A circuit that converts contiuous pulse signal to a stable level signal, and reverse if pulse not present
&quot;Simple solutions&quot; probably require some active devices (and a bias supply): some might regard this as &quot;not simple&quot;.
One possible solution requires a transistor. The input signal comes from the tacho wire in the fan, that is either shorted to ground, or is left open-circuit.
The coupling capacitor C1 allows the &quot;stall&quot; condition to be either a short-to-ground or an open-circuit. The transistor keeps discharging C2 while the fan rotates. When it stops (in either short-circuit or open state), C2 charges to the DC supply voltage through R3:



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Potential &quot;gotchas&quot;:

During power-up, you may have a fan:fault condition at the output until thing stabilize.

A fan is most often used to cool things. The delay until the fault condition where C2 to charges up toward 5V might be acceptable.

Output voltage here assumes a very light loading, such as a CMOS logic gate input.

A +5V DC supply is needed (or perhaps a +3.3V supply). Not +12V



A +12V DC supply requires the fan pull-up resistor to be divided into two resistors, else Q1's base-emitter junction is at risk of breakdown. If output signal needs to be active low when faulted, a MOSfet inverter works nicely. Might as well use the same type MOSfet for Q1 too (circuit at right):



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