Hi all,

I've signed up, as I am trying to find a solution to a 12v automotive problem.

I have been asked to somehow convert the signals sent from the radio controls on the steering wheel, to seperate realy outputs.

The circuit is something like this:

--------[resistor]-------------[resistor]------------------- Output
| | |
| | |
Switch1 Switch2 Switch3
| | |
| | |
------------------------------------------------------------- -ve

There are 3 switches, which all share a common earth.
There's then resistors between the switches, to change the Output resistance, depending on which switch is pressed. (Sorry if i'm teching you to suck eggs!)

Anyway, I wish to gain 3 seperate outputs, either -ve or +ve (doesn't matter), to operate 3 seperate relays.

Again, the circuit is to be used in a 12v car.

Does anyone know how this can be done?

Cheers,
Stu

 

You can do it with a series of comparators, but probably the easiest way is to use a bar graph IC like the LM3914, this has the comparators already built in, and gives a possible 10 outputs, just choose the ones that work for your application.

A quick google finds LOAD'S of information, for an example try http://www.electronics-tutorials.com/devices/lm3914.htm.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Thanks for the info! I'll do a little reading on them.

Another quickie though.... Will the LM3914 work with the -ve input I have? Its not a voltage, its a ground with varied resistance.

 

If it's just grounded resistors you simply add an extra one from the top of the resistors to 12V, this converts it to a voltage.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I only have access to the "output" wire from the radio controls.. It has been disconnected from the radio.

Are you saying I should connect it to the LM3914 input, and also to a resistor that is +12v on the other side?

[-ve output from r/c]---->[LM3914input]<----[resistor]<-----[+12v]

Like that?


Sorry for being so lame!

 

Yes, it makes a 'potential divider', which will give a changing voltage everytime you press a button.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

just make yourself an R/2R ladder and use the switches as your inputs.

In fact, the electronics section of http://www.play-hookey.com shows you how to do it.

__________________
-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-

 

mstechca, read. surely gaining direct access to the switches would require tearing apart his steering wheel, which makes no sense when he can just use the existing setup with a purpose-built circuit.

__________________
EEgeek.net

 

No, the steering wheel will not be coming apart...
I've been asked to put together a harness that can be plugged in by someone with little knowledge of car electronics.
I plan to pickup the wire from the steering wheel controls at the rear of the radio, as this will eliminate any problems with the guy unplugging/messing with his airbag etc..

The radio will no longer be controlled by the steering wheel controls, but instead they will be used to control the cruise control, via relays (which I have already worked out how to easily do)

 

I will give Nigel's solution a try later this week... it looks to be the best option so far.

But, how do you set up the circuit for the resistance levels?

I did a little testing with the wire last night, and with my multimeter set to 2000 ohms, I got the following readouts;

When testing the wire with the buttons at rest (not pressed) I get a readout of "1"
Then whilst pressing each button in turn, I get;
[+ Button] = 200
[- Button] = 118
[O Button] = 320

Not sure if that makes any sense to anyone? I've not done much with electronic components etc for a good few years! Forgot most of the things I leant back in school!! I need to get reading again!

Anyway, will these resistance values work ok with the LM3914 setup? And how do I configure it? i.e. how does it know that 22ohms should be output 1, and 320 should be output 3... etc etc.


Sorry again for my lack of basic knowledge, and thanks again!

 

As I said before, connect a resistor from the wire to 12V, with no button pressed the wire will be at 12V, as you press buttons the value will change, depending on the resistor value you use (you could make it variable to allow accurate adjustment).

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Are you certain the switches and resistors are configured as shown in your first post? I can't see any way you can get those resistance readings with that circuit.
To get 1 ohm with all switches at rest, Sw3 must be normally closed. When it is not pressed, you will always get 1 ohm - or am I missing something? :? Is the reading of "1" not a measure of resistance, but an overrange indication?

__________________
Ron

 

Ron, I think it is an overrange indication, that's how it is shown on a couple of the meters that I have.

it seems they used three resistors (basically added one between the switch on the right and the output) and they seem to be about 100 ohms each, since the values are close to 100, 200, and 300 when switches are pressed.

if, for example, you used a 2k resistor between the output and +12v, you would get output voltages of 0.57v for the 100 ohm output, 1.09v for the 200 ohm output, and 1.57v for the 300 ohm output, and the maximum current draw of the resistor divider would be only about 6mA which is completely insignificant in a car's electrical system.

all you have to do is have a circuit that can detect the difference between +12 (no switches pressed), 0.57, 1.09, and 1.57 volts. As others have already stated, a comparator circuit would work, or you can use a bar-graph type driver chip, which is little more than a pre-made bunch of comparators set up for such a purpose. A very good reason to use the LM3914 type driver chip is that it can be operated in "dot" mode, where only ONE of the outputs will be active at one time. If you just used a few comparators with no logic circuitry on the outputs, the outputs for all 3 switches would appear to be on when you pushed the button that makes the output 1.57 volts, because that voltage is also greater than the thresholds for the other two switches.

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EEgeek.net

 

If he uses the LM3914, it would be smart to use the internal reference to provide the reference to his switched resistor network and to the 3914 divider string.

__________________
Ron