1975 with Bluetooth

 In 1981 I had a Radioshack 150 in 1 kit, and it came with a circuit for a siren. You could replace the capacitor to make the frequency go up more slowly, and a button for it to go either up (charge capacitor) or down. I also had a Siare 12 Ohm oval speaker, which, when hooked up to this circuit, was quite loud.

On the first Monday of the month, at noon, the air raid alarm would play, and I would open the window and then make the same sound this way.

I still have the speaker, as you can see, and I was wondering what to do with it. I had already made a regular active monitor for my synthesizer using a TDA2030A and imagined I could do something similar here, and then realized it might be more fun to make it into a bluetooth speaker.

For the amplifier I decided to use the LM386, which should be powerful enough for this speaker, and is from approximately the same vintage (1975). I used the standard circuit in the datasheet, with a 220uF capacitor for decoupling and a 470uF capacitor for the speaker, mostly as these were available in my older drawers, to stick with the older components. However, the rest of the system needed to be newer.

To power the amplifier I used a MT3608 buck-boost converter, which can be bought as a ready made module on Amazon. This was set to slightly below 12V and also the source of my star ground configuration, to prevent ground loops. The power for this came from an Arduino breakout board that my sister had given me in Australia (from the excellent Altronics) which uses the TP4056. I decided to desolder the SMT LED from it and instead have regular LED on wires, so I could show them externally. I also got a 12mm USB-C jack to attach to it, so I could hook that up externally and the cheapest LiPo battery I could find on Amazon, which remarkably had a capacity of 3200mA and came  with a charger and a tester. I used double sided tape to mount it into the box, same as the MT3608. For the TP4056 I mounted it on tiny wood blocks, so it could remain cool.

The finally part was the Bluetooth module, for which I chose the VHM-314. I also tried to desolder the LED here, but it damaged the board, so instead I decided to drill a hole in the box for a lightpipe instead, and mounted the PCB using a screw, again from  1975 or before (it came out of my grandmother's cake box that my father filled with screws). This Bluetooth module is promiscuous, so it will hook up to anything that wants to.

The box itself was a standard enclosure of the size of the speaker. I also bought 3D mesh speaker cloth for it. Drilling the oval hole was involved, using lots of drilling and dremeling, but it worked. I had to add some double sided tape offset to not have the center magnet stick out. I finally added a switch to turn the thing on and off.

You can see the end result working here.