DC signal condititoning

[Algojervia]

Hello there.
I have been working a small circuit that's just two 555 timers acting as a servo motor controller and that works great but now I want to control it via a NTC resistor. As you can see in the schematics bellow in my latest try I use an op-amp as a comparator and that in turn manipulate the voltage of pin 5 of the second 555 timer via the pots R7 and R8 that set the upper and lower voltage level. This works OK but can get a bit hysteric when the temperature changes slowly, making the servo go back and forth like crazy. I don't think this is particularly bad for the motor but it is not the nicest design. And what I really wanted to do was to make to servo open gradually over a span of a few degrees.

And thereof the title but do any of you have a completely different design, i'm interested as along as it does not involve a µC. If I have a signal from a voltage divider of a NTC I can easily clip the voltage level with a diode but how do I set a lower voltage limit without distorting the signal. So I have a input signal that can swing between something like 1-4 volt but I'm only interested in the range of 2-3 volt and when the in signal is higher I wan't it to cap a 3 volt and when it is lower I want it to stay at 2 volt. I also want to be able to adjust the max and min levels. I think the problem I run into is that there is a lot of information on mixing ac components of a signal but I can't find anything about this type of DC mixing, maybe it is so trivial it is not worth a note? Also I do know how to clip the upper part but my attempts to set a minimum has only resulted in me adding a minimum level to the input signal instead of superimposing it.

//A

[Ammar]

A completely different design could involve an ADC connected to an FPGA. Doesn't even use a uC 

[Cliff Matthews]

Maybe it's just me, but that schematic needed some help.. Here ya go!

[Ian.M]

That's a grossly over-complex circuit.  Ignoring passives, you only need one 555 and a couple of steering diodes to produce a voltage controlled servo signal, and to make it thermistor controlled, simply add a single supply rail to rail opamp, configured as an inverting amplifier, with  the thermistor in a bridge feeding its inputs.  The amplifier gain sets the transition range and the resistor in series with the thermistor sets the midpoint.   When the output is railed or below 1/3Vcc it will have no further effect on the timing.   If the servo movement is in the wrong direction, swap the thermistor and the resistor in series with it in the bridge. See attached LTSPICE sim for details.