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Pressure or Temp sender as switch

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quadfour

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Hi.

I've have a couple of automotive sender units, one for pressure, and one for temp. I'm want to setup a 'switch' action in addition to gauges to work with the existing tell tales on the car. Aside from a spot of soldering, I'm pretty unskilled with electronics.

After some reading, looks like I need to use a comparitor. I think the best one to use would be a lm339 That is based on there being two sender units I'd like to do this for ( and maybe another down the line ). One sender operates from 10-180 ohms, and the other is 23-287ohms.

After some playing around I've come up with this

My real concern is burning out the comparitor... I don't really know what I'm doing, and am not running large resistance the circuit, which I gather allows a bit of current. There will also be a gauge in parallel ( which apparently has 1Kohm effect ).

Am I heading down the right track? Sorry I don't have any proper electronics simulators etc.
 
Don't see a particular problem.

Themistor resistance is nominal.

The current(s) in the circuit look acceptable.

Might want to consider using 1/2 or 1 watt resistors, but again, the currents listed for the circuit don't really warrant that.

Cool sim, by the way.
 
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I don't quite get what you are doing in the simulation? I assume it just represents a single channel using one comparator for either pressure or temperature? The gauge is just across 12 volts to gnd. Does the latter pot represent the gauge out? Anyway, yes, a comparator circuit would likely be the solution.

Ron
 
I don't quite get what you are doing in the simulation? I assume it just represents a single channel using one comparator for either pressure or temperature? The gauge is just across 12 volts to gnd. Does the latter pot represent the gauge out? Anyway, yes, a comparator circuit would likely be the solution.

Yup, this is just a simulation of one sender. Resistances would change a little with other senders, but the rest is the same. I have no idea how the gauge works... but I found some documentation for a rather expensive version of what I'm trying to do which stated it needs a 1kohm resistor if no gauge is in use. Based on that, I just put the gauge in as a 1kohm resistor.

I'll try to do up a complete sim for all senders, and perhaps a relay for one sender ( to mimic what the existing circuit does ).
 
This is generally done using a comparator as you mentioned. I suggest you give this link a read to get a good handle on using a comparator. They cover them fairly well. The idea here is the sensor will provide one input voltage to the comparator and the comparator will also have a reference voltage in. Depending on the configuration when a sensor voltage crosses the reference voltage the comparator voltage will toggle. The trick is getting the voltages from the sensors to scale to a range of whatever units, temperature for example. For what you have in mind using a pot like about 10 K is a good way to set the reference. If the circuit would be used in an automotive application things get a little more complex as the Vref needs to be stable and automotive systems vary. The link should be useful. Your circuit also used hysteresis which is covered in the link. If you need help, just ask. :)

Ron
 
Finally got around to wiring up my design on a breadboard. It changed plenty from what was discussed here. Sadly doesn't seem to work as I expected.

In this setup I expect the left comparator to be sinking output...

R4 = 87Ohm

But it's not doing as expected. Measured voltages on the comparator are:

+ = 12v
- = 9.4v
out = 11.7v

There appears to be no voltage drop across R1 which seems strange to me. I would expect the + side of the comparator to be somewhere around 7v in this setup. Obviously & sadly, I still don't really know what I'm doing with this. Are there any blindingly obvious problems here? All comments welcome!


View attachment 64170
 
If your comparator is a LM393 dual comparator the outputs are open collector and will require a pull up. I don't get how you have things configured? You may want to give this link a read for some useful comparator circuits.

Ron
 
If your comparator is a LM393 dual comparator the outputs are open collector and will require a pull up. I don't get how you have things configured? You may want to give this link a read for some useful comparator circuits.

Ron

I've proven the comparator is working exactly as expected. If I swap the inupts, the comparator switches on, sinking current from the transistor and hence the LED switches on. Sorry Ron, I really don't know what to say...
 
OK, I assume D1 & D2 go to the wipers of a pair of 200 ohm pots with one side to common? This becomes the ref for the comparator? R1 and R8 should have a drop across them if that is the case. While I don't quite understand why the diodes? Does R8 behave with a drop across it?

Ron
 
OK, I assume D1 & D2 go to the wipers of a pair of 200 ohm pots with one side to common? This becomes the ref for the comparator? R1 and R8 should have a drop across them if that is the case. While I don't quite understand why the diodes? Does R8 behave with a drop across it?

Ron

D1 & D2 do indeed go to the wipers of 200 ohm pots. The reason the diodes are there is because there's another circuit ( existing, working circuit ) connected to the wipers of those pots. Didn't want that other circuit to interfere.

I didn't see a voltage drop over R8 either...
 
While I don't know what else you have out there across the pots I suggest removing it. You should be seeing your reference voltage on those pot wipers. I would also eliminate the diodes. Unless whatever else is out there on the pot wipers has a really low input impedance you should see Vref on those pot wipers.


Ron
 
I got it partially working this morning. Had a faulty link wire on the breadboard.

Forgot to mention, my comparator circuit relies on the low impedance of this parallel circuit via the wiper of the 200ohm POT. Even I forgot this while testing this morning ;). But anyway, after testing with it I think the diodes were in the wrong place.

First I had an issue with the LEDs. The transistors were allowing 3mA of Collector-Emitter current in its OFF state, enough for the LEDs to glow bright enough. So instead I've connected the LEDs to the outputs of the comparator. Each LED only passes about 8mA at 12V, so shouldn't cause any problems for the comparator with it's max of 16mA. And the darlington transistors will add bugger all to that.

I still have a problem with the other circuit in parallel through the 200ohm POT, and I'm concerned about this. I have no idea how this other circuit is wired... it's a standard electronic automotive temp gauge. When I remove power from the gauge through its +12V terminal, the gauge still operates, meaning it has to be powered through the comparator circuit via the 200ohm POT. Not sure if I'm harming it or not. Under normal circumstances there would be nothing operating in parallel with the gauge.

Is there anything I can do to prevent the gauge being powered from the comparator circuit? A diode between the gauge and the 200 ohm POT works, but adds too much impedance which throws the gauge right out in its reading.
 
Without seeing and understanding the other circuit I can't say what or where the problem is beyond the possibility the other circuit is providing a loading effect. I can't begin to guess why the other circuit (gauge) is powered through the comparator circuit pot. Maybe another member has some thoughts?

Ron
 
Did a bit of research figuring out how these needle gauges typically work. Seems they typically use coils on either side of the input to setup an electric field. Based on a couple of the designs from the web, I can understand why it is being partially powered through the 200 ohm POT and the comparator circuit when + is off ( which should actually happen anyway ). And there's bugger all I can do about it anyway :).

I'm pretty sure it wont cause any harm. As a precaution though, I'll drop the current used in the comparator circuit down a bit, should lessen any impact on the gauge.

Cheers for all the help everyone. Once I actually finish the design and build this thing, will give it out freely for all.
 
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