Is there a component, similar to a .100" jumper, but holds a resistor or capacitor...

**h3rk** · 07-13-2008, 04:59 AM

Formerly titled:

Various thoughts and concerns about connecting to existing circuitry (analog)

I have been working a bit on this one circuit in my car and it led me down some thought-paths. I thought I would share them with everyone and see what others can offer on this. The main point I’d like to cover in this discussion is the precaution and preparation necessary to connect to your automobiles existing circuitry for utilizing a sensor or other analog parameter in your Fusion Brain project. This could apply to: temperature measurements, speedometer readings, tachometer readings, pressure measurements, etc.
For starters, I wanted to build an interface board where I can put capacitors for filtering, resistors for bringing voltages that may rise past +5V down to a level where they may never go above +5V, and possibly provide something to protect the brain by clamping at +5V. I wanted the circuit to be easily configurable so that only changing resistors and moving jumpers would be all that is necessary to change from one type of application to another. I came up with this circuit.

The intent of this was I could put several of these on a board and insert jumpers and resistors later, as needed to fit the application. The drawing on the left is for applications where there was either no sensor before, or the sensor is to be completely disconnected from the rest of the car’s circuits to be connected only to the Fusion Brain. The 2 left jumpers, that are used, connect +5V and GND to the circuit the other 2 jumpers bypass R2 (which may not even be installed in this case), complete the divider circuit through J1, and connect to the SIG pin through the current limiting resistor R3. I know, it looks like overkill, but I wanted it to be flexible. I’m not sure if using a current limiting resistor is recommended here, that’s part of what this post is for, to get some discussion and clarification.
The circuit on the right does not utilize the +5V and GND from the Brain, its intent is to divide a voltage that is already available in the car’s existing circuitry. The third jumper connects that voltage to a two resistor divider circuit, R11 and R12. The values of these resistors are intended to be part of a later discussion here in this thread. Once again the voltage that is available to the SIG pin is current limited through R13. Are R3 and R13 necessary? Can a universal circuit, like what I intend, be achieved more easily? Should another component be available in the circuit to clamp the voltage at +5V just in case?
Now, to provide an example of an application of the second circuit, and the concerns I have stated in the opening paragraph, I’ll walk through a small part of my HVAC project:
I was working on where to input and output signals, connecting through my existing control head and sensors. In this particular circuit, I was working on the compressor request and intake (Evaporator outlet) temperature. I both wanted to understand how this control head evaluated when to ask for the compressor from the ECM, knowing that intake temperature was part of the equation (anti-freeze function for the evaporator), I started probing around. There’s a 100 pin controller chip in there, so I figured I was out of luck for the most part, but as it turned out, it wasn’t entirely the case.

(This is where, on the control head, most of the circuit I’m explaining resides)

In fact, once it all got laid out in schematic form, it became fairly easy to understand. And for starters I believe intake temperature is not used for any other purpose than provide a signal that prevents the compressor from turning on. I have more in store for that sensor, but what affect will my use have on its intended function?

So basically what is going on here is the intake sensor is part of a voltage divider fed to a comparator. Current, I1 going through R751 sets up the voltage to be compared, the I2 portion sets up the reference voltage. I’m not entirely sure but I think the I4 branch was used for biasing. As the temperature heats up, the resistance of the intake sensor goes down, which Raises the total current going through I1, more voltage drops over R751 and the comparison voltage goes down. The way the circuit is initially, the comparator will shift to +10V at the output at about 27F. It was pretty fun to model it out and see it operate (I used a potentiometer). It works.

But now, I want to add one more parallel branch, another voltage divider comprising of R11 and R12 from the first picture. My concern was; will that bias the compressor comparison circuit even more?
And the answer is yes, most definitely. Resistor values in the 1-10K range changed the set-point from 27.5F to 11.1F (10K used for both R11 and R12 in that case). Using resistors in the 200K+ range seem to minimize the bias so that the set-point only changes by <1F.
So that is my example of; how connecting to a certain point in your vehicles electronics can adversely affect important operating parameters. And in this case, the voltage I am using is definitely capable of going above +5V, so something must be used.
The thought occurred to me to use an opamp, or something with very high input impedance that is capable of scaling down and buffering, but I’m not sure if that’s necessary at this point.
Your thoughts? Comments? Questions?

I would also like to reiterate my request for feedback on the top circuit, with regards to its multi-use intended function, complexity, and whether or not current limiting resistors are reccomended. Feel free to criticize.

**2k1Toaster** · 07-14-2008, 12:41 AM

The current limiting resistors arent needed, but why not, they are cheap. And I would recommend to put an opamp, something like a LM358 (you can get non-duals too, or even quads) and run it in buffer mode. Even if the input is huge, it can only output the rail which should be 5v. Extra precaution there.

As for using the existing temp sensor, Depends how it is read. If it relies on resistance and you hook up a huge resistor to it, you throw off all the readings (if you intend to keep the OEM working correctly as well). If it is current, then pulling current off of the line will again result in some weird readings. Voltage is what you hope it is. In which case you can run a buffer line off of it. For current, use a small resistor to get a voltage, and run through a buffer.

And now as I read further down I see you already experimented with this!

And you already have the answer, buffer opamp.

I like the idea of the univeral OEM sensor connector. Very nice. Took a while to stare at it to see what was what. Looks daunting with the wires everywhere at first

**h3rk** · 07-14-2008, 02:30 AM

Originally Posted by 2k1Toaster

The current limiting resistors arent needed, but why not, they are cheap. And I would recommend to put an opamp, something like a LM358 (you can get non-duals too, or even quads) and run it in buffer mode. Even if the input is huge, it can only output the rail which should be 5v. Extra precaution there.

As for using the existing temp sensor, Depends how it is read. If it relies on resistance and you hook up a huge resistor to it, you throw off all the readings (if you intend to keep the OEM working correctly as well). If it is current, then pulling current off of the line will again result in some weird readings. Voltage is what you hope it is. In which case you can run a buffer line off of it. For current, use a small resistor to get a voltage, and run through a buffer.

And now as I read further down I see you already experimented with this!

And you already have the answer, buffer opamp.

I like the idea of the univeral OEM sensor connector. Very nice. Took a while to stare at it to see what was what. Looks daunting with the wires everywhere at first

Sorry, I probably should have put a simplified schematic up near the one with the jumpers. I know what it's like to stare at something that looks so unnecessarily complex just to find out it's a simple little voltage divider. So for everyone else

here it is.

I added another 3 pin connector to allow one more use, like for the 3pin humidity sensor I plan to use. This allows me to filter the ouput a little bit with a small capacitor.

I have to think about the opamp thing. I've never really thought about scaling down with an opamp.
In order not to draw current, the J11 circuit would have to not be a voltage divider. The line tap would have to go straight to an input pin of the opamp. Set up like a buffer, the opamp would probably just clip the signal at 5V, not scaling, but clipping, ruining any semblance of linearity. So something with feedback needs to be done.... but, just thinking about the +1 in the gain equation makes me think I can't do it as easily as I would like... yikes. Anything you can tell to elaborate on what you were thinking or to clear up my misunderstanding would be appreciated.

Maybe just really high resistors (100xK or M) is the best compromise. What negative effects, if any are there to using such large resistances in a real-world divider?

After that, a rail to rail 5V buffer may still be a smart addition.

I just wanted to put something out there for that person who unknowingly taps a voltage signal coming into or out of their ECM for a speedo input, and unknowingly changes their shift points, or something drastic like that.

In fact, the Altima has a pressure transducer that sends analog to the ECM, which decides, based on that input, when to turn the clutch on and off (provided other conditions as well, but this isn't my HVAC thread). I want that signal for my project, and I'm not going to tear down my ECM to see how this one works quite yet. But if I had to scale it down, I would have to be super cautious, as any current drawn by my tap may change the on/off setpoints, possibly destroying expensive hardware.

I think it's 0-5V already, so I'm in luck. But, be careful.

**2k1Toaster** · 07-14-2008, 02:57 AM

With an opamp you cannot get below 1 gain.

But after the voltage divider on your 2 top circuits, then put the opamp. Just incase there is a big spike and the divider doesnt put it down enough.

But on the same way, if you connect a buffer opamp to the car's +12v power, it wont clip it, but will buffer it. Then from there you can do whatever you want to it, and then at the other end buffer again with a 5v rail.

Big resistors are definately a good way of doing it, but if you make them too big (multi MOhm) then it wont really work as the Fusion Brain has a MOhm input resistance, and that throws everything off. A voltage reading assumes infinite input resistance in. So ifg you have a few kOhm or less, multi-MOhm reader looks huge like infinity so no problem. Put in huge resistors, that reading resistance is no longer so big and throws off everything. You wont read in what the actual voltage in is. I dont think something in the 100k to 200k range will make a difference at all assuming the signal levels are a few mA or more since it is in a car and not a cell phone where they use uA or fempto-amps and 1 resistor screws everything royally...

**h3rk** · 07-14-2008, 03:19 AM

Originally Posted by 2k1Toaster

With an opamp you cannot get below 1 gain.

But after the voltage divider on your 2 top circuits, then put the opamp. Just incase there is a big spike and the divider doesnt put it down enough.

But on the same way, if you connect a buffer opamp to the car's +12v power, it wont clip it, but will buffer it. Then from there you can do whatever you want to it, and then at the other end buffer again with a 5v rail.

Good stuff, the chips are starting to add up now though... I think the safety factor will be worth it... I'll put a redesign up soon.

Originally Posted by 2k1Toaster

Big resistors are definately a good way of doing it, but if you make them too big (multi MOhm) then it wont really work as the Fusion Brain has a MOhm input resistance, and that throws everything off. A voltage reading assumes infinite input resistance in. So ifg you have a few kOhm or less, multi-MOhm reader looks huge like infinity so no problem. Put in huge resistors, that reading resistance is no longer so big and throws off everything. You wont read in what the actual voltage in is. I dont think something in the 100k to 200k range will make a difference at all assuming the signal levels are a few mA or more since it is in a car and not a cell phone where they use uA or fempto-amps and 1 resistor screws everything royally...

So in regards to the J11 divider resistances:
Too small = possibly bad for car electronics,
Too big = bad for Brain electronics.

Reminds me of the three bears.

Ah, that's what all of that Thevenin stuff was about, I should have read harder.
(Reference to "Art of Electronics" by Horowitz, early chapter.)

**h3rk** · 07-19-2008, 06:21 AM

I'm finishing up on my universal analog input interface, after adding some 12V and 5V opamps (quad opamps), so the design will be in packs of 4, which complements the brain in some way. I also added another optional path for a wheatstone bridge configuration.

Then a question came up:
Is there a component, similar to a .100" jumper, but holds a resistor or capacitor (insulated and secure)?
It would be nice if the piece could hold different through hole resistors, since there are so many different flavors.

I realize I could just use a 2 pin female connector and pins, running each lead in to each pin, but I was thinking maybe someone already thought of this before, and made something (insulated and secure). Kind of like shift-light pills, used for setting different shiftpoints, but smaller like PCB jumpers.

**2k1Toaster** · 07-19-2008, 11:05 AM

I know they make resistor arrays, but the resistors are already inside and usually either the same size, or the same factor of size. I dont know of resistor sockets so to speak. But The thick lead ones like 1/4W or 1/2W or more usually fit into the DIP socket arrays.

**h3rk** · 07-19-2008, 11:08 AM

hmmm, I guess I'll just use female header connectors, crimping pins to them. I need them to work interchangibly (nice word huh?) with .1 jumpers. I hope it wont lead to high resistance problems. And then something to insulate them.

Oh yeah, I can mold them in clear epoxy! easy as cake!

**h3rk** · 07-19-2008, 11:19 AM

Here's where I'm at, I'm still running it through all of the lineups, but I've been doing that for a couple hours now... actually I've been staring at it blankly, mostly watching TV.
But the plan is to remove most of the resistors from the board, replace it with a header pin pair, and condense where possible (either/or resistor choices can be a 3 pin header (R1,R14), etc).

That would allow every use I intended, some I haven't, and only one pcb template.

**greenman100** · 07-20-2008, 03:06 PM

We went through designing this, but there are some problems.

If you use cheap op-amps, you can't power them from the 5v header as they don't go rail to rail then.

Op amps' minimum gain is 1... so you need a resistor divider for anything greater than 5vdc.

also the input resistance of the PIC is more like 5k ohms IIRC, I can look it up later in the datasheet

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