Making a Bluetooth adapter for a Car Phone from the 90's

[deleted]

This was a response to a post that turned out to be spam, and has been deleted now. I deleted my response and the quote of the spam post to avoid preserving the spam.

Here's a tour and demo of the completed car phone system in my car!

It's good to see it all come together! Really great end result.

My project has been featured in a few articles!

• Hackaday
• Jalopnik
• Japanese Nostalgic Car

I've received several comments about my prototype circuit board layout looking "like art", so I decided to put it on display like art with a clear polycarbonate lid for the project enclosure. Now the circuit will be on full display whenever I show someone the car phone system in my trunk.

I replaced the rest of the electrolytic capacitors on the 30-year-old "hands-free controller unit".



The 3 remaining on the board are the ones I replaced when repairing the board to make it work.



None of the capacitors I removed had any obvious visual signs of failure (no bulging, no crud/residue from massive leaks, etc.), and they were still in range of rated capacity according to my multimeter. I don't have an ESR tester.

But clearly there was something wrong/degraded with at least some of these old capacitors, because the microphone audio quality is much better now! Both in volume and clarity, and less "cutting out".

It's still not perfect quality like talking directly on a handset or using a modern Bluetooth hands-free system in a modern car. It's a bit less clear, a bit more "distant" sounding, and consonants are softened a bit (especially at the ends of words). I suspect this is mostly due to the 30-year old MC34118 speakerphone audio processing chip/circuit?

According to my wife, it is good enough now that it's in the realm of "normal" and not obviously "wrong" or distracting. That's what I strive for!

Here's a demonstration of what my voice sounds like on the other end of a call from my car phone:

Small update:

1. I swapped a resistor in my circuit to increase the external microphone amplification, so now the volume of my voice on the other end of the call is much more consistent between the handset microphone and the hands-free microphone. The op-amp circuit is now using 1M and 5.1K resistors for approximately 192x amplification!
2. I've been having occasional issues with all sound being completely non-functional. I was able to probe the circuit while it was happening one time and confirmed that my analog switch IC was not letting any sound pass through any of its 3 switches. It was driving me crazy, but it would rarely happen, and randomly resolve itself pretty quickly. Stupid me didn't read the data sheet well enough. There's an "enable" digital input pin that enables all switches when low, and disables all switches when high. I originally left this pin floating, so it would sometimes have enough stray voltage to disable. That pin is now grounded, and I'm glad I found such a simple explanation and solution for my seemingly random and rare problem.

dropped a reply on YT - doing similar with a 1990 Olds Toronado Trofeo. emulating a handset though instead of a transceiver.
( email address removed) if you want to reach out.

Jeff,
got email . will remove mine now. will respond there.

Nice job.

If you want to boost the treble, add 33 to 39 nF across 5.1k resistor with a series R up to 2k2 in series. This OpAmp will be using up all the Gain BW if a typical 4 MHz GBW type so the plot will roll off the high frequency worse than this plot if only 1MHz. Then you need a better Op Amp.

Tony Stewart said:

Nice job.

Click to expand...

Thanks!

Tony Stewart said:

If you want to boost the treble, add 33 to 39 nF across 5.1k resistor with a series R up to 2k2 in series.

Click to expand...

I think I'm going to leave it as-is, because all evidence points to the original "Hands-Free Controller Unit" being the source of microphone audio quality issues, but "working as intended":

• Microphone audio quality is great with a simple microphone plugged into my circuit (the microphone that came with these car phones for a generic aftermarket install in a car, where you would clip the mic onto the sun visor).
• I already refreshed the "Hands-Free Controller Unit" with all new electrolytic capacitors, removing a major potential source of audio quality degradation due to 30 years of aging. Audio quality did improve significantly when I did this, but still not up to modern standards (as shown in my video).
• I got some feedback from someone that actually had this phone system in their car back in the day. He said the hands-free mic audio quality on the other end of the call sounds like what he remembers, and possibly even better than what he remembers. According to him, the audio quality is authentic to the original experience. My overall goal with this car and the car phone project is to restore/maintain it to function and appear as originally intended.

Hi, sorry if I missed an update in the journey about the power supply to the handset, but at last look it seems like you're powering the original transceiver, charging and drawing from the NiCad batteries, etc. all to simply provide power to the handset. Is there a reason you had to maintain the power to the handset through all the old transceiver gear? Was there any thinking of getting rid of the old power (and being able to get rid of the old transceiver box altogether) and supplying power to the handset direct by way of a new means?

MKS said:

Is there a reason you had to maintain the power to the handset through all the old transceiver gear?

Click to expand...

It's a mixture of laziness and justification of my laziness

• The original transceiver provides functionality that I was able to reuse, keeping the scope of my already-ambitious (for me) project more manageable.
  • Support for both battery and external 12V power supply.
  • Battery charging circuitry.
  • Battery level detection/reporting (for presentation on the handset and over Bluetooth to the paired cell phone), and low-voltage power-off.
  • Power on/off functionality with a mix of power button on the handset and the ability to have vehicle ignition turn the phone on/off.
• I like that I have retained the original transceiver as a functional component of the system. When installed in my car, every piece of the original car phone system is still in use. Nothing is there as a "fake prop" just for appearance.

MKS said:

Was there any thinking of getting rid of the old power (and being able to get rid of the old transceiver box altogether) and supplying power to the handset direct by way of a new means?

Click to expand...

It is something I am considering as a future improvement, but only if I can package my circuitry inside of the original transceiver case (replacing the original circuit board). I would not want to eliminate the original transceiver entirely. One challenge is that the transceiver case is thick aluminum, which will be a problem for the Bluetooth antenna AFAIK.

The ultimate goal would be to carry the car phone around in portable mode, fully functional, with no externally-visible signs of modification.

My project this winter is to build a new prototype with some minor hardware changes:

• Returning to a pair of INA105 precision unity gain op-amps to generate the differential audio output, because the DRV134 I'm using now has noticeable noise from the serial data handset communications.
• A simplified breadboard adapter for the BM62 Bluetooth module that does not have a bunch of unused audio jack and microphone jack circuitry on it. I want to ensure that my audio circuitry works well with "direct" connections to the BM62.

Once I'm happy with the audio quality, I'll move on to other goals at my leisure.

MKS said:

Was there any thinking of getting rid of the old power (and being able to get rid of the old transceiver box altogether) and supplying power to the handset direct by way of a new means?

Click to expand...

I have to thank you for asking this question, because it got me seriously thinking about it again.

I did a lot of searching/reading about li-ion battery charging components/circuits, how to deal with charging multiple cells that are connected in series (I need 3 cells in series to get to the operating voltage range of the handset), how to support external power to the project while simultaneously charging the batteries, then switch to battery power when external power is disconnected, etc.

Here's some of the info I was finding:

• Hackaday article about Li-Ion battery circuitry
• A Stack Exchange question and related discussion about using TP4056 boards (with integrated discharge protection) to charge 3 cells in series.

I was already starting to research components (TP4056 boards, DC-DC isolating converters, P-FETs, etc) and trying to come up with a plan to prototype. While I was searching for Li-Ion battery cell holders, I stumbled upon this:

LBB-3v2 3 x 18650 Lithium Battery Charger Board/Module 12V with Charge Protect



From what I understand so far, I think this provides exactly what I need! I can use the ~12V output of that board to power the rest of my project. It handles everything else about battery charging, switching between external and battery power, over-charge/over-discharge protection, and it even has some external LED indicator outputs that I can likely adapt to digital logic inputs to my MCU to be aware of when external power is connected, when the batteries are charging, etc.

I think all that's left for me to worry about is:

• Implement my own power on/off functionality.
  • Detecting the handset PWR button press is easy (it's basically just a momentary switch that connects to ground when pressed)
  • Detecting vehicle ignition is easy (~12V on a specific pin of the vehicle power supply connector for the phone). Combined with the battery board's external power indicator output, I should be able to implement the auto power on/off with ignition.
  • Figure out how to properly use the different power modes of the MCU so I can put it into a low power mode while monitoring inputs for power-on.
  • Figure out exactly how I'm going to power on/off the rest of the circuit. I think it may be as simple as running the ground of most of my circuit and the handset through a transistor so that I can use a digital output of the MCU to switch the rest of the circuit on/off? The entire circuit + handset draws well under the 1A current limit of a basic transistor, even when in a call and producing the loudest possible volume of sound.
• Figure out how to package everything inside the original transceiver.
  • I'll probably try to fit everything except the Bluetooth module inside the aluminum transceiver case. At minimum, that's where the connectors for the external power, handset, and external microphone need to be.
  • The Bluetooth module will need to go in the plastic "transportable cover" where original large NiCad battery goes. I hope that a wired connection between the Bluetooth module and the "main" board will not introduce any significant noise issues.
  • Finally learn how to design a PCB so that I can build a main circuit board that positions all the connectors exactly where the original transceiver connectors are, with mounting holes positioned to reuse all the original mounting points.

I was briefly concerned that I wouldn't have enough I/O pins on my MCU to add support for all of this because I currently have only 2 unused pins. I'm already monitoring the handset PWR button with one pin, but I need 4 more:

• External power connected indicator input.
• Vehicle ignition on indicator input.
• Battery voltage analog input (so I can calculate/display/report approx. battery level).
• Output to control power on/off of the rest of the circuit.

But then I realized that I'll be freeing up 2 I/O pins for the UART communication with the transceiver that I won't need any more. I think I'll end up using every single pin on this MCU.

Nice find! This place is selling them at $15.49 so pretty good value as well!
(I was expecting them to be a silly price).

rjenkinsgb said:

Nice find! This place is selling them at $15.49 so pretty good value as well!
(I was expecting them to be a silly price).

Click to expand...

It's also available on Amazon (from the same seller) for the same price and shipping.

I'm pretty sure it would cost me more in parts + shipping to try and fail to build my own custom power supply and battery charging circuit, and it would be about 10 times as big on prototype board with through-hole components

BTW - The same company (Dayton Audio) has several other modular boards, like audio amplifiers, a 5-cell battery board, etc.: https://www.daytonaudio.com/category/233/audio-amp-boards-modules

I'm disappointed with the lack of technical details on their product page and owner's manual. I'm waiting for a response to some questions I sent Dayton Audio about some details, but I might give up waiting for a response to the web form I submitted and just call them tomorrow:

• Is the output voltage regulated in some way, or is it directly the voltage of the batteries in series? I'm hoping for unregulated so that I can estimate battery level from voltage. If it's regulated to 12V, then I'll have to make use of the battery charge status LED indicator outputs to get a very rough estimate (in 25% steps), or make some custom connections to the board to find raw battery voltage.
• What's the voltage output for the external LED indicators, and is the "LED-" pin connected to the output ground? I need to figure out how to convert those LED outputs to digital input for the MCU.
• What is the output voltage behavior when external power is supplied and batteries are low/charging? I'm hoping that the output is a regulated 12V derived from external power supply in this situation. And it's my understanding that li-ion charging circuitry doesn't work properly when something is drawing power from the batteries at the same time, so I think the output must be "decoupled" from the batteries while charging?
• What is this strange connector for external power input?
  • I think it might be a quick disconnect for bare wire? But then I need to know what gauge wire to use.

The implication is that the output is regulated, The status connector appears to have a battery voltage pin as well as the LED and switch connections, so metering should be easy.

You will have to wait & see what the LED pin voltage is - it depend how they are switched and from which polarity.. It looks to me like they are switched to 0V by the LM339 next to the connector though.

It says it provided 12V while charging, so it appears to have battery bypass.

The input connector looks like some type of cage clamp - and suitable wire is supposed to be included with the module?

It's rated 5A maximum so I'd expect it to take 22awg at a minimum & probably at least 18awg.


I found a place in Europe that has them, so I've ordered one myself.

I called Dayton Audio and got some answers about their "LBB 3V2" battery/charging board:

• Output voltage is NOT regulated. It varies with battery charge level: about 12.6V at full charge, and discharge cut-off happens around/below 10V.
• Could not answer specifics about the LED indicator outputs. They are intended for use with optional pre-assembled plug-n-play LEDs (with wires, connectors, and resistor).
• Could not give exact details on output voltage when charging, but a general observation is that it will be somewhere in between current battery voltage level and max voltage at full charge.
• For this connector, use a 22-18g wire, tin it with solder, and insert it into the connector (could not identify the exact make/model of the connector, but described it as similar to a Wago connector):

I also got a part number for a Molex Micro-Fit connector (0436540200) for connecting to the black output connector on the board.

Thats enough info for me to order the board and have confidence that I can make it work for me. I'll figure out the details of the LED outputs on my own.

I confirmed a suspicion: these modules from Dayton Audio are primarily marketed as a plug-n-play interconnectable modules that are designed to work with each other and other accessories that they offer. Most customers just mix and match the modules with each other and don't worry/care about detailed specs. I suggested that these products could be more widely marketed to DIY/maker/Arduino customers if they provided more detailed technical specs. They agreed and said they would pass on the suggestion to produce more detailed datasheets and make them available.

At initial glance, I thought these were JST XH connectors on the Dayton Audio LBB-3V2 board. But when I started putting together an order for parts and looked more closely, I found I was wrong:


I called Dayton Audio again today, and they were unable to tell me what type of connector these are. They recommended I submit the question via their website "contact us" form, and they will look into it and respond when they get an answer.

For now, I think I'll just have to buy their kit of pre-build accessories that plug into these connectors so I can cut the wires and use them as pigtails: