Question about input (button) and variable resistance

s_mack · Joined: 04 Jun 2009 Posts: 107

Hi.

PIC18F4480

PIN_B4 input is connected to a simple momentary pushbutton switch and our code treats it as a simple on/off. Can it be used for a variable resistance type switch? I'm not sure exactly how to describe/word my question. Basically, a customer has a different physical switch that they want to hook up to our device. They sent me the switch for testing and I see (using a simple ohmeter) that instead of going to zero ohms like our original switch, it brings it down to 85. With our setup as-is, it doesn't recognize the button push. This switch also has two other buttons on the same wires and they bring the values to 215 and 400 respectively. I'm really intrigued because if I can make 3 switches out of one, it opens up a whole world of possibilities for our device. But I don't want to change anything physically... just with firmware.

I'm sorry... I'm not the engineer that designed it and I really have very limited knowledge of the electronics side. I have modest skills with the firmware programming though. My hope here is that, with guidance, I can make the changes necessary if it is in fact possible to do what I describe with the current hardware.

I'm sure I need to post more info for you to answer but I don't know what to post.

Thanks for any guidance and pointers.

Steven

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Right, but what's manufacturer and part number of the switch ?

Post a description of the existing external circuit used with the switch.

Does your client want some kind of 3-way switch, or is this simply
a switch that has a higher "on" resistance because it's a very low cost
switch ?

s_mack · Joined: 04 Jun 2009 Posts: 107

As stated initially:

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Sometimes you put multiple switches on a single line by putting resistors in series with the switches to give different voltages against the common pull-up resistor. But this arrangement has to be read by an A/D or by careful timing measurement with a capacitor.

Is this the sort of arrangement you are looking for?
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s_mack · Joined: 04 Jun 2009 Posts: 107

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

B4 is an analog Pin, so it can basically work. You have to connect a resistor externally (if it's not already there),
according to the switch operation mode either pull-up or pull-down, that allows to detect the different switch resistances
by a different voltage drop. You can check this with a multimeter, without writing code.

Humberto · Posted: Sat Apr 03, 2010 8:39 am

This arrangement is used widely in automotive industries. The windshield wiper delay selector,
cruise speed control buttons are examples of using of this technique, where minimizing the wiring
is a must.
I do not see major problems in replacing a single pushbutton with this kind of switch in an Analog
input. Of course there are a little job to do it, it is not a matter of suppressing some pull ups resistors.

regards,

Humberto

s_mack · Joined: 04 Jun 2009 Posts: 107

So what I think I'm getting from the last few replies... is that it is possible (I figured that) but not without hardware modification, right?

I was hoping it could be done with the existing hardware and just access it by changing the firmware. Sort of like using the code to ask, "what is the voltage of" rather than, "is it on or off".

- Steven

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Also a single on/off switch will use a pull-up or pull-down resistor. Depending on the resistance value and other
circuit details, the multi-level switch function may be implementable without hardware modifications.

s_mack · Joined: 04 Jun 2009 Posts: 107