Output depending on impedances

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orwikcons

Joined May 2, 2008
22
Greetings!

A circuit with 2 inputs and 2 outputs. Inputs are (-), but with impendances, R1 and R2 (0 - 1000 Ohms). When R1 > R2, then +12V goes to Output1, and when R1<R2 then +12V goes to Output2. Circuit works on 12V (in car)
 

mik3

Joined Feb 4, 2008
4,843
Greetings!

A circuit with 2 inputs and 2 outputs. Inputs are (-), but with impendances, R1 and R2 (0 - 1000 Ohms). When R1 > R2, then +12V goes to Output1, and when R1<R2 then +12V goes to Output2. Circuit works on 12V (in car)
Its just a comparator circuit using an op-amp, i guess:)
 

SgtWookie

Joined Jul 17, 2007
22,210
Something like the attached circuit. R1 and R2 represent the thermistors, set at slightly different values.

It's simplified. There is no hysterisis provided, so when the two resistors are about equal, the outputs would likely toggle back and fourth.

Also, there is no adjustment; R3 or R4 should be a trimmer.
 

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SgtWookie

Joined Jul 17, 2007
22,210
If you're actually going to build it using an LM111/211/311, you'll also need a pull-up resistor on the output to +12v of around 2k Ohms, you should short the compensation pins together to reduce the probability of unwanted oscillations, add a 2MEG resistor from the output to the + input (adds hysteresis), and you'll also need to put some 100 Ohm precision resistors on the ground side of your thermistors, because the LM111/211/311 does not have rail-to-rail inputs.

Something like Texas Instrument's TLV7211 would be a much better comparator to use:
1) It's CMOS, and has far lower input current. This would enable you to increase R3 and R4 to 10k save on power consumed.
2) Being CMOS, it uses less current internally than the 111/211/311. This will save fuel.
3) It has push-pull outputs, so you would not need a pull-up resistor.
4) It has rail-to-rail inputs and outputs. You would not need the 100 Ohm precision resistors on the ground side of the thermistors.
5) No compensation pins to worry about.
This device is not available in a DIP, however - it's an SMT package. You would need to make a PCB in order to use it.

If you don't wish to manufacture a PCB, you could instead use a TLC372, which is a dual CMOS comparator that IS available in a DIP package. You will need to use a pull-up resistor on the output of this IC, but other advantages are similar to the TLV7211.
 
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