Every second pulse inhibit circuit

Hi, I have this little circuit. This uses a 3-pin computer fan, which shorts one of the motor pins to ground by the internal hal sensor.
After shorting, the capacitor is discharged and when the sensor is off again, the resistor charges the capacitor up to a point.

The problem is that these motors short (pulse) twice per revolution. I need a circuit that prohibits the second short per revolution, but not the first.
Then the cycle repeats again. Allow the fist pulse but prohibit the second on the next revolution and so on.
*It can be the other way round i.e. inhibit the first pulse and allow the second. I do not mind, as long as there is one pulse per revolution.

Is there any simple (I prefer discrete components, unless circuit is very big) circuit that can do this?

A simple flip-flop would do the trick using the pulses as the clock. (I consider things like the 74xx series ICs as 'discrete' components these days.)
Susan

If the fan speed is constant, use a monostable, otherwise use a two transistor bistable (like a multivibrator with crossed resistors instead of capacitors), it will divide by two.
Note that if the Hall sensor really grounds its output pin, you will probably destroy it if the resistance to 5V is set close to zero.

Brian.

betwixt said:

If the fan speed is constant, use a monostable, otherwise use a two transistor bistable (like a multivibrator with crossed resistors instead of capacitors), it will divide by two.
Note that if the Hall sensor really grounds its output pin, you will probably destroy it if the resistance to 5V is set close to zero.

Brian.

Click to expand...

Note that this 5 to zero short pulse is not exactly what we would call a square wave. Will this circuit apply to short falling pulses as well?

Regarding the hal sensor, I thought that, but I tried it with several motors and it does not get destroyed. Perhaps it is not shorted to ground really all the way, but has an internal resistor.

A bistable will respond to very short pusles at its input. Carefully crafted bistables will toggle in the 10's of GHz region!
Depending on design, they can be made to trigger on falling or rising edges both not normally both. If all you are doing is counting every second pulse it doesn't matter which edge it triggers on. Note that a bistable will produce a square wave output, changing state at every input pulse. If you need to restore the original pulse length you can gate the input with the output. As the output changes state on every second input pulse, only alternate pulses will pass through the gate.

Brian.