If you have an older car, with a cassette player in it, I KNOW you'd want it to be able to play the music from your phone. You might have even tried different ways to do that and neither one was perfect.
This one is. It's the best way to do it. I'm not bragging, I'm stating the facts. I hate to see a good amplifier (the car player is also an amplifier, a good one, too) rendered obsolete because people can't connect their phones to it.
But you need to know your way around with a soldering iron. If you do this right,, not only you'll get crystal-clear AUX IN sound (on any cassette player!) but you can also upgrade it to a Bluetooth-connected sound system! Read on and do it, it's worth it.
If you're afraid of ruining your car's tape player, buy a used one and work on that (just get its anti-theft code with it). They're cheap now and still put out around 40W of good quality sound. The sound settings will still work. You'll make yourself a cassette player that puts out solid sound from you phone using a cable or a Bluetooth connection.
Step 1: Car Cassette Players. If Only Those Had AUX IN ...
Well, they could have had it but back then phones as music sources weren't the norm at all, you know. So nobody thought about that. Maybe a few. But the norm was most of the old car cassette players had no AUX IN.
If you're like me, the owner of an old car that had a factory fitted cassette player you would long for a stereo input jack enabling you to play all that music you have in your phone. Or drool whenever you saw a phone streaming music over Bluetooth to the new car's sound systems. Yep, I've been there.
All sorts of AUX IN devices sold over the internet tricking some CD output of some car players but those were newer than mine so I didn't bothered with those. Besides, many users reported low quality playback using those devices.
So here's how I did it. I consider it the best way there is. I gathered this information from a lot of sites with different procedures. Zero noise, perfect digital quality, no volume degradation, on the contrary you get additional signal gain so your music is now loud and clear. But you have to be skillful with the soldering iron. And you have to have some understanding of what we're doing. Also, a multimeter is mandatory. So here goes.
Step 2: Here Goes Nothing ...
First of all, be aware of the PIN code of the car player. You have to have it. Because if you don't, the next time you put the player back into the dashboard it won't work without it. At all.
So, get the cassette player out of the car. Open it. It'll look like this, or similar. Take a photo using your phone. Carefully take the tape player part out. It's mostly mechanical stuff, with springs and levers. But it also has micro switches with wires. (and a reading head). Some are depressed and some are not when a tape is in so try the mechanism with a tape and see how the switches state change (also, take photos...). Make a note of it all.
Take it all out, (make photos of every step) you can cut the wires but label the pairs for each micro switch and the ones that power the small electric tape motor. Those switches tell the radio it has a tape and it changes the source from radio to tape. Then, cut the wires and solder a normal switch (the kind that stays pressed when push it and you have to push it again to release) to those wires in order to change the source just by pressing it. Great. Now we have a switch that tricks the player into playing a cassette that doesn't exist anymore. We'll take care of that soon.
For now try and find a place for your switch so that you can access it. I re-used the eject button for that and didn't ruined the car player's look. I advise you do the same.
Step 3: The Hunt for AUX IN Lines
It's a hunt, but an easy one. You already saw where the tape head ribbon (or wires) connects to the mainboard. It's a connector, visible here. Follow the traces from it and it will ALWAYS go to a preamplifier integrated circuit. This small IC (also visible here) amplifies the weak signal from the tape head to deliver a proper volume to the main amplifier.
Once you found that integrated chip, search Google for its datasheet. In that you'll see the stereo OUTPUTS of that chip (usually left out, right out and ground). Make a note of those. Those pins are the ones you'll use for the AUX input. Let me repeat, we need the stereo OUTPUTS, not INPUTS. Many people on the internet mistakenly used the stereo inputs, ending with horrible sound quality. So don't make that mistake.
In fact, once you find those OUTPUT pins, the best way is to de-solder the entire IC from the board. We don't need it anymore, there's no tape head to be amplified anymore. And the phone's stereo output is louder than its output anyway. So take it off and throw it away. Solder a stereo IN jack to those wires and place it in a hole somewhere on the player's faceplate. Or place as a very ugly wired connector dangling out of the cassette door. Your choice. I did it different, you'll see it soon.
You could leave the IC on the mainboard but you'll get noise. A lot. Because that IC still receives power from the player and it has internal noise, despite the lack of signal from the tape head.
The easiest way is to take a nail clipper and cut the power pins of the preamplifier IC, check the datasheet for that. Then, it'll be silence. I recommend this way, it's less work.
Step 4: Feast Your Ears
Now that you have a switch and a stereo input jack, it's time for a test. Place the player intro the dashboard, input the code if necessary and turn it on. Radio should be unchanged. Now press the switch. The player should indicate "TAPE". Connect you phone and play a song. If you did it right, pure digital sound will flow, making you wonder why did it take you so long to do this. You'll discover new life in those tired factory-fitted car speakers, believe me.
If you hear nothing, check the following - is the car player indicating "TAPE"? If not, you messed up the wires from the micro switches. If it does, and you still hear no sound with the phone connected, you messed up the stereo OUT pins of the preamplifier IC. Taking photos of the entire process is the only way to go back to the drawing board. In fact, this instructable is made out of my "backup" photos of the entire process.
As you see, I got creative in placing that switch and stereo connector. This is a SEAT car player I modified for my brother. Try to do the same, it's more work but everyone will think that car tape player had those factory-fitted.
If this is all you wanted, stop here and enjoy your connected phone music. Despite the required cable connection, it's simple to do and reliable. Make a photo of your achievement and let me know you made it.
But I wanted more. I wanted Bluetooth connection. The next step is for the brave ones.
Step 5: Bluetooth Car Music on a Budget
Bluetooth enabled car players are expensive. Or they come with an expensive car. It shouldn't be like that, but it is. Well, I wanted it cheap. So I found a good stereo Bluetooth receiver in one store. It must be a reputable brand, cheap Chinese stuff sounds horrible, believe me, I tried. So in my case, a Philips AEA2000/12 fit the bill. And the sound quality was great. It was powered by a small 9V power supply.
I opened it up to see it's actually using a 5V regulator (7805) that can easily take 12V but first I replaced the 10V rated capacitors around it with higher voltage ones. So now it works on 12V.
Remember the power wires from that electric motor? Measuring voltage on those lines with the player in TAPE mode revealed it was a 12V one. Good! Here's the power supply ready! And it turns on only when the player is put in TAPE mode (using the switch). Just perfect.
I soldered my stereo AUX IN wires from the earlier steps (with a multimeter I found better soldering points than those small IC pads) to the output of this Bluetooth receiver and powered it with the electric motor's wires (mind the polarities). I placed everything inside the player (we do have the space now). One quick pairing and the sound coming out of my car with me standing outside was all the rage of the evening. I measured about 25 meters range for my connection. I only need about 2-3m so I was extatic.
I can connect to it with any android/Windows/IOS phone and tablet. The sound is perfect. No hum, no static. It's way better than the FM stations I get on the radio side.
The last photo shows my brother's setup, he purchased a different Bluetooth receiver powered from USB 5V so I sacrificed a car power supply to power it from those electric motor wires. Insulate it well, you don't want it to touch anything in there.
My Bluetooth receiver uses Bluetooth 2.1 EDR standard, my brother's is 4.0 with auto connect. On mine, I have to manually connect each time (cumbersome, I know, but there are apps for that) but I got 25 meters range! His setup self connects to his phone but only has about 3-4 meters range. Bluetooth 4.0 has a shorter range but increased data speed. For music 2.1 is enough, but higher is always better. You can use any brand, but it has to be a good one. Test it before using it for this project.
That's about it. If you have questions, just ask, I'll try to answer them.