the next second, the idea of turning them into wireless ones came into my mind. And here is our project: turning any wired headphones into wireless ones. Headphones and earphones both work with this custom device. Simply plug your device into the 3.5mm jack, slide the on/off switch, connect the Bluetooth to your device, and start listening. The overall story here is that I need a Bluetooth transceiver IC that can connect wirelessly to a device and some supporting circuitry. I have covered some projects in recent articles, see if you can relate.

Well, this is not a new idea; the DF Robot already has one in production. First, I ordered one, and it works perfectly as I wanted. However, after using it for a day or so, I put my engineering mind to work and noted some modifications that can be made. That’s how I have added features like USB data support, USB Type-C power input, a Battery charging circuit, a switch, and a small battery. It is relatively easy if I simply add these things externally using modules, but I prefer a minimal system; that’s why it is better to build a PCB.

About the Bluetooth IC :

I recently posted an article about making wireless earphones using Bluetooth technology from Jieli, China. They made Bluetooth cards and audio processors. This company accounts for nearly 80% of the wireless Bluetooth market, and the product's datasheets are not available on the internet. That's why the products are cheaper; a 32-bit DSP with an ARM core is available for just $2.

However, the problem is that I do not know the IC internals, as there is no online data available from the official vendor. It is challenging to find PINs/functions. However, the modifications I want are relatively simple, so I reverse-engineered the circuit of the original board from the DF robot. All the information about the IC is shared here in the documentation I found on a random website after surfing through the web for almost 1.5 hours.

Components required:

I want to pair this IC with the TP4056 lithium battery charger module, so I desoldered all the components from the TP4056 board for this 5.0 Bluetooth. Find the full BOM file in the shared files below.

• TP4056 IC
• 8205A protection circuit (removed in update)
• JL 5.0 BT
• USB Type-C
• 3.5mm AUX Female jack
• 220u, 100n, 10u, 1n capacitors (all taken from old Bluetooth board)
• 24Mhz Crystal Oscillator
• 0603 LED Indicators
• 1K, 10K, 100R Resistors
• PCB from NEXTPCB

Circuit Diagram:

I redraw the circuit on paper and then finally on the EDA software. The modified circuit is given below, which contains the battery charging circuit, a switch for on/off, and a USB Type-C port with data handling capabilities. The primary challenge here is to arrange it on the PCB with a compact form factor. Because the circuit has components like an IFA antenna and an audio IC, I need decaps and grounding to maintain proper signal returns and power integrity. I have to keep the antenna and audio section away and isolate it from noise sources because it will affect the audio performance. I will explain how I redesigned this circuit to include Bluetooth audio.

PCB designs:

I used the standard battery charging circuit TP4056 with the protection IC, which is removed in the updated version due to the overall wide BOM. Because the Bluetooth IC itself has the features of undervoltage lockout. To minimise noise as much as possible, I have placed the analog components in the centre, away from the antenna and the battery charger.

I followed the exact same placement rule as implemented in the original design. The PCB is slightly larger in size due to the addition of the battery charger, which is positioned away from the antenna. Via stitching is essential, as the board uses only two layers. To keep the inductance path as low as possible, the PCBs are fabricated with a thickness of 1.0mm.

The battery charging circuit, which is placed on the bottom layer, has been moved to the top layer in the updated design. You can download all the required files from here. As the PCB is small and there are no high-speed parts, there is no need for impedance matching at this level. The designs are shared here; I find some difficulties in replicating the same antenna. These PCBs are beautifully fabricated by NEXTPCB, and parts are supplied by HQ Online. NextPCB also supports component assembly services if you want to try it and give it a go.

Soldering the Components:

The design features almost all SMT components, including the Bluetooth IC, USB, Capacitors, and charging circuit. The first priority is given to SMT; it is better to get an SMT stencil if possible. Soldering these parts by hand is a challenging task and requires a lot more courage because any misplacement or cold joint can cause damage/noise in the circuit.

After soldering the SMTs, I soldered the THT, which is the switch in this case. There are bare pads on the back side of the PCB to enable data transfer through the USB feature. It is better to have parts on a single side of the PCB because it helps in soldering (modified in the newer version). Download the Gerber files and get fabricated with the maximum discounted price using my link.

Testing procedure:

Do not plug the battery in first. Check the battery terminals with a multimeter in continuity mode to ensure everything is okay. Then take a 5V charger and plug it in. If you have a USB power meter, it's better to see the current in it. If everything goes perfectly and the current is not more than 100mA. Connect the battery. This is the basic test procedure that you must follow, as there may be shorts in the circuit due to soldering that we need to identify before connecting the battery. The parts are susceptible to voltage fluctuations and may pop out if a misconnection occurs.

Working and Sound test:

Now that the PCB is ready, simply connect your wired headphones to the 3.5mm jack and turn on the switch. Now connect the device to a phone or laptop. The Bluetooth will draw more current when establishing the connection, and once done, there will be around a 20-30mA typical usage. The battery, which runs at 200mA, lasts about 7 hours. And can be charged at a rate of 500mA/hr. This will enable it to charge quickly.

I have tested the Bluetooth for 2 days, and it works well. The 5.0 connection is good, and the antenna is working fine, even for long ranges. Moreover, there is no such noise and hum in the circuit. If you want an assembled and thoroughly tested one, just comment below. See the 30 seconds sound test video attached within here.

Outro:

I just love this device; I can carry it in my pocket with wired headphones plugged in. It comes with a great battery backup, I have removed the onboard LED to somewhat it a bit more. Users who have any earphones or headphones can use this device, even if it doesn't have a 3.5mm jack. I have not tested the latency, but it is a 5.0 version, so it might be good enough. In my testing, the sound was processed in real-time, as it happens with AirPods. If you like this concept, please like and share this project, and check out the services offered by NEXTPCB.