Author Topic: electronics help needed  (Read 2117 times)

Offline Gadabout

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electronics help needed
« on: April 07, 2019, 08:09:00 PM »
Hi, I am in need of someone more electronic savvy than myself(that means everyone :-) )to help me with a small electronic job, its just telling me what resistors I need to get a voltage drop at certain points, am going to try and attach three diagrams to hopefully explain. I need to know the value of the resistors shown in the diagrams to achieve the voltage outputs in each example please
thanks
Mark

Offline RussellT

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Re: electronics help needed
« Reply #1 on: April 08, 2019, 05:03:15 AM »
Hi Gadabout

Interesting problem, presumably you're trying to recreate a cruise control switch.  Unfortunately I don't think it's quite as simple as you think as it depends what's in the cruise control ECU CCS connection.  As your diagram shows you get the highest voltage when all three resistors are in series - so the highest resistance.  If the effective internal resistance of the ECU connection is for example 10,000 ohms then having 10,000 ohms in the switch would halve the voltage.  I think to do the calculation you need to measure the voltage at the CCS connection with nothing connected to it and probably with the cruise control engaged.  You also need to connect a resistor in the circuit and see what the voltage changes to.  From that it should be possible to calculate the resistors you want.

On the other hand it's probably easier to connect a variable resistor in place and reduce the resistance until you get the voltages you want and take the resistor out and measure the value.  I would have thought you'd be safe enough starting with a 10k variable resistor - which should give a maximum current of less than 2mA at battery voltage.  That shouldn't be enough to damage anything.  Make sure you start with the resistance at maximum.

Maybe others more expert than me will have different views or more to add.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline mc

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Re: electronics help needed
« Reply #2 on: April 08, 2019, 05:43:46 AM »
You should be able to get a reasonable resistance measurement, by simply measuring between CCS and GND with the module shut down (that old a vehicle, it'll probably shut down pretty quickly, but would still be wise to give it a couple minutes with the ignition off before measuring).
Try measuring the resistance with a multimeter connected both ways, in case there are any diodes that may upset resistance readings.


Once you know the internal resistance of the module, some series resistance calculations should give you the resistances needed.

Offline philf

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Re: electronics help needed
« Reply #3 on: April 08, 2019, 05:50:18 AM »
Hi Gadabout,

I agree with Russell and think there's not enough information given to work the values out.

I would try to apply 7.2v to the CCS connection and then measure what current is flowing. This would tell you the input impedance of the Cruise Control ECU which has to be part of any calculation. (Voltage/Current=Resistance - Ohm's Law)

I came up with 3 formulae for the three scenarios:

I used the mean voltages and A, B & C are the three resistors working from top to bottom. D is the input impedance of the Cruise Control ECU.

(B+C)/(B+C+D)=(12-3.45)/12

C/(C+D)=(12-1.6)/12

(A+B+C)/(A+B+C+D)= (12-5.65)/12

Once you know D, C can be evaluated, then B and finally A.

Phil.

[Edited]
« Last Edit: April 08, 2019, 08:16:31 AM by philf »
Phil Fern
Location: Marple, Cheshire

Offline RussellT

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Re: electronics help needed
« Reply #4 on: April 08, 2019, 08:46:10 AM »
I thought about trying to measure the internal resistance of the cruise control ECU, but I thought that it's likely to have transistor switches so it might be difficult to get the correct value.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline Gadabout

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Re: electronics help needed
« Reply #5 on: April 08, 2019, 02:45:21 PM »
Thanks guys for all the input, yes I am trying to fix my cruise control on my Landcruiser. I am updating it by using a later ( same series model but just two years later) cruise control. The later uses a three wire switch where as the one in the vehicle is a 5 wire switch. The diagrams I attached are from the factory service model just changed to show the different switching. They are momentary switches, and the voltages shown are what Toyota say you should get when testing the various modes with everything hooked up and the ignition on.
thanks
Mark

Offline Gadabout

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Re: electronics help needed
« Reply #6 on: April 08, 2019, 10:59:58 PM »
Ok so thinking about it after reading what you guys have said I need to work backwards from the cms(module) to find the resistance of that then to the next resistor to get that voltage and so on??
Am I on the right track??
regards
Mark

Offline philf

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Re: electronics help needed
« Reply #7 on: April 09, 2019, 04:26:21 AM »
You've got it Mark!
Phil Fern
Location: Marple, Cheshire

Offline RussellT

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Re: electronics help needed
« Reply #8 on: April 09, 2019, 04:30:18 AM »
Hi Mark

I still think the easiest way to do this is with a variable resistor - no calculation needed - except for some subtraction for each stage.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline philf

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Re: electronics help needed
« Reply #9 on: April 09, 2019, 05:27:01 AM »
Hi Mark

I still think the easiest way to do this is with a variable resistor - no calculation needed - except for some subtraction for each stage.

Russell

You're probably right Russell  :thumbup: but I like mathematics! :smart:

Phil.
Phil Fern
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Offline Gadabout

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Re: electronics help needed
« Reply #10 on: April 09, 2019, 08:46:39 PM »
thanks guys, not good at mathematics......whats subtraction.. :-)
am waiting for another module to come and will set up a test rig to sort it all out
regards
Mark

Offline tom osselton

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Re: electronics help needed
« Reply #11 on: April 10, 2019, 06:24:04 PM »
Subtraction is what happens when the wife or gf gets hold of your wallet! :beer:

Offline AdeV

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Re: electronics help needed
« Reply #12 on: April 11, 2019, 03:26:50 PM »
Subtraction is what happens when the wife or gf gets hold of your wallet! :beer:

And division is what you get when there's nothing left to subtract!  :lol:
Cheers!
Ade.
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Offline Gadabout

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Re: electronics help needed
« Reply #13 on: April 29, 2019, 12:53:05 AM »
Hi guys, got the spare module today and I have measured from pin 18 to ground on the chassis of the module and get 7.56k ohms resistance. So can one of you experts help me with what the value of the other resistors need to be to get the required voltages in the original post? Thanks heaps
Regards
Mark

Offline philf

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Re: electronics help needed
« Reply #14 on: April 29, 2019, 04:31:02 AM »
Hi Mark,

I tried working out the values but I got a negative value for a resistor. Looking again at your diagrams I think you must have the pin voltages for Cancel & Resume/Accleration mixed up. Can you check.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline Gadabout

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Re: electronics help needed
« Reply #15 on: April 29, 2019, 05:25:16 AM »
Hi, have just checked the diagnostic pages and what I have posted is the same as the pages, have attached photo
Thanks
Mark

Offline Gadabout

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Re: electronics help needed
« Reply #16 on: April 29, 2019, 05:27:33 AM »
Sorry that should have been resized, not sure how to that on this old iPad 😟

Offline philf

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Re: electronics help needed
« Reply #17 on: April 29, 2019, 06:20:53 AM »
Hi Mark,
You're right but then I can't work it out! I think it's not just a simple divider and you may be better wiring in a 100k potentiometer as has already been suggested and seeing what resistance is needed to arrive at correct voltages.
Phil.
Phil Fern
Location: Marple, Cheshire

Offline seadog

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Re: electronics help needed
« Reply #18 on: April 29, 2019, 06:36:39 AM »
Resized for you

Offline John Swift

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Re: electronics help needed
« Reply #19 on: April 29, 2019, 08:44:36 AM »

by combining the circuit diagram with the info in the manual

as the   main switch on the cruise control grounds the ECU CMS terminal 19

and cancel , set/coast  & resume/accel switches  selects a resistance to ground the ECU

the only way for the voltages to agree with the table in tlhe manual is the ECU inputs to have pull up resistors built into the ECU

                               john

Offline Gadabout

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Re: electronics help needed
« Reply #20 on: April 29, 2019, 08:11:55 PM »
Thanks all! Will get a 100k pot and experiment!
regards
Mark