Turn a car analog into an external srf meter for a cb radio.

Thread Starter

bortweb

Joined Jan 11, 2023
11
Hi Guys,

I'm a CB radio club president and id like to make a little project for my self. Our group is using Cb radio to play Hide and seek in car. To do that we have external "jumbo" srf meter that are connected to our CB radio. Now to my idea:
Instead of using those big rectangular box with the meter in on the dash I'd like to use the in dash tachometer from my car.

The specs:
the cb radio meter output for the analog meter is 200ua

Do you guys think it would be doable to connect strait to the meter leads bypassing all the system for the rpm? If yes, how would i go about doing it and retain the possibility the still use the RPM meter as the RPM with something like a double throw switch or something like that?

Thank in advance for your time.
 

Irving

Joined Jan 30, 2016
3,987
The tachometer in a modern car isnt a 'meter' in the conventional sense and is deeply embedded in the dash & ECU circuitry. In many cars these days, particularly those in the VAG stable, its actually a form of stepper motor driven by the dash microcontroller in response to an RPM value from the ECU, so there's no easy way to break into that chain.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
579
Depending on the car, the tacho may be an "air core" meter. You may be able to find that out from someone who knows the car, or you could supply details in case anyone here knows?

If you can get behind the instrument cluster you may see the four connections to the back of the meter, if you are lucky, marked SIN+, SIN-,COS+,COS- respectively. There are two orthogonal coils inside and the pointer angle is determined by sine and cosine currents through them. There are special ICs for generating these meter inputs, like the onsemi CS8190 (although it needs a frequency input) but I've driven the tacho from a 2005 MX5 (Miata) directly from an Arduino UNO with code I could share if anyone interested.
 

bassbindevil

Joined Jan 23, 2014
836
A different approach might be an LED bar (or arc) display mounted behind the instrument cluster that's only visible when lit up. If you see what I mean. It would be safest to avoid any direct involvement with the vehicle instrument cluster electronics.
 

Thread Starter

bortweb

Joined Jan 11, 2023
11
Depending on the car, the tacho may be an "air core" meter. You may be able to find that out from someone who knows the car, or you could supply details in case anyone here knows?

If you can get behind the instrument cluster you may see the four connections to the back of the meter, if you are lucky, marked SIN+, SIN-,COS+,COS- respectively. There are two orthogonal coils inside and the pointer angle is determined by sine and cosine currents through them. There are special ICs for generating these meter inputs, like the onsemi CS8190 (although it needs a frequency input) but I've driven the tacho from a 2005 MX5 (Miata) directly from an Arduino UNO with code I could share if anyone interested.
Well the car is a 2004 Honda accord.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
579
Well the car is a 2004 Honda accord.
The pictures I could find of the instrument cluster are very similar to the 2005 Miata instrument cluster, so it's a fair bet that the tachometer is an air core meter and if you can get behind the cluster it'll have a flexible pcb across the back and the four terminals will be marked plus and minus sine and cosine. If I were attempting this I'd drive the tachometer from an arduino for both your application and the tacho pulse also being read and converted by the processor to sine/cosine so a single pole switch can select one or the other. Not for the faint hearted. Maybe worth buying a used tachometer and testing this approach first, before attempting to modify your instrument cluster! Your full scale 200uA output will need to be converted to 5.0V to be read by the Arduino. The existing meter is probably a moving coil type with a relatively low resistance so this (measured) resistance will need to be put in it's place and a rail to rail Op Amp can convert to voltage.
 

Thread Starter

bortweb

Joined Jan 11, 2023
11
The pictures I could find of the instrument cluster are very similar to the 2005 Miata instrument cluster, so it's a fair bet that the tachometer is an air core meter and if you can get behind the cluster it'll have a flexible pcb across the back and the four terminals will be marked plus and minus sine and cosine. If I were attempting this I'd drive the tachometer from an arduino for both your application and the tacho pulse also being read and converted by the processor to sine/cosine so a single pole switch can select one or the other. Not for the faint hearted. Maybe worth buying a used tachometer and testing this approach first, before attempting to modify your instrument cluster! Your full scale 200uA output will need to be converted to 5.0V to be read by the Arduino. The existing meter is probably a moving coil type with a relatively low resistance so this (measured) resistance will need to be put in it's place and a rail to rail Op Amp can convert to voltage.
Thanks I'll be looking for an used dash to test the system for sure before doing anything with the one in my car. Thanks also for the info. I'll be looking into that.
 

BobTPH

Joined Jun 5, 2013
9,160
Another approach would be to determine what the tach signal looks like, then build a circuit that converts your 200uA signal to a corresponding tach signal. That way, you would not have to touch the instrument cluster, and the old function could easily be restored if necessary.
 

Thread Starter

bortweb

Joined Jan 11, 2023
11
Another approach would be to determine what the tach signal looks like, then build a circuit that converts your 200uA signal to a corresponding tach signal. That way, you would not have to touch the instrument cluster, and the old function could easily be restored if necessary.
I like your idea too! Do you have any idea on how could I find that information?
 
Thanks I'll be looking for an used dash to test the system for sure before doing anything with the one in my car. Thanks also for the info. I'll be looking into that.
I've just read the suggestion by BobTPH which I have to admit it much simpler! A wiring diagram may help you to track down the sensor which is probably putting out a square wave (with fixed or frequency related duty cycle) and mimic that.
 

MisterBill2

Joined Jan 23, 2018
18,997
With the OEM tachometer in that car it is undoubtedly fed by the engine control computer. So there is no telling what is driving it.
But the idea of using a tach as the display is interesting. Does that tach have the large round scale that is about 3/4 of the way around? I am familiar with the tachs that use a moving coil that moves around on a ring magnet. Those are actually fairly standard meter movements. I would not mess with the in-dash tachometer, but rather fond an after-market tachometer with the suitable scale.
Then you can use a simple op-amp powered by a 9 volt battery and have a much nicer looking signal strength display..
 

Thread Starter

bortweb

Joined Jan 11, 2023
11
With the OEM tachometer in that car it is undoubtedly fed by the engine control computer. So there is no telling what is driving it.
But the idea of using a tach as the display is interesting. Does that tach have the large round scale that is about 3/4 of the way around? I am familiar with the tachs that use a moving coil that moves around on a ring magnet. Those are actually fairly standard meter movements. I would not mess with the in-dash tachometer, but rather fond an after-market tachometer with the suitable scale.
Then you can use a simple op-amp powered by a 9 volt battery and have a much nicer looking signal strength display..
I like the idea but the ultimate goal is to remove the large box from the top of my dash.
 

MisterBill2

Joined Jan 23, 2018
18,997
I've just read the suggestion by BobTPH which I have to admit it much simpler! A wiring diagram may help you to track down the sensor which is probably putting out a square wave (with fixed or frequency related duty cycle) and mimic that.
Given that the car has an electronic control module it is unlikely that any of the tachometer system is separated from the ignition system. The ECM processor knows exactly how fast the engine is turning, and the various cranks haft sensor signals are buried deeply within the wiring. Connecting to those will probably upset the control processor.
So if the TS wants to drive that same gage, disconnecting it from the processor is the way to go. That is,presuming it is a type of gage that can be driven simply. Thesame scheme with a battery driven op-amp would apply. That is required because the actual voltageson the two connections to the CB radio's signal strength meter are unknown. so isolating the remote display circuit is the way to go.
 
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