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

[rjenkinsgb]

The cable photos show JST connectors; note that the cable set is for the five cell battery module, as far as I can tell?

Edit - the five cell unit appears to have similar connectors.. It may be that they take JST cable sockets, but eg. both keys on the larger ones just fit in the one large recess in the board connector wall??

 

[UselessPickles]

That is the only kit of accessories they offer for any of their battery modules. It appears to me that all of their battery modules (3, 5, and 6-cell) have the same connectors, so I assume the same kit is compatible with all 3 battery modules and they just haven't updated the product info? Maybe the 5-cell was the first battery board they introduced?

Yeah, maybe it is a JST-compatible connector on the board. I'll order the accessory kit to guarantee I have compatible connectors to start my testing/development. If they turn out to be JST, then that's great. I can order parts and assembly my own wiring harnesses from scratch for any future needs.

 

[Chrroger]

depending on how quickly you need this I am fairly certain I have used these in something. its on the machine at home and I have part numbers.
will look when I get back in around the 28th.

 

[UselessPickles]

I started playing around with KiCad, because I'm going to need to make some custom PCBs if I want to fit my electronics inside the transceiver of the original car phone (and line up the connectors with the existing holes in the case). I've decided to split my project into two separate boards:

• Main board will go in the metal transceiver case, which is where the power supply socket and handset socket need to be. This will contain almost everything except the Bluetooth module.
  • I hope to also fit the Dayton Audio battery module in there, although I will probably need to separate the battery cell holders from the board for a lower profile.
• A "daughter" board containing the Bluetooth module and some supporting components.

The main reason is that the Bluetooth module cannot be enclosed in the metal case. I need to put it in the attached plastic cover that makes the phone portable (has a cradle for the handset, battery charging circuitry, and a place to hold the giant 10-cell NiCad battery pack).

There's conveniently a hole in the metal lid for the transceiver case for a connector to pass through to the battery circuit in the plastic cover. I can remove all the original battery charging circuit, battery, and connectors, so that the hole is open for me to run wires through to connect the Bluetooth board to the main board. The Bluetooth daughter board will go in the large empty space in the plastic cover where the original battery would be installed normally.

Another benefit of this split is that this will allow me to easily load configuration/firmware onto the Bluetooth module by disconnecting it from the main board and connecting it to a UART USB adapter.

So here's my first attempt at designing a schematic and PCB in KiCad for the Bluetooth module daughter board:

Yeah, it's probably ugly and could be organized a bit better. Yes, I'm still using through-hole components, because that's what I'm comfortable with hand-soldering. But I think it should work?

The 8-pin connector includes (by pin number):

1. Audio output
2. UART Tx
3. Ground
4. UART Rx
5. Mic input
6. Ground
7. Reset (allows MCU to turn module on/off)
8. +5V (BT module regulates this to 3.3V for itself and outputs at the SYS_PWR pin)

The connector is laid out so that a single row of 4 pins matches up exactly with the 4 pins on my UART USB adapter, so I can easily connect the daughter board to the USB adapter with a 4-wire ribbon cable for installing firmware/config.

I have a ground fill on both sides of the board (except for on the front side underneath the BT module), and 2 pins/wires on the connector dedicated to ground for good measure (otherwise I would have a wasted pin).

The 3 dip switches are used to put the Bluetooth module into various programming modes for firmware/config updates over UART.

The LEDs are for status indicators (the BT module can directly drive them without need for current limiting resistors).

Resistors are for dropping the MCU's UART signal from 5V to under 3.3V. The Tx going from BT to MCU is fine at 3.3V but I still run it through a 1k resistor for consistency/symmetry with the Rx signal going through a 1k resistor (don't know if it's necessary, but symmetry seems good, right?). Having this on the daughter board allows me to use the UART USB adapter at 5V safely.

The 47uF capacitor is recommended by an errata data sheet to avoid EEPROM corruption when the BT module loses power while it's in the middle of writing to EEPROM.

The 0.1uF capacitor is a coupling cap for the mic input to the BT module. I figure the coupling cap should be nearest to the component that is "consuming" the audio. So the coupling cap for the audio output will go on the main board.

 

[UselessPickles]

Before I invest too much time into my new plan to fit my custom electronics with its own power supply inside the original transceiver, I should probably confirm that the Bluetooth module will work in the location I plan to put it. I was able to test this with my current prototype, thanks to the fact that the Bluetooth module is mounted to a breadboard adapter and plugs into a socket. It was easy to temporarily add some wires between the Bluetooth module and my prototype board.

Transceiver on the left, bottom side of the plastic "transportable" cover on the right with the NiCad battery installed:

Metal transceiver case cover removed:

Battery and charging circuit removed from the transportable cover:

Bluetooth module temporarily "installed" in the battery cavity:

Metal transceiver case cover seated into the plastic transportable cover with the Bluetooth module wires routed through the hole:

Connected and ready to test:

Good thing I have plenty of spare phones to make tests like this easy:

Good news! The Bluetooth module works fine mounted in the plastic cover, above the metal transceiver case cover. It was able to connect to my cell phone about about 15 feet away in another room with a wall between them. I can proceed with my plan.

 

[rjenkinsgb]

OK, my LBB3v2 battery module has just arrived.

I can confirm the odd looking connectors do take JST 2mm cable sockets.
The black vertical battery out connector is a Molex Minifit, I believe - there is a connector with wires supplied for that.

There is also a four way cable assembly with a JST at each end, possibly intended to power another of their projects.

For info, the output does not enable until it's been connected to a charge power source momentarily; it then all comes to life.
The charge status LEDs show continuously while it has power connected & only work when the button is pressed when operating from the battery.

 

[UselessPickles]

the output does not enable until it's been connected to a charge power source momentarily

That's interesting. I wonder if that's a side effect of the low voltage cut-off behavior being in an initial default state. For example, after charging the battery, then running on battery power until the batteries are drained, it would cut off power output until external power is supplied again.

I received my battery module today. It's going to be a challenge to get everything to fit in the original car phone transceiver. I'm going to need to remove the battery holders from the board and assemble the battery cells into a shrink-wrapped battery pack that connects to the board with wires. It's unlikely that I'll be able to fit the battery, battery board, and my main circuit board all in the metal transceiver enclosure. I'll probably need to mount the battery in the plastic transportable cover (in the big battery cavity, along with the Bluetooth daughterboard), and cut a new hole in the metal enclosure cover for the battery wires to pass through.

But I have plenty of time to think about that. First I need to work on a breadboard proof of concept of how I will integrate the battery module with my microcontroller (circuitry + programming of power on/off behavior).

 

[UselessPickles]

I'm been spending more time getting comfortable with KiCad and revising my Bluetooth module daughterboard.

I organized my schematic into subcircuits with labeled nets, and added an onboard reset button to help with loading firmware/configuration onto the BM62 (need to reset after reconfiguring the dipswitches). I also slightly rearranged the connector pinout so that the 2 ground pins are directly next to each other.

I refined the PCB layout a bit:

I filled out the BOM with specific part numbers and links to datasheets, and have a "project" setup on mouser.com ready to order all the parts. I confirmed dimensions of all parts match up with the footprints used for the PCB layout. I think I'm ready to start choosing a print shop, then figuring out if I need to make any layout adjustments to meet limitations of the print shop.

I'm glad I had this small circuit to practice with. I'll be much more comfortable getting started on the main board after this.

 

[rjenkinsgb]

It's not necessarily obvious from the PCB images, but have you "stitched" the two ground planes?

The PCB should have a few VIAs around the outer edges, then as each side has breaks for tracks, a few either side of each break, so the opposite side plane provides continuity past the break, as far as possible?

 

[UselessPickles]

It's not necessarily obvious from the PCB images, but have you "stitched" the two ground planes?

No, I haven’t. I was wondering if that was necessary, considering that some of the through-hole components accomplish that “for free” in a few places.

What approximate spacing should I be aiming for between the “stitching” vias? I could go wild with it, but I assume that would increase the manufacturing cost of the boards unnecessarily.

 

[rjenkinsgb]

Near any points or corners on a plane, then 0.4 - 0.8" or something like that around the edges of shapes.
It's not critical for low frequency stuff, just enough to avoid anything floating or resonant due to open ends.

 

[UselessPickles]

Thanks for the tips. I scattered some vias around as suggested, and also used them to fill in some "island" gaps in ground fill between traces.

 

[UselessPickles]

I really need to stop working endlessly on this PCB layout and start figuring out how to get it printed. I decided I didn't like how closely a couple pairs of traces were running parallel to each other (UART TX and RX, speaker and microphone). So reorganized a bit so they are separated by ground fill and no longer cross over each other.

 

[UselessPickles]

I played around with my Dayton Audio LBB3v2 battery module today. Here's what I learned:

• All grounds are connected (inputs and outputs), so I don't need to worry about treating the input and output as isolated circuits.
• It can provide power without batteries installed. When any external voltage between 5v - 24v is supplied, the output voltage is a steady ~12.4v.
• With batteries installed and external power supplied (charging), the output voltage is not much higher than battery voltage alone (~0.2v higher, at least with the batteries at their initial charge level as they were shipped).
• Only the "external power indicator" LED is a simple circuit: one pin is ground, the other pin is about 24v when the LED is on. Not a useful voltage level for me to use as an input to my MCU. I may as well directly detect the external power supply voltage (~12v).
• All other indicator LED connections are NOT simple circuits (no ground pins; neither constant ground nor switched ground). I'd guess that some LED drivers may be involved?
• The black output connector is NOT a Molex Micro-fit connector! It's very similar, and the Molex plug will insert into the socket on the board, but the position of the indentation on the latch is different, so the Molex plug will not fully latch onto the socket on the board.
  • Not a big deal, since the battery module includes the correct connector with wires. And I have a spare that came with the accessory pack of wires and LEDs.

So my plan is to:

• Detect external power connection and vehicle ignition switch by using small relays to pull MCU pins low, directly triggered by the external power and ignition switch voltage.
• Figure out how to approximately map battery output voltage to charge level (I only need granularity of 20% steps, and a lower threshold when I trigger "low battery" behavior (display message and beep).
  • If I need to detect "charging" state, I can probably reasonably infer it from a combination of external power supply detect and battery level (if below a limit with external power connected, then it must be charging).

 

[camerart]

Hi U,
My walk about phone is a dumb phone, and I'm happy that it's not making noises all day, unless there's a call or text. It's small and fits in the little jeans pocket.
Anyway, I hear that as my phone is 3G, it is going to be switched off, so I'll have to use a smart phone.
I'm wondering if your car phone will be switched off, and spoil your modification plans?
C.

 

[UselessPickles]

I'm wondering if your car phone will be switched off, and spoil your modification plans?

My car phone is 1G. Service has been unavailable since 2008. That's exactly why I started this project. So, no, the shutdown of 3G will not affect me. My project makes the car phone work via a Bluetooth HFP (Hands-Free profile) connection to any modern cell phone. Here's a simple diagram from the README on my GitHub repo that illustrates it:

I'm currently working moving the Bluetooth Adapter inside of the original Transceiver so that the car phone externally appears completely original, but is fully functional.

 

[camerart]

My car phone is 1G. Service has been unavailable since 2008. That's exactly why I started this project. So, no, the shutdown of 3G will not affect me. My project makes the car phone work via a Bluetooth HFP (Hands-Free profile) connection to any modern cell phone. Here's a simple diagram from the README on my GitHub repo that illustrates it:

View attachment 143692


I'm currently working moving the Bluetooth Adapter inside of the original Transceiver so that the car phone externally appears completely original, but is fully functional.

Hi U,
Ah, good, I like it.
I once gave a lift to someone, with an early mobile phone, with a lead acid battery in the base. It was heavy
C

 

[UselessPickles]

I got started figuring out if I can actually physically fit everything in the original transceiver case. First step was to replicate the original circuit board shape/size in KiCad.

Here's the original circuit board and lid of the transceiver case for reference:

To do this, I took a photo of the original circuit board from about 15-20 feet away across the room and zoomed in to minimize distortion from perspective and not being at the perfect angle. My Nikon P900 came in handy for this (crazy amount of optical zoom).

Then I loaded the photo into photo editing software to straighten it, carefully trace a selection all around the edge of the board, delete everything around the board, and crop the image dimensions to the edges of the board.

Then I added the image to the KiCad PCB layout and scaled it to exactly the measured width of a board (125mm as measured with digital calipers). Now I have a pretty precise scale image of the original board to trace and mark mounting holes, and place connector components that need to match up with original placement.

The lighting of my photo wasn't great, but I think it was good enough.

I then similarly brought in a scale image of the bottom of the transceiver case so I can mark "keep out" zones where for the divider walls on the transceiver case lid. I'll also use it to avoid placing any components/pads/vias on the bottom side of the board where there's no clearance due to structural supports and dividers on the bottom.

Here's the end result of placing the major connectors, the Dayton LBB3v2 battery module, and a few battery cell holders:

The battery module is way too tall to fit as-is. I only have about 16.5mm height clearance between the PCB and the case lid. There's not even enough room to fit some bare 18650 li-ion cells. So my plan is to remove the battery holders from the battery module and downsize to 14500 li-ion cells and holders (same size as AA batteries). I had to rotate one holder horizontally due to an are of the lid that drops even lower (look back at the first pic).

I'll have to solder wires to the battery module in place of the battery holders, use a connector to connect to my circuit board, and run traces to the battery cell holders on my board. I can at least run those traces under the battery cell holders, through the "keep out" zone, and under the the battery module to avoid eating into precious space for the rest of my circuit.

The "mounting" of the battery module board itself will likely be pretty ghetto. The mounting holes of the battery board unfortunately overlap with some structure on the case bottom that will prevent me from actually mounting with screws or anything. I'll probably just end up sandwiching the battery board with some type of soft foam board that will both prevent contact/rubbing, and gently "squeeze" the battery board in place when the lid is mounted.

I've decided I probably need to go with surface mount components to increase the chance I can get everything else to fit. I did recently hand-solder a couple SMD LEDs that were about the same size as metric 3216 resistors/capacitors, so i should be able to handle all the SMD resistors/capacitors in that size.

For now, I think I'm fairly confident I can get things to physically fit. Now I need to focus on designing/testing some new sub-circuits and microcontroller code to deal with powering on/off, detecting external power supply connection, detecting battery voltage for approximate battery level calculations, etc... back to the breadboard for a while.

I also ordered a few 1100 mAh 14500 size li-ion batteries so I can test them with the LBB3v2 battery module before I fully commit.

 

[UselessPickles]

Here's my plan for hardware to deal with all the power management stuff...

The connector for the transceiver external power has ~+12V (constant when installed in vehicle), ~+12V "ignition on" indicator, and ground.

I'll basically connect the constant +12v directly to the LBB3v2 battery module's DC input, then use a couple 5.1V Zener diodes and resistors to produce a digital signal for my MCU to detect when external power is connected and when the vehicle ignition is on:

I already tested this particular arrangement/values of diode and resistors, and it results in an output signal voltage range from ~3.9V-5.0V across the entire supported 5V-24V DC input range for the LBB3v2 battery module.

Power for the rest of my circuit is all derived from the "+BATT" output voltage of the battery module. This is basically a "constant" ~12V power supply for the circuit, regardless of whether external power is connected or not.

First, there's the "constant" +5V power supply derived from "+BATT":

This is only used to power the MCU. The MCU needs "constant" power so it can stay running in sleep mode when the phone is powered off, ready to be woken up and process inputs that power the phone on.

For the rest of the circuit, I need MCU-controlled "switched" power. I decided to keep it simple with small relays to switch from the power side, rather than transistors that switch from the ground side. I was afraid that ground-side switching would create grounding issues, because entire sections of the circuit would have their grounding go through a "choke point". With the relays on the power side, everything can share the same ground (large ground fills, etc.). But I am using a transistor to switch the relays themselves.

The "+BATT_Switched" will power the handset, while the "+5V_Switched" will power the rest of my circuit.

And some of my components require -5V or 2.5V (for audio signal biasing), which will will be also "switched":

And finally, the battery voltage detection. I just need to divide down the battery voltage to get it within the 0-5V range that the MCU ADC can handle. This will be based on "constant" battery voltage rather than "switched", because I want to be able to program the MCU to test the battery voltage before switching power on to the rest of the circuit. If the user tries to power on while battery voltage is too low, then the MCU will abort the power-on.

I'm using high value resistors (and can probably go even higher?) for the voltage divider to minimize battery drain, but then have to buffer it with an op-amp to meet the "maximum 10kOhm impedance" spec for the MCU's ADC input. I'm still not 100% sure on this, but I think my logic is sound? The op-amp shouldn't be drawing the power necessary to provide that 10k impedance output to the MCU unless the MCU is actively "reading" the ADC value, right? Part of me is concerned that I added a ton of complexity that will end up drawing just as much (if not more) amperage as if I did a simple 10k impedance voltage divider without an op-amp.

 

[UselessPickles]

I made some good progress on designing my new PCB that will install in the metal transceiver case of the original car phone:

I am still using a few through-hole components:

• The NJU7031D op-amp is available in a couple surface mount options, but they are all "not normally stocked" by mouser.com (where I order a majority of my components) and require a minimum order quantity of 2000. I don't really want to deal with trying to find another source, or a different part that will work as well (especially since the NJU7031D was somewhat "randomly" chosen and seems to work well, and I don't know what specs matter for my use cases).
• The transistors and diodes didn't seem worth the effort. They have a reasonably small footprint already, and I know the ones I use work well.
• The relays, because it was convenient to order through-hole components that I could plug into a breadboard for testing.

Here's a closer look at the top half of the layout:

This is all power supply stuff. Connections between my board and the LBB3v2 battery board in the top left (6-pin connector for the 3 battery cells, then an external DC input on the left, and DC output from the LBB3v2 to my board on the right).

Below the connectors, crammed into the smallest divided area of the transceiver case, is the 5V converter, a pair of relays and a transistor for switchable 12V and 5V, battery voltage detection for the MCU (resistor divider and op-amp buffer), and a 2.5V converter (built from a resistor divider and op-amp buffer).

And here's the bottom half of the layout where most of the magic happens:

MCU with pin headers for UART I/O and programming on the left (IC1).

External power and vehicle ignition signal connector on the bottom left (J1), along with components to allow the MCU to detect presence of external power and vehicle ignition as digital input.

The 4x2 8-pin header in the middle bottom-ish is for connecting to the Bluetooth module daughterboard.

Below that is some components for converting/adjusting the UART I/O between the MCU and the handset.

To the right from there is the RJ45 connector (J2) for the handset and the external microphone connector (J3).

Immediately above those connectors is all of the external microphone circuitry (signal biasing/amplification, and insertion detection; IC10, Q1).

IC5 is the op-amp buffer for the MCU audio output, with a low-pass filter nearby.

IC6 is the analog switches for selecting audio output source (MCU audio vs Bluetooth audio) and microphone source (handset mic, external mic, or muted), with several resistors around it for biasing signals to 2.5V.

IC7 is the digital potentiometer for volume control.

IC4 is the -5V converter, needed to power IC8 and IC9 to convert the audio output to differential audio.


I think I arranged everything in a logical way, but I'm sure I'm probably violating several PCB design rules and best practices that I don't even know about. Some notes:

• I don't really like that all the power supply circuitry is pretty much as far as possible away from the external power connector. But this is a consequence of the LBB3v2 battery module only fitting in that one area in the upper right, and the upper left is then the only option for connectors that interface with the battery module board.
• I still need to decide on the exact type of connector I want to use for connecting the Bluetooth module daughterboard, and verify its exact placement on the PCB.
• I still need to go through and stitch all the ground planes together and fill in some "islands".
• I don't really know what I'm doing with track widths, routing, spacing, etc., beyond trying to do things that "make sense" (minimize messiness of routing).
  • I'm hoping the default 0.25mm track widths are sufficient for most of the tracks (all the digital I/O and audio tracks).
  • I did at least do some basic verification that power supply track widths are more than sufficient for the expected amount of current flow through them. In addition, I generally start with a wide track from the "source" of power, then branch out to a couple different areas on the board, reducing track width a bit.
  • NOTE: Since creating the screenshots, I have already increased the track width of the main external 12V track running from the bottom to top of the layout from 1.5mm to 2.5 mm, since that will be powering the battery charging. Dayton Audio doesn't specify how much the board may draw during charging, but recommended at least a 1.5A supply. At 2.5mm wide, that track will handle 4.6A with a 10C temperature rise. The rest of my circuit (including the handset) draws well under 1A (under 500mA, IIRC). I could be wrong. I will definitely test to confirm.