Need small adaptation of this thermostat circuit

Discussion in 'General Electronics Chat' started by DMR, Mar 9, 2011.

  1. DMR

    Thread Starter New Member

    Mar 9, 2011
    4
    0
    Hello,

    I successfully built this simple 4-12 volts dc thermostat, I run it on 5 volts:

    [​IMG]
    The NTC gets a lower resistance at higher temperatures this closes T1 which then opens T2 that lits the LED D1.

    Instead of the LED I need an approx 4 to 5 volts output when 'hot' and approx 0,5 volts when cold. How would I go about adapting the circuit above for this?

    It is important that the output voltage switches abruptly (like the LED does now) instead of a gradient sliding voltage.

    Bonus question: I would love the 'cold' voltage output to be variable.

    Many thanks!
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,575
    230
    Google: voltage comparator

    Ken
     
  3. DMR

    Thread Starter New Member

    Mar 9, 2011
    4
    0
    Thanks!

    Would it be possible by just connecting a trimpot instead of the LED or below T2 maybe?
     
    Last edited: Mar 9, 2011
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,575
    230
    D2 is used as a fixed reference voltage. Replacing it with a trim-pot won't give you what you want. I would replace everything to the right of P1/R1/R2 with an LM311 and a trim pot and a resistor to get your 5v logic level output.

    Ken
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    How much current is output to deliver? First requirement could be met by replacing LED with a PNP transistor or P ch FET, base to R4, emitter to 5V supply, collector is output. Now bypass collector to emitter with a 4.5V zenor, anode to collector. To make 1/2 V a variable ??
     
  6. DMR

    Thread Starter New Member

    Mar 9, 2011
    4
    0
    Thanks for your ideas!

    A few microamps? The output is to an advanced house ventilation controller, I guess very high impendance because he manual dial (in the kitchen) is just a 470k trim-pot on 5 volts. The output of the trim-pot is directly used to set fan-speed, no active components.

    I want to put the NTC on a shower water line. When the pipe runs hot the circuit should output approx 5 volts. If pipe cools down, circuit should output low voltage so fan speed drops. (1 big fan for the whole house)

    The central fan "computer" has 2 dial inputs, the thermostat needs to have an abrupt switch because if it would slide during normal room temp variations the fan computer would make the shower input active and ignore the kitchen input.

    Why whould this circuit have 2 transistors? Could I remove T2 and create an output between R3 and T1 ?
     
    Last edited: Mar 10, 2011
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    T2 is needed to provide positive feed back to give ckt the snap action. The extra transistor is needed to invert the output at T2 collector so output goes to 5V when hot. To adjust cold V output , R5 & R6 is added along with D3, which should allow adjustment of cold V output from 0V to about 1V- why?? Small change: Leave D1 as is, connect base of D3 to T2 collector via 1k resistor.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Modified ckt slightly, still forgot to lable T3 & base resistor R7-1kΩ.
     
  9. DMR

    Thread Starter New Member

    Mar 9, 2011
    4
    0
    Thank you very much, that is very kind!

    I'll get an extra transistor and build it that way, it looks exactly like I need with the snap action and all.

    Thanks!
     
Loading...