Car Stereo Tape Player to Bluetooth Hack

In this Instructable I will show you, or better yet take you on the journey of how I managed to modernize my 2002 Suzuki by hacking the tape deck into a modern age Bluetooth stereo. If you'd like to skip the journey, the last step has a quick summary of the key steps. But you will be missing out...

As much as i used to love buying random 80's cassettes for 50 cents from thrift stores and jamming out to the early days of synthesized music, i found that recently i use my deck exclusively for an auxiliary adapter plugged into my phone so i can listen to far superior non-80s music via Spotify. This is great, except for the terrible frequency range of the tape adapters, the static noise, the inevitable broken 3.5mm plugs, the wire that gets tangled around everything in the front seat, and just the constant reminder that my car is 15 years old.

Now, i could have been a normal, boring human, and gone to the local car audio section of my favorite department store or online retail giant, and bought a compatible bluetooth stereo. I could have even bought a slightly more interesting bluetooth cassette that would have plugged right in, end of story. I might have even been able to get a pretty penny for this old hunk-o-junk. But no. I am a DIYer. Why would i pass up an opportunity to tear something apart? To see whats inside? No, this has to be done the hard way.
And so the journey began...

Step 1: Buy a Bluetooth Dongle

The idea is simple. Feed the line level output from an off-the-shelf Bluetooth receiver to the tape decks output to the amplifier inside the car stereo.

I'll admit, I had no idea if I would be able to do this; to find the right point in the circuit, let alone be able to splice into it, but it seems simple enough. The worst case scenario: I break something, then I have to buy that new tape deck anyway, plus I am bound to learn something. Worth a shot right?
I actually had this idea for awhile before trying it. The hold up was the cost of the Bluetooth dongle. The only ones I saw were at least 20 dollars, which is a lot to spend on something that i had little confidence will actually work, especially since i could buy a whole new stereo for not much more. That was true until I found this one, and at a 6 dollar price point it was finally worth it to try. So i did.
(And yes, dongle is a real term)

Step 2: Take Out Your Car Stereo

Take Out Your Car Stereo
If it isn't obvious, someone on YouTube will show you how to do it for your make/model. Fortunately mine was straight forward, with some screws exposed. The stereo was easily removed from the car and brought to the chopping block.

Step 3: Take 'er Apart

Take 'er Apart
So. Many. Screws. With each screw I took out, the voice inside my head telling me I shouldn't be doing this grew louder. Luckily that voice was muffled by the loud non-80s music I was listening to.
Once all the screws were out, the sides and top came off like like a puzzle, with some hidden interlocking tabs.the front control panel also easy pulls off. Once those pieces are off, the goodies are exposed.

The CD player lifted off as its own unit, attached only by a sturdy ribbon cable. With this placed to the side, the tape player was easily accessible. It took a bit to familiarize myself with the guts of the tape deck. It is an impressive little machine, with an array of tiny levers, motors and gears.

At this point I actually used a 12 volt lead acid battery and some alligator clip jumper wires to power it up, mostly just to see it working, and because I will need to test my connection point. I had some trouble finding the harness cable pinout online, so I took a voltmeter to my car and just measured the pins on the harness there. With the car on, I found three pins at 12v, and from other pinouts online I figured these were the ON power (the main power source, only on when car is on but not necessarily started), permanent power (connection straight to battery for persistent memory) and ignition power (only on after engine has been started). I found that i only needed the ON power and the permanent power for the stereo to work. The ignition power just dimmed the lights. I then reasoned through some more wiring harness pinout diagrams to figure out which pins were for the speakers, and I connected one set to a speaker I had in the room. Finally I turned it on and was able to hear the reassuring static of an antenna-less car radio. Wahoo! I didnt break anything yet!

I next switched to the CD player, it works! My music started playing, even with the CD player propped up on its side. Ok, now for the tape deck. I press the button... it didn't switch. Hmmm. What is wrong? Oh ya, there is no tape in the deck! It doesn't allow me to switch to the tape deck if there is no tape. But i don't want a tape, so what to do? Inside the tape compartment i found a lever that gets pushed back when a tape is inserted. I just used a screwdriver to shove it back into the "in" position, and voila, i can now switch to tape mode.

Step 4: Find the Solder Points

Find the Solder Points
Feeling a new sense of motivation and accomplishment at having just assembled the world's stupidest home audio system, I got on to the task at hand. Finding the tape deck audio lines. It wasn't hard to figure out which circuit board contained the tape deck electronics, as the tape head was directly attached to it. Hmmm...how to identify the audio traces... oh, how about the 5 parallel traces in the top center labeled FL, FR, RL, RR and common! One for each car speaker (front right, front left, rear right and rear left), and a common. Each with a convenient solder bead right there! Well that wasn't so hard. Could it really be that simple? Could all I have to do is splice into this connection? Well, almost. It does get a bit more interesting, so stay tuned.

Step 5: Strip a Headphone Jack

Strip a Headphone Jack
If you don't already have a handy stripped headphone jack lying around, make one. Any pair of broken headphones should work. Better yet, if you have a 3.5mm to RCA cable, i have found they are easier to use because the channels are split and the wire is usually a thicker gauge. Trying to strip a tiny headphone cable can be cumbersome.

Inside the outer insulation there should be two insulated wires (probably red and white, or other colors), one for left speaker and one for right, and one or two non insulated wires or foil for the "common" (this might be insulated too, if it is it will most likely be black).

Step 6: Test It Out

Before i dove into the Bluetooth dongle, i plugged the jack into my phone to do a quick test. Turning my phone volume very low to try not to blow anything if I was doing this wrong, I took one set of speaker wires (one colored and one common) and touched the colored wire to the FR on the circuit board and the common to the common. It took a couple of tries, because i guess my speakers weren't connected to the front-right output. I moved the colored wire to the FL, then the RR, and finally it worked! I heard some crackly noise, and once my hand steadied i heard my phone playing through the stereo! Wow this might actually work!

Step 7: Freak the EQ

Freak the EQ
The first stumbling block had to do with the frequency response. I noticed that, while playing through my phone, the music sounded overly loud, with lots of clipping, and extremely heavy on the base.
Let me preface this section (and the rest of the instructable) by telling you that i am not an electronics engineer. I am just a concerned citizen. So if any of you have corrections/clarifications/suggestions, i'd love to hear them in the comments.

A little background here: Tape decks work by reading the analogue audio signal encoded with magnetic particles on the thin strip of tape inside the cassette. The quickly changing polarity of the magnetic particles corresponds to the actual sound wave. This magnetic strip is run passed the "head", a sort of electromagnetic microphone, to convert the magnetic signal to an electrical one sent to the amplifier. Ask wikipedia for more details.

Why am i telling you this? Well, the signal coming of of the tape deck is really small. It is not quite the "line level" coming out of my phone. There is usually a preamp stage to bring it up closer to line level before the main amp. I don't actually know how this specific amp is set up, weather these leads i am trying to use are before the preamp, or whether the main amp is just configured for lower input levels, but either way, my phone's signal is too high for this point in the circuit.

The solution? Throw in a resistor. I chose a 15k resistor inline with my audio signal. This seemed to help remove the clipping. Later i will realize it is still a bit louder then the radio or CD player, but definitely better. I also later thought about this more, and a voltage divider might have been a better solution here than just a resistor inline, since this is likely an input to a very high impedance input. If i ever take it apart again i will experiment.

Next there is the overpowering bass problem. Cassettes dont have a flat frequency response, so the amplifying circuit will likely do some adjusting to flatten it out. As you can see from that link, the bass is usually weak, so to equalize the response, the circuit will need to boost the low frequencies. So, when i plug my already equalized phone output, the boosted bass becomes overpowering.
The stereo has a digital equalizer, but i don't want to have to keep adjusting the bass all the way down when i switch to the Bluetooth. How can we attenuate the bass level before it is spliced into the circuit? An inline capacitor should do.

An inline capacitor forms a high pass filter, meaning direct current will be blocked and low frequencies will be attenuated. Smaller capacitors have a higher cutoff frequency then bigger capacitors, meaning they will reduce the bass more.

Here is a tangent about my way of thinking of capacitor filters, in case it helps. Capacitors absorb energy, to a point. It can only hold so much energy at a given voltage. This is what it's Farad measure indicates. A small capacitor gets "saturated" with energy quicker than a larger capacitor.

 If a frequency is high, meaning it is reversing polarity very fast, it wont be bothered by a small capacitor, because it has turned around before the capacitor gets saturated. A low frequency, however, doesn't turn around quick enough, and thus will saturate the small capacitor and be "clipped". After the capacitor is saturated, no more current will flow. A larger capacitor, however, will take longer to saturate, and thus a lower frequency will have ample time to turn around without being clipped. However a very low frequency will still be clipped, and so forth.

There are lots of calculators out there to calculate the size you need based on frequency cutoff, but i decided to just try a few different values that i had lying around. The first one i tried was a relatively big capacitor, at 33 uF. This didn't reduce the bass enough. Next i tried a .1 uF "104" capacitor. This actually seemed to work very well.

The photo shows the headphone leads after the resistor and capacitor have been soldered.

Step 8: Solder the 3.5 Mm Cable

Its finally time to solder the headphone cable into the board. Be sure to read the rest of the instructable before attempting this step to avoid the same mistake I did.
I soldered one wire to both the FR and RR, and the other wire to the FL and the RL, and then twisted both commons together and soldered them to the common. I plugged into my phone, and it worked like a charm... or so i thought....

Step 9: Finding a Power Source

Finding a Power Source
The bluetooth dongle is powered by 5 volt USB. I hadn't even plugged it in up to this point, so i figured I should test it to make sure it works ok. i plugged it into a wall adapter and plugged in the jack, and then connected with my phone. All works as expected. Easy to connect, good quality sound, and most importantly, i haven't broken my stereo yet. Excellent.

I want this to be hidden inside the stereo, so I need to find power to pull from inside. It needs to be at least 5 volts, able to source enough current, and not interfere with audio signals. Hmmm... where to look... Hey what is this thing labeled SDC2l? It has a red wire going to it. A quick google search reveals it's a motor. Must be the one that spins the cassette tape. That'll do. Actually, this might solve two problems. I noticed when in tape mode, the tape-spinner mechanism still spins (because, of course, it thinks there is a tape in there). This spinning mechanism makes a bit of noise which i would rather do without. Could i disconnect this wire from the motor and use it to power my bluetooth? Lets try!

Snip.

Fail.

After snipping the wire and switching to the tape player, the deck did some robotic looking movements then ejected the ghost tape. I suppose it has some sensor inside that notices when the wheel stops spinning, which would happen at the end of a tape. By snipping the wire, the wheel no longer moves, and it thinks the tape has ended so it ejects. Bummer.
I reattached the wire, and also soldered another wire that i will use to power my device.

Step 10: Powering the Bluetooth Dongle

Powering the Bluetooth Dongle
The power coming off of the motor lead is 12 volts. I need a converter to lower this to 5 volts. I could order some fancy adapter, but i dont want to wait! Luckily i have an LM7805 in my goodie box. Lets build one.

The circuit is incredibly simple. 12v in, 5 volt out, ground to ground. Throw in a couple smoothing capacitors (in parallel this time) to filter out noise (especially important when dealing with audio applications) and we've got a power supply. I used two capacitors, a 330uF and a .1uF because I had them lying around. See the first picture.

I soldered this to the power wire from the last step and found a ground to solder to on the board (actually the whole chassis is grounded, so i could have throw it anywhere). I plugged in the dongle and tested it out.

Aaaaaaanddddd.... fail.

Something isn't working. The light comes on, but nothing is playing on my test speaker. Hmmmm.... the circuit looks right... could it be unable to supply enough power? I don't know how much this bluetooth chip draws, but it can't be much. Maybe i screwed something up? Maybe i shorted something or burnt the chip while soldering? But the light comes on... Oh well, i could sit here and troubleshoot, but this isn't a power supply project. I have another idea.
I had this little red cigarette lighter adapter in my car. You can get these for 5 bucks at walgreens. This is nothing more then a 12v to 5v adapter with a USB plug. I took off the casing and soldered some leads to the 12v and ground pins, and soldered into the stereo. Done.

Step 11: More Fail

More Fail
At this point i plug the bluetooth dongle into the new power supply in the stereo and test it WITH MY HEADPHONES (this is an important point, as you will see). I had already disconnected the test speaker and didn't want to set it back up. I assume all is well and good, and that was the final step, so i reassemble the stereo, put a million screws back in, and go excitedly install it back in my car.

I test the radio: works.
I test the CD player: works!
I test the tape player.... nothing.
What happened? Is it something about being connected to my car? It had just worked! Uggh...
I sadly remove it from my car again. Back to the workbench.

I open it up. A million screws.

I get down to the circuit board. I plug in headphones, connect power, and... it works! I reconnect the test speaker... it doesn't work... hmm. Did i blow something in the amplification circuitry? I remove the dongle and plug it into the wall adapter, still connected to the test speaker through the stereo... and... it works! HMMMMMMMMM.

Ok, this is weird. For some reason, when powered from the stereo power, it only plays on headphones. When powered from non-stereo power, it will play on the speakers. It must be something with the power source.

I was stumped. This was weird. Maybe something with the grounding levels.... I was about to put it away for the night, but in a stroke of genius i decided to try one more thing. I had soldered the signal and the common coming out of the bluetooth dongle to the circuit board. The dongle is also connected to ground through the power supply. The common on the circuit board goes... who knows where. But maybe the issue is that i am tying these two together through the bluetooth module. I snipped the common wire and... IT WORKS! Hooray! My music is playing through bluetooth on the test speaker while the dongle is powered from the stereo!

So to recap, the trick is that the bluetooth module output must only be connected to the FR, FL, RR and RL pins, no common. Without tracing out the circuit board, I still don't quite understand why, something about the signal common being at a different level then the stereo ground. Whatever. It works. Lets move on.

But hmm... this probably also explains why i though my first power supply didn't work!

Step 12: Success!

Success!
Time to install it.. .again. Yadda yadda, million screws, back in the car, test it out and ta-da! It worked like a charm. A bit louder than the other stereo functions, but other than that a complete success! There was still some static noise, perhaps from the cheap power supply to the bluetooth module or maybe from the tape player running empty (after all, the tape player is still fully connected... i wonder what would happen if i insert a tape?) but it is better than before!

Just as a side note, when hiding all of the components inside the stereo, i used a small plastic bag as a layer to separate it from the circuit boards and stuff, so when driving if i go over a bump it doesn't bounce and touch the wrong thing and fry my stereo.

Thanks for taking this verbose journey with me. Hopefully you leaned something about car stereos along with me. This was a fun project and, besides for the time, saved me at least 40 bucks! Plus i now have a nice talking point in my car.

Step 13: Summary

Summary
If you didn't want to take the whole journey with me, or just want a recap, here is the highlight reel:
  • Remove and disassemble the stereo until you get to the tape deck
  • (optional but reccomended) set up the stereo with a power supply and speakers to test it on your work bench
  • find and set the mechanism to trick the deck that a tape is inserted
  • strip a headphone cable
  • find the amplifier inputs on the tape deck circuit board, for me these were labeled FR, FL, RR, RL, one for each speaker in the car.
  • Solder the headphone jack wires to these points on the board, one wire to both left channels and one wire to both right channels. DO NOT CONNECT THE COMMON (ground) WIRE. Consider adding resistor/capacitor inline to attenuate the signal for a more balanced output.
  • find 12v coming out of the stereo (i used the wire to the tape deck motor, but don't disconnect the motor!)
  • solder the power supply to the stereo power
  • hide everything inside the stereo and reinstall.
Thanks for reading!
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Hi, I`m Sostenes, Electrical Technician and PLC`S Programmer.
Everyday I`m exploring the world of Electrical to find better solution for Automation. I believe everyday can become a Electrician with the right learning materials.
My goal with BLOG is to help you learn Electrical.
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