2

I am struggling with this problem: I have a fan whose output tacho signal is a square wave when it is running properly.

If the fan stops the signal remains at its last value (either high or low). I need to a simple circuit to light up a red LED if the fan stops and a green one if is running properly.

Do you have any suggestion? I thought about using a monostable IC but I am quite unexperienced.

ocrdu
  • 8,705
  • 21
  • 30
  • 42
amsi
  • 41
  • 2
  • 1
    Welcome to Electrical Engineering Stack Exchange. What is the frequency of the square wave when the fan is running? You may be able to put the signal through a simple RC differentiator (or a high pass filter), (possibly) rectify it, and use the result to drive the LED. Or you might use a 555 timer in a mode where frequent positive (or negative) edges of your signal keep the output on. – Math Keeps Me Busy Dec 16 '20 at 14:30
  • Thank you, it is 180Hz approximately – amsi Dec 16 '20 at 15:55
  • What voltages/currents do you have available for your circuit? What is the voltage of the square wave? Does it matter what size LED's are used? – Math Keeps Me Busy Dec 16 '20 at 16:01
  • Another question. You write "I am struggling with this problem: ". Is this a homework assignment? If so, you need to show what efforts you have already taken to solve the problem. – Math Keeps Me Busy Dec 16 '20 at 16:03
  • I only have a 24V supply voltage and no it is not a homework assignment. The output tacho signal amplitude can be set by using a pull-up resistor. I could not figure out how to easily generate the 2 signal. In the end I did it with two voltage comparators and some logic gates. – amsi Dec 18 '20 at 11:26

1 Answers1

2

This will keep the MOSFET M1 conducting (and thus M2 off) as long as regular pulses are coming. If the pulsetrain stops high or low, the FET M1 will shut off, which will turn on M2.

Depending on the amount of current the TACH signal can sink, one could also possibly omit M1. But then diodes would have to be re-arranged.

schematic

simulate this circuit – Schematic created using CircuitLab

tobalt
  • 18,646
  • 16
  • 73