# How can I have different circuits relays based on a particular voltage input?

I want to build a circuit that is dependent on an input from a thermistor which is in a circuit that is a voltage divider.

Now if the thermistor is showing low temperature, then I want to increase my temperature supply voltage thus providing more temperature! But if the resistance of the thermistor is close to the correct value, I want to decrease the temperature supply or stop it!

Maybe nested relays but that does not really work, nested Schmitt trigger will only be more accurate but does not really deliver what I am looking for, maybe a certain OpAmp configuration but how?

Your thermistor has a voltage related to it that changes with temperature, yes? Compare that voltage to a set voltage point with an op amp/comparator, and drive your relay from there, maybe using a transistor to buffer the op amp output if it doesn't have enough drive current itself.

simulate this circuit – Schematic created using CircuitLab

• yes ee, but this can be achieved with a schmitt triger or inverted opAmp shmitt what i am trying to acheive is multiple voltage points. sorry if my question was not clear – KARAM JABER Dec 18 '18 at 19:39
• You can have multiple of these running in parallel, each opening another relay to turn on another heater. – CrossRoads Dec 18 '18 at 20:46

Now if the thermistor is showing low temperature, then I want to increase my temperature supply voltage thus providing more temperature! But if the resistance of the thermistor is close to the correct value, I want to decrease the temperature supply or stop it!

Rather than say "increase my temperature supply voltage thus providing more temperature" we say "increase the heater power to increase temperature". The way we do that is to use proportional control. We switch the heater on and off periodically.

Let's say that your heater takes 20 s to rise 1°C at full power. If we switch the relay on every 5 s we should be able to control within about 0.25°C.

          0           5           10           15           20         25 s
|           |            |            |            |          |
__          ____         ______       ________    ___________
on time  _|  |________|    |_______|      |_____|        |__|           |____
power     20%           40%            60%            80%           100%


Now, to get 20% power we switch on for 1 s out of 5. For 40% power we switch on for 2 s out of 5, etc. This is time proportional power or very slow pulse-width modulation.

The big advantage of this control is that only one power supply, one relay and one heater is required so it is simple and low cost.

Note that you need to make the time period short enough that the temperature doesn't rise or fall too much during one cycle but long enough that you don't wear out your relay by switching too often. Solid state relays are often chosen for temperature control as they have no moving parts.