Tuesday 12 March 2019

Bluetooth Audio Transmitter Repair

We recently had a problem with our Insomniacs Bedside Radio.

The Bluetooth pairing button seemed to have shifted position within the box, causing it to be jammed on.

So time to open it up and reposition it, or so I thought.

I built the Insomniacs Bedside Radio for my wife during the summer of 2015. The idea is that she can reach for the Bluetooth headphones during the night, and listen to the BBC World Service, until she is bored back to sleep.

The radio itself is just a WiFi enabled Raspberry Pi connected to a small Bluetooth transmitter module, which I must have bought via the internet. The module is just a little black box with a 'pairing' button, which also flashes red and blue to indicate 'I-know-not-what' status conditions.

The button can only be depressed by pushing through a small hole that I drilled in the plastic case of the radio's box. So its a tight fit, and the module is just clamped down via a plate with the button aligned as close as possible to the hole.

insomniacs bedside radio raspberry pi bluetooth
Top button: Bluetooth. Front button: radio/music selector.

While investigating the problem, I initially just slackened the plate and moved the module back into place by putting side pressure on the switch. I couldn't see the module itself at this stage. But happy that it was re-aligned, I powered up and tested it.

I still could not pair with the headphones, so I plugged in a wired set to the Pi audio output to confirm that the Pi was still working.

When I finally removed the Pi and the base plate, I was very surprised to see that the Bluetooth transmitter module had opened up. The internal battery had inflated to the size of a German Zeppelin and split the module case wide open.

insomniacs bedside radio raspberry pi bluetooth
the Bluetooth audio transmitter

I think the last thing you need to sleep next to, is a swollen Lithium battery that has been pressured inside a small box.

So with the skill of a 'B' Movie bomb disposal expert, I carefully cut the red and black wires, before very carefully picking it up by its wires and carrying it outside.

Upon powering up the radio once more, I has happy to see that it was working and it 'paired' with the headphones. The module receives 5Volts via the charger connector, so really does not need its own battery in this application.

It was only after I'd fully re-assembled the radio that further testing revealed another problem. After the system had been paired for about 5 minutes, the connection would drop and I had to re-pair. I checked this several times and it always seemed to run for 4-5 minutes before dropping out.

I wondered if the circuit relied on the battery to hold up the supply in the presence of noise, so I fitted a 100uF 16V capacitor, effectively in place of the battery.

insomniacs bedside radio raspberry pi bluetooth
battery replaced by a capacitor

This bodge seems to work fine. As the capacitor was a little too round to fit inside the Bluetooth module, I just pressed to 2 sides of the case together, over the capacitor wires, and reassembled the radio.

Its been working now for over a week, so it looks like a valid fix! I don't know whether a capacitor of lower value would work. There didn't seem much point in experimenting, but if the system seems to frequently lose connection with the headphones in the future, a higher value may be necessary.

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