Car interior fan automation

Discussion in 'General Electronics Chat' started by Dan2, May 28, 2010.

  1. Dan2

    Thread Starter Member

    Oct 16, 2008
    I recently built up a temperature guage for my car. it measures the ambient temp inside the car. I used a ICL7107 chip and LM35 sensor, and it works great.

    now i want to connect my interior fan to the circuit. the resistors for slowing the motor down are blown, so i am looking at building a PWM circuit to drive the motor.

    if i do that, i might as well try to control it with the temp sensor.

    my plan is to have a setpoint, say 20 deg C, the fan must run faster the further away it is from the setpoint. the heater and AC controls i will leave manual.

    i want to use an op-amp as a comparitor between the temp sensor output (0-200mv) and a defined setpoint which will be fed into the pwm controller. the problem i have is that i need the fan to speed up in both the positive and negitive deviation from the setpoint. so if its very hot, the fan runs at full speed until the temperature reaches 20 deg. and if its cold, i will put the heater on but i want the fan to run fast until the temperature reaches 20deg.

    so how can i "rectify" the output of the op-amp so the output is positve whether the input is positive or negative??
  2. BMorse

    Senior Member

    Sep 26, 2009
    Here is part of a circuit I built (Part of a much larger project) this is just an example you can go by, the output is always positive, even if the input is negative or positive.... altough you will need to power the LM324 from an 8 volt power source....


    B. Morse
  3. Dan2

    Thread Starter Member

    Oct 16, 2008
    thanks for the reply. if the input is negative, the output is not proportional to the input but positive, right?

    I need a circuit that will amplify the positive input, as well as invert then amplify the negitive signal this possible??
  4. SgtWookie


    Jul 17, 2007
    You can use a precision full wave rectifier circuit to keep the output positive.
    You would use your setpoint voltage as the noninverting input.
    As far as opamps to use, a TL072 would not be suitable for your application. An LM124 or LM224 might be used. An LM324 is not rated for automotive temp ranges.

    Rod Elliott of Elliott Sound Products has posted a number of precision rectifier circuits on his page: