I have a design challenge that I have been thinking about.
Is there a passive and inexpensive system that I can get a square wave (frequency not decided yet, but less than 8MHz) that will output a 5V DC signal? If the input changes to a fixed voltage the output must go low.
My solution was to DC null the square wave with a decoupling cap and then smooth out the gaps in the remaining square wave with a passive low pass filter. Essentially take advantage of the capacitor to GND and allow it to charge and discharge at a frequency that will result in a DC signal.
The problem with this design that I can see is that this is wildly inefficient as the DC null will half the power and the passive LPF will also result in a loss of power.
Is there a way I could use transistors to active what I want? (I am trying to stay away from IC's)
Thank you for your help.
This is my simulation response with a slow frequency of 2kHz.