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DIY Bluetooth Retrofit CM5908

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    DIY Bluetooth Retrofit CM5908

    My cassette deck belt finally broke on my Alpine CM5908 radio in my 91 vert, so I figured it was time to upgrade. I really love the look of the OEM radio, and I didn't really want to have to replace it. I considered just replacing the belt in the cassette player, but I was tired of using those janky casette-to-3.5mm adapters. So I decided I wanted to try to retrofit Bluetooth into my CM5908 radio. I found some options online, but didn't find any that fit my desired use case. I was after something that was (1) simple, (2) cheap, (3) DIY, (4) retained radio receiver functionality, (5) reversible if I ever wanted to go back to the cassette deck. This is what I came up with.

    The design is very simple. I just took out the cassette deck, and did the bare minimum in order to get the Bluetooth module to function in-place of the cassette deck. This means there is no extra integration with the radio display/buttons. The only thing you can do from the radio itself is enter/leave cassette mode (aka Bluetooth mode). In order to do this from the radio itself, I disabled the original volume-knob push-button functionality (I'm not even sure what it did originally), and I used that as the way to turn on/off the Bluetooth mode of the radio. I figured some people may not want to do this, so I also designed a simpler version which uses a simple external SPST switch to enter/leave Bluetooth mode.

    Here are the main required parts. Note that for most of these, there are many options that will do the job.



    Bluetooth receiver:
    These are the ones I bought. They work great. https://www.amazon.com/gp/product/B07W4PJ469

    12V to 5V converter:
    I harvested mine from an old car USB adapter. Other options would be: https://www.amazon.com/HiLetgo-LM259.../dp/B00LTSC564 or https://www.amazon.com/PoiLee-10pcs-.../dp/B07FS73T9C

    Update: another user on here tyeler18 just built one of these using a combined bluetooth receiver+ voltage regulator board. If i were going to do it again I would just buy one of these for $12. This board will take place of the previous two parts (bluetooth receiver + 12v to 5v converter) https://www.amazon.com/gp/product/B07P94Z9XR

    Switching circuit:
    If you want to use the volume knob push button, you need a way to convert the momentary pulse into a bi-stable latching circuit. You could build this yourself, or just buy one of these like I did: https://www.amazon.com/gp/product/B08593HHDG
    If instead you want to mount an external switch, pretty much any switch will do.

    So, all in all, we're talking maybe $20 in parts...less if you decide to use up some of those spare parts you have been hoarding for years.

    I built my circuit on a piece of perf board, 3d printed a bracket (available at https://www.tinkercad.com/things/63HjVIQ4z0e) that shared the mounting holes with original cassette deck, and used header pins as a way to plug into the existing plugs in the radio. The only soldering on the radio itself was soldering a wire to the volume potentiometer board, and soldering a small wire to the cassette ribbon cable. Both of these mods could be easily un-done to revert to original.

    I'll let the pictures do the rest of the talking.

    Here is the design based on using the volume knob:
    Click image for larger version  Name:	e30_bluetooth_diagram_1.png Views:	5 Size:	200.8 KB ID:	9955924

    And here is the design without the latching circuit and volume knob integration (disclaimer: I didn't actually build this one...so it may not work.. but it should..)
    Click image for larger version  Name:	e30_bluetooth_diagram_2.png Views:	5 Size:	187.4 KB ID:	9955925

    Here are some build pictures with annotations:


    Click image for larger version  Name:	combined_build_images_2.jpg Views:	73 Size:	76.0 KB ID:	9955931

    And the final product (3d printed bracket available here: https://www.tinkercad.com/things/63HjVIQ4z0e )
    Click image for larger version  Name:	IMG_1142.jpg Views:	0 Size:	94.6 KB ID:	9955930


    Well I hope this helps someone who is also trying to retrofit Bluetooth on the cheap, but still keep most of their radio functionality.

    -bbowman
    Last edited by bbowman; Yesterday, 06:02 AM.

    #2
    "Very nice!"
    -Borat
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      #3
      Wow this is super clean! I ended up gutting my radio and sticking a bluetooth amp in it.

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