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

[UselessPickles]

I need a way to test my battery module prototype. After some searching and reading/watching reviews, I ordered a Kunkin KP184 DC electric load. The lowest price I found was on Banggood.

It seems to have a lot of good features for the price (optional remote voltage sensing, constant current/voltage/power/resistance modes, battery discharge mode, over-current protection test mode, remote/automated control features, etc.) and good accuracy. One "negative" I've seen reported in a review actually seems like a good thing to me: while the load is "on", adjustments are immediately effective. As long as you are aware of this and don't do stupid things, it seems useful to be able to manually sweep the current and monitor voltage readings.

I hooked it up to my power supply and multimeter to play around with it and get familiar with its features, and also learned that my power supply under-reports current draw quite a bit more at lower amperage compared to higher amperage (-2.3% @ 200mA, -0.4% @ 500mA, -0.2% @ 2+A). But voltage output/readings all seem to be pretty close, and the electric load seems to have pretty accurate control of amperage.

I also made some custom cables for testing the battery module:

Cables for connecting the power supply and electric load to the input/output of the battery module, potentiometers that I can connect to the board to simulate temperature readings from thermistors, and cables for eventually connecting the battery module to the car phone Bluetooth adapter motherboard for a full integration test.

The potentiometers include a resistor to offset the lowest possible value (giving me a more useful range of adjustment), and intentionally bare wire for connecting a multimeter to monitor the resistance as I adjust the knob.

The assembled boards have shipped already, so I should be able to start testing by the end of this week.

 

[UselessPickles]

My battery module unfortunately does not work

Here it is all assembled and hooked up for testing with the LEDs lighting up as expected and giving false hope.

First, the protection IC wouldn't enable charging/discharging of the battery. I spent a long time trying to figure this out, double-checking everything I could think of, and eventually discovered some strange voltage readings at the per-cell connections for cell balancing. This led me to discover I had a mistake in my schematic where I mixed up two connections because I simply placed two "hierarchical sheet pins" out of order, but connected them as if they were in order:

So I corrected the mistake on the PCB:

Once again. I had an initial false sense of hope. The protection IC was now enabling charging and discharging. It generally appeared that external power was able to charge the battery, and I could draw power from the output both with and without external power connected.

But I found that I was still able to draw power even when I simulated a temperature fault to force the protection IC to disable charging/discharging, although with substantial voltage drop. Then looked at voltages more closely when discharging was enabled and found that there was a substantial (but less) voltage drop in this situation too. The charge-enable MOSFET was never fully opening or closing and was the source of the voltage drop.
After some more research, I discovered that I did not understand MOSFET specs when I chose a MOSFET. I only looked at specs like "max continuous current", etc., but had not looked at the "safe operating area" chart:

Despite having a "max continuous current" rating of 3A (which I thought was plenty for my target of supporting a max of 1~2A, but realistically only using a max of about 300mA in the car phone project), it only handles about 100mA at the battery voltage I'm working with.

I definitely had cranked up my load to 2A at some point, so this seems like a reasonable explanation: I must have fried my MOSFET with too much current.

So then I assemble my second PCB and correct the mixed up connection before I install batteries or apply input power. My plan was to test with low loads (about 50-100 mA) to stay within the limits of the MOSFET so I can at least verify that everything else is working properly with the charging/discharging and battery protection for everything other than over-current protections.

It initially looked good. I installed the batteries, and the protection IC initialized into a low voltage fault as it is designed to do: charging enabled, but discharging disabled. I was unable to draw any current at all from the output, and the voltage reading of the output was very low: the MOSFET appears to be working properly when "closed" this time.

I connected external power, which should clear the under-voltage fault and allow discharging, but now I encounter a completely different problem with the second board. The "input OK" LED flickered crazily for a second before remaining solid, then the charging LED started flashing (indicating charging). But my power supply was showing that it was not supplying any current at all.

The protection IC did, however, enable discharging, and without any substantial voltage drop, so it now seemed that the discharge-enable MOSFET was working correctly.

However, at some point while trying to investigate what was going on with the charging IC (disconnecting/reconnecting external power, removing/reinstalling batteries, etc.), the MOSFET fried again. Same symptoms as the first board (MOSFET never fully opened or closed, allows current draw when gate signal is low, etc.)

I still have no idea what's wrong with the charging IC on my second board, but it has a very unstable "startup" for a second or two every time external power is connected, and it never draws any current from the power supply to charge the battery or provide output power. The batteries are always supplying output power, even when external power is connected (external power is supposed to take precedence over batteries for supplying output power).

• Is there a manufacturing defect from JLCPCB that is shorting out some pins on the IC or something?
• Did some inrush current from the battery take out both the MOSFET and the charging IC?
  • This doesn't seem likely because the protection IC did not enable discharging until AFTER the charging ID finished it initial glitchy power-on, so I'm pretty sure something was damaged/faulty before there was any chance for battery inrush current.
• Was I too careless about ESD risk while handling the PCB to solder my connectors, and damaged the charging IC myself with static?
• Did I just receive a faulty charging IC?

And I'm also less confident about the cause of my MOSFET failing, because I thought I was staying within its limits this time, but it still failed. Maybe the unstable glitchy voltage from the charging IC contributed this time? Maybe there was still some current spike at some point when disconnecting/re-connecting things that briefly exceeded limits enough to fry it?

The best I can think of doing next is to choose a MOSFET that handles much higher continuous DC current at 12-14V, and order a new batch of PCBs (with my mixed up connections fixed too, of course), and hope that those were my only significant problems, and that the charging IC issues on my second board was just bad luck with manufacturing.

I'm currently leaning toward this MOSFET: Vishay SUM70060E

• Over 10A continuous current at my battery voltage levels.
• It specifically lists "Battery management" as one of the intended applications.

 

[UselessPickles]

I'm currently leaning toward this MOSFET: Vishay SUM70060E

I just had a brilliant idea. I looked at the Dayton Audio battery module I'm currently using in my prototype to see what MOSFETs it's using. There's a pair of UMW 50N06 next to each other, arranged like they are probably used for charge/discharge enable/disable. If that MOSFET is sufficient for 3 larger 18650 batteries in series, then it should be more than enough for my smaller 14500 batteries with less capacity and discharge current.

 

[rjenkinsgb]

Be careful with the FET polarities - they are often used "backwards" in applications like this, to avoid the internal diode passing current when the FET is supposed to be shutting off the power.

This is a typical connection, with separate charge & discharge enables & the two FET drains linked.

 

[UselessPickles]

Be careful with the FET polarities

Thanks for the tip. Fortunately, this is one detail I double/triple/quadruple checked because I am aware that there are a variety of pin arrangements for MOSFETS. I was careful to select the correct symbol in KiCad that matched the pin order of the MOSFET I had selected, and double-checked the pin arrangement of the PCB footprint.

It seems very reasonable that my problem is simply an under-sized MOSFET. I also realized that even when I thought I was testing within the limits of the MOSFET (<= 100mA), I probably still exceeded the limits as the MOSFET warmed up, because that 100mA limit is at 25C. This makes sense now that I think about it: the MOSFET seemed to work fine until after it had been switched on for about about 30 seconds. It was probably initially within limits, then warmed up and its current capacity decreased below the 100mA I was drawing.

I also have come to the conclusion that the additional diodes I placed across the MOSFETS are unnecessary. I initially did this because the current rating of the body diode of the MOSFET was lower than the amount of current I wanted to support. But the protection IC datasheet specifically explains that it protects the body diodes by opening each MOSFET in various conditions based on the direction of detected current flow.

 

[UselessPickles]

Now for a bit of a side quest. Once I have the new battery/power management circuitry worked out, I plan to position the 3 batteries and all the battery charging/protection circuitry in the area currently occupied by the red battery management circuit board in my current prototype:

This will free up a lot of space in the lower section of the PCB near all of the connectors/ports, including the space where the original car phone could include an optional RJ11 "data" port for connecting to modems and fax machines:

This port is a small PCB that mounts via 2 pin headers/sockets:

I'm creating a replica of the overall shape of that data port PCB so that I can add an RJ12 port in the same position (same body/connector as RJ11, but with all 6 positions populated with contacts) and use it as a programming/debugging port:

I need 5 pins for programming/debugging of the MCU, and I can use the 6th pin for my I/O UART TXD that I use for logging to a terminal on my computer. I'll just have to make a custom adapter from an RJ12 cable to a pair of connectors that connect to the PICKIT4 programmer and my UART/USB adapter. This will allow me to fully debug and program the MCU without opening up the car phone at all, while still keeping the car phone 100% original in appearance. Plus, the completionist in my likes the idea of having all possible ports populated on the car phone.

The existing programming and I/O pin headers on my PCB are on the opposite side of the board from where this programming port will be, so I'll run some ribbon cable between those pin headers and the the right-angle socket on the programming port PCB. The pair of pin headers/sockets used to mount the programming port to the main board will be purely structural (no electrical connections).

In the future, I may consider a major reorganization of my PCB layout that could move the MCU closer to the programming port so I can make use of the mounting pins for the electrical connections, and possibly move power-related circuitry closer to the external power supply socket. But for now, I'm taking things incrementally.

Conveniently, the exact RJ11 port originally used by the car phone is still in production, and there is a 6-contact variation of it available: Hirose TM5RE1-66(20)

The pin headers will be a bit trickier, because the exact JAE parts are no longer available, and I need to get the total height of the mated connectors right. The dimensions of the male pin headers on the main board seem to be pretty standard/common, but the female part will require some creativity:

That is about 8.4mm tall with a 1.1mm spacer for a total height of about 9.5mm. Typical height of female pin headers seems to be 8.5mm. I haven't yet found any options that are either 9.5mm, or have options for spacers that make it 9.5mm. But I can buy sheets of 1mm thick ABS plastic and make my own spacers.

 

[UselessPickles]

I updated my battery module design to use the UMW 50N06 MOSFET. Conveniently, this is a part that is in stock at JLCPCB. Unfortunately, I still need to pre-order a couple more of the battery charging ICs before I can place an order for the updated boards, so it will probably be about 3 weeks before I have these new boards in hand.

These MOSFETS are substantially bigger than the ones I used the first design. Hopefully I have enough copper area for heat dissipation (it's on both sides of the board with many vias). But considering I'll be running much less continuous current through these than they are rated for, I expect heat to be a non-issue.

 

[UselessPickles]

Today I started learning how to create 3D models, using "onshape" (free web-based 3D CAD design software). I made this model of the antenna connector on my car phone so I can better visualize my PCB layout and ensure I don't place any components or traces too close to it.

In my custom Bluetooth conversion, this antenna socket isn't electrically connected to anything, but it is still physically installed (with the wire cut off) so I can still attach the original antenna for the sake of complete appearances.

 

[UselessPickles]

And here's a sneak peak at what my board will look like with the programming port added on, and the battery holders relocated. I still need to fine-tune the positioning of the pin headers for the programming port a bit more.

I really need a better 3D model for the RJ45 port and the special connector that sticks up out of the case. Sounds like some more fun 3D modelling practice

When I place an order for a new batch of battery module prototypes, I plan to also order some bare boards of the programming port and the motherboard so I can partially assemble them with ports, pin headers, and battery holders to test overall fitment/alignment of everything.

 

[UselessPickles]

Here's my second attempt at creating a 3D model of a part: the RJ45 port that the handset plugs into. It's much better than the basic cube that came as the model for this part from SymacSys.

Here's the real part:

It's slightly different from the RJ45 port on the original car phone, because the exact part is no longer available (Molex MXJ 52018-8816). But I found a part from a different manufacturer that is compatible (Kycon GLX-N-88M).

 

[UselessPickles]

I created a 3D model for the special obsolete connector (JAE DRA-8SC-F0) on my car phone's PCB that is originally used to connect the portable battery circuitry, but I am reusing in my Bluetooth conversion to connect to the Bluetooth module.

The 3D render of my PCB is finally pretty accurate.

 

[UselessPickles]

I'm hoping for some guidance here...

While waiting for my next batch of prototype boards, I'm thinking about what I should do to make my main motherboard PCB more robust before I consider selling my car phone conversion to anyone else. My prototype has survived just fine in my car all last year, but I don't have any kind of ESD protection on any of the connectors, no reverse polarity or surge protection on the external power supply connector, etc. Since this will be directly connected to a vehicle's electrical system, I think it should have some protection, at least for the power supply connector if nothing else.

I'm overwhelmed by general information I can find about if/how to implement these protections. If I am believe some sources, I need TVS diodes on every connector pin that is not power supply or ground (but just selecting an appropriate TVS diode seems non-trivial), and I need one of many varieties of complex circuitry for over-voltage, reverse-polarity, and surge protection on my external power supply connector.

So I'm looking to the original car phone to see what Mitsubishi thought was necessary for protection, and I'm not quite sure how to interpret it.

Here's a schematic of the external power supply to the original car phone. I determined this through visually following traces on the board and testing for continuity between various points on the board:

The ZNR Surge Absorber makes sense to me, and I'm able to find one that is similar to that in the original phone, and the voltage ranges work for me (guaranteed to cut off at a voltage lower than the max for my battery charger IC, guaranteed to allow voltages within range for vehicle electrical systems): ERZV10D220

The double diode (D2) makes sense to me, basically allowing power to be supplied from either external power or the battery (whichever is higher) without back-feeding into each other. This is irrelevant for me because my battery charging IC handles all of this.

What I really don't understand is D1 and C1:

• I've seen reverse polarity protection designs with a reverse-biased diode like this, but it usually also involves a resettable fuse between the power pin and the diode so that reversed external power won't just short-circuit with unlimited amperage through the diode. There is no such fuse on the original car phone. I wonder if they left that out based on the assumption that the external vehicle wiring to the phone would be fused?
• C1 is a very small SMD capacitor (unknown value) right next to the connector pin like a bypass capacitor. In fact, I see small bypass capacitors next to nearly every pin of every connector on the original car phone. For reference, I was able to get a reading of 0.3uF on several bypass capacitors for the handset connector. I have found mixed information about whether a bypass capacitor next to a connector pin offers any useful ESD protection, with some sources saying it is a myth - use TVS diodes instead.
• BTW - I can't find any TVS diodes near any connector pins on the original car phone. Only small bypass capacitors.

So I'm not sure what's worth doing in my PCB design. Should I keep it simple and follow the example of the original car phone (ZNR surge absorber and a reverse biased diode in parallel with the connector, relying on the vehicle wiring fuse to protect against excess reverse current?).

Would it be better to add MOSFET-based reverse polarity protection like this?

Should I slap small value bypass capacitors on all connector pins (handset connector, microphone jack, etc.) for some debatable level of ESD protection? Or dig deeper into understanding TVS diode selection? Or just ignore ESD protection?

 

[Diver300]

A couple of thoughts.

The only reason to use the MOSFET-based reverse polarity protection is if the voltage drop due the diode would cause a problem.

The reverse parallel diode was used a lot in the past but less so nowadays for the reasons you mentioned. There was also a big problem where the reverse voltage is large enough to cause problems but too small to blow the fuse. You are far better to use a diode or a MOSFET and you know that it will stand any negative voltage up to the diode breakdown for any period of time.

The small capacitor will also help with all types of high-frequency noise so they are a good idea if the TVS is there.

Also, the ESD test involves a voltage up to 25 kV, and only a few hundred Ohms in series, so the current can be around 100 A. TVS diodes have resistance so the very large current can mean that the peak voltage during an ESD test is still far above the breakdown voltage.

The ESD test only involves a small capacitance, maybe 330 pF or similar. If a capacitor of 0.3 μ is added to the inputs, that is 100 times the size of the test capacitance, so the voltage will never get beyond 1/100 of the test voltage, and that can improve things a lot.

Actually proving what is needed is often more expensive that just adding capacitors everywhere.

 

[UselessPickles]

Actually proving what is needed is often more expensive that just adding capacitors everywhere.

That's really the key here. I do not have the resources or ability/knowledge to do full/proper testing/analysis, potentially destroying multiple iterations of prototypes, to meet any particular standards for ESD protection, etc. I'm just an amateur trying to do whatever is reasonable to give my electronics a high chance of surviving normal/reasonable usage and handling in the real world.

So do you think it is worthwhile to throw a small bypass capacitor (around 0.3uF? smaller?) on every pin of external connectors (except ground pins, of course)? If I understand correctly, this will offer at least some level of ESD protection, but it will slow the rise/fall time of digital signals. All of my digital signals are running at pretty low bitrates, so that shouldn't interfere with signal integrity (assuming my digital outputs are all fairly low impedance, which I will need to double check). In fact, it may even reduce some noise on the board.

The ESD test only involves a small capacitance, maybe 330 pF or similar. If a capacitor of 0.3 μ is added to the inputs, that is 100 times the size of the test capacitance, so the voltage will never get beyond 1/100 of the test voltage, and that can improve things a lot.

BTW, it's better than that. 0.3uF is ~1,000 times 330pF

The only reason to use the MOSFET-based reverse polarity protection is if the voltage drop due the diode would cause a problem.

I think a series diode on the power input would be perfectly fine for reverse-polarity protection in my use case. A small voltage drop won't be an issue. My battery/power management handles a wide range of input voltages with buck-boost regulation to the desired output voltage, and I don't need to optimize efficiency to the extreme when it is connected to external power supply.

I think I'm leaning toward:

• Small bypass capacitor next to every non-ground pin of every external connector for some basic "better than nothing" ESD protection.
• Schottky diode in series with the input power pin for reverse-polarity protection.
• ZNR surge absorber (after the diode? or between the power pin and the diode?) for surge protection, because I am vaguely aware of potential issues with vehicle electrical systems that can introduce surges (e.g., failing alternator or voltage regulator).

 

[UselessPickles]

Here's a demo of how the original car phone can beep the car's horn to alert you to an incoming call when you are away from your car.

Could you imagine sitting in a restaurant eating lunch while listening for your car horn, then running out to answer an important call?

I'm considering replicating this feature in my Bluetooth conversion. It will be a pretty useless novelty demo of how old car phone tech worked. With my conversion, you'll have your modern cell phone with you when you go away from your car anyway (no need to go back to the car to answer the call), and you would have to stay within about 20 feet of the car to maintain the Bluetooth connection. So there's absolutely no practical use for this. But I still want to do it.

Technically, it is very simple: just a transistor used as a low-side switch for the horn relay circuit in the car. I have one unused pin on my MCU that I could use to trigger a transistor, and I found the specs of the (obsolete) transistor in the original car phone that I can use as reference to confidently select a suitable equivalent.

The only downside is that the factory car phone wiring in my car does not include the necessary wired connection between the car phone and the car horn relay, so I would have to dismantle the interior to run a wire from the trunk to the dashboard area, and splice it into original wiring. I'm conflicted between keeping my car original, with only the originally-supported car phone integration, versus adding a custom wired connection to fully take advantage of an original feature of the car phone. I'll probably at least update the hardware design to support it, then I can decide later whether to follow through with the software implementation and custom wiring in my car.

 

[UselessPickles]

My updated battery/power management circuit is working! I did a lot of testing with a power supply and electric load, and have moved on to testing it with my car phone Bluetooth adapter.

The larger MOSFETs seemed to solve all my problems. I was able to draw over 2 amps for an extended period of time without issues (way more amperage than my car phone will ever draw, around 250-300mA max). Nothing got noticeably hot to the touch.

I was also able to successfully test all the various safety features (over-current, under-voltage, over/under-temperature). Most importantly, I was able to confirm that when connected to the car phone circuit, an under-voltage fault can be recovered by simply connecting external power (no "load removal" required).


I also got new PCBs to test the fitment of the new battery holders in their new location, and the new programming port. Everything seems to fit well, except I think I need to move the programming port to the right about half a millimeter.

Here's a comparison of my custom PCB/connectors (top) to an original car phone with the optional data port installed (bottom):

And here's my custom RJ12 programming/debugging cable connected to a PICKIT4 and a USB/UART adapter (for console logging output to a terminal on a PC), and plugged into the programming port:

Next up is many hours of transferring the battery/power management circuit design over to my main car phone motherboard design, reorganize some things (using the extra space freed up by moving the batteries, add support for the horn beeping circuitry, and make some other improvements before I can place an order for new PCBs.
I'm going to try some major reorganization to position the MCU near the programming port so I can eliminate the need for a ribbon cable.

 

[UselessPickles]

I did some testing of temperature thresholds with real thermistors (instead of simulating them with potentiometers)

It wasn't the most accurate testing. I was using an IR thermometer to monitor the temperature of the battery while warming it up with a hair dryer to test high temp thresholds. For low temp thresholds, I cooled the batteries down in a freezer, then monitored the temperature as they warmed up outside of the freezer (which I then determined was a bad idea because of condensation). The charging/discharging generally seemed to enable/disable somewhat near the intended temperature thresholds. I think that's about the best result I can get with this method. I'll consider this a successful sanity check and trust/hope that my calculations were correct when choosing thermistor/resistor values.

I like the very thin film style thermistor better because it is easy to tape securely to the battery, and seems to get a more accurate reading of battery temperature (the other one is affected by ambient air temperature more). I will try to find another one of those thin film thermistors with the specs I need for the other thermistor

Also, check out the nice clips that can secure the batteries in the battery holders:

 

[UselessPickles]

I'm trying to nail down the hardware design for the external horn alert feature, and I think I may be able to improve the robustness of mine compared to the original car phone.

For reference, here's the wiring diagram from the car phone owner's manual:

The car phone acts as a low-side switch to trigger the car's horn relay. Internally within the car phone, it's a pretty simple transistor, like this:

My understanding is that it is necessary to add a diode across the coil of the car's horn relay to protect the transistor from voltage spikes when turning off.

But I don't like the idea of requiring additional external modifications to the car's wiring to protect the car phone's electronics. Could I eliminate the need for the added diode if I used a relay inside of the car phone instead of a transistor? I would still need to use a transistor to trigger the internal relay, and would include a flyback diode on the internal relay to protect the transistor from voltage spikes. But my assumption is that the relay itself would not require any protection from the voltage spike of the car horn relay coil.

Am I over-complicating this and just creating more potential sources of problems? A relay I'm considering using has a rated "switching voltage" of 277 VAC, 125 VDC. I'm not sure if that's enough to withstand the voltage spike of the car horn relay coil. I can't seem to find any clear guidance on what kind of voltage spikes to expect from common automotive relays. The best I found si this forum thread where someone suggests it can be "often up to 100V": https://www.eevblog.com/forum/projects/automotive-relay-help/

Is there a simpler way I could just use a MOSFET or transistor inside of the car phone to trigger the horn relay, and protect it from voltage spikes without adding a flyback diode to the car's horn relay?

Or should I stop trying to be clever and just go with the design of the original car phone? Use a transistor (or mosfet) in the car phone, and require a diode to be added across the car horn relay coil for protection.

 

[rjenkinsgb]

The diode is between the yellow horn output and red positive vehicle power - it could be wired like that inside your adapter?

 

[UselessPickles]

The diode is between the yellow horn output and red positive vehicle power - it could be wired like that inside your adapter?

Two concerns with that approach:

1. Everything I read recommends placing the flyback diode as close to the and directly across the inductive load as possible. If the diode was in the car phone, then would that expose other circuits/components in the vehicle to some kind of side effects? In my car specifically with factory wiring for the car phone, the power supply to the car phone shares a fuse/circuit with other electrical components on the vehicle, which is on a different fuse/circuit from the horn relay. When car phone turns the horn relay off, the flyback current would flow through the car phone, back up through the power supply wire, to a common connection point (at the battery), then finally back down the horn relay circuit. What side-effects could this cause?
2. This would technically allow for the car phone to be powered directly by the horn output in "non-ideal" situations. For example, if the horn wire is connected to the car phone, but the main power supply is disconnected/broken (e.g., its fuse has blown). It seems bad that the car phone could directly draw power through what is intended to be an open-drain output. It would hide the fact that the power supply is not properly connected. Then if/when the car phone triggers the horn, the voltage would drop and the car phone would shut off.

I read about another possibility: use a MOSFET to trigger the horn relay, and use a combination of a high-value gate resistor and/or a capacitor across the MOSFET to slow down the switching so that current flow through the relay coil ramps up/down gradually enough to reduce the voltage spike within the limits of what the MOSFET can handle.

I'm turning a horn on for a full 1 second at a time, with 3 seconds between beeps, up to 5 times in a row only, with no critical timing/synchronization needed. I can live with some inefficiency and "sluggish" switching, maybe on the order of tens of milliseconds? I'm aiming for slow enough to reduce voltage spike, but still fast enough for the horn relay to open/close quickly/strongly. I'll, probably just have to try it and measure voltage spikes on my scope.