Andy Lamb

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Banana Phone Handsfree Kit

My 4 year old son, Cameron, has been asking for a "real banana phone" for the his birthday and Christmas since he was 2. So I thought it was about time I built one for him... His requirements were:

  • It must look like a banana.
  • It must be yellow. Of course.
  • He must be able to speak with Grandad on it.
I didn't want to let him loose with an actual phone of his own, so I thought some how linking it to my mobile was the best approach. I knew of the Hands Free Protcol (HFP) Bluetooth profile used by car handsfree kits so decided to start there. Using HFP would allow the device to pair with any mobile phone, meaning it's not reliant on my phone alone.

I settled on a BK8000L single-chip bluetooth audio module, it has an integrated analogue-digital converter for a microphone and an integrated digital-analogue converter for a speaker. The development board I settled on included a 3.5mm audio jack for output, but no jack for a microphone. It also runs on anything from 4V to 12V, making it easy for me to power the device using a USB powerbank. Best of all, the module comes pre-loaded with an HFP Bluetooth profile! So no software required for this project... which makes a change. Here is an image of the board I used. I ordered it from Ebay, where there are various vendors selling various development boards for the BK8000L chip.

BK8000L development board

I prototyped the microphone circuit on a breadboard, based on the diagram below using an electret microphone, but, being cheap, I used just two different capacitors. I replaced C26 with a 0.1uF capacitor. It works fine so that's what I've stuck with. The speech is perfectly clear, but a little quiet. I think that's down to the microphone I used rather than the circuit, though.

Microphone circuit diagram

To make it look like a banana, I used a plastic case designed to protect a banana from bruising in your bag.

To keep things simple I made use of the 3.5mm audio jack on the development board and a pair of old ear buds. This isn't a great solution. It's a little quiet as the buds are inside the case but your ear is not, and the headphone wires take up a lot of space in the banana case, but it works. If I was to build another banana phone I'd get a small speaker and wire it up properly.

To power the device, I was looking to use a small USB powerbank, like those used to charge mobile phones. They output 5V which is suitable for powering the BK8000L. Unfortunately, all the powerbanks I looked at were too bulky for the banana case. So I used a stand-alone charge/discharge module connected to two 18650 Li-ion batteries.

I'm always cautious of these batteries and ad-hoc charging circuits, I only charge them when I'm nearby!

When powered on, the device pairs to any mobile phone like any other handsfree kit. Because I'm using the pre-loaded software, it's listed as "BK8000L" on the phone. I've read that it's possible to change the name by downloading the image, editing the appropriate resource string and uploading the modified image to the chip, but I've not looked into that.

Cameron was over-the-moon when I gave the banana phone to him and he realised it actually worked. So, from that perspective, it's been a very successful project.

Banana phone internals
The device just fits inside the banana shaped case.

Cameron using the banana phone
A happy Cameron using the banana phone.

© 2021 Andy Lamb