Low Pass Filter design

Hey vin vino, Welcome to EP!

Is this a strictly analog circuit? It would be best if you posted a schematic AND if the schematic does not clearly illustrate the purpose of the circuit, then you will likely need to give some details about the circuit's purpose. From the very few details you have provided, perhaps an OP-Amp configured as an inverting amplifier? or if it is a digital circuit, perhaps a simple logic inverter like a 74x04 or a schmitt trigger version like 74x14? But w/o more details these are just shots in the dark....

Fish

 

Is your input signal just a single frequency? In that case, a filter probably isn't the best option. One option is a frequency-to-voltage converter and a comparator with an adjustable threshold. Another option is a circuit using one or two monostables. Either of these can give a fairly accurate detection threshold.

 

The LM2917 looks like a really good option. It has a frequency-to-voltage converter and an uncommitted op-amp/comparator driving a transistor, which can be used as a threshold detector.

There's a circuit to do exactly that in the data sheet for the LM2907/2917 at http://www.ti.com/lit/ds/symlink/lm2907-n.pdf. Look for the "Speed switch" circuit on page 9.

You can use either the 2907 or the 2917; the 2917 is smaller and has all the features you need, so I'll assume you will use that one.

You will need to drive the input (pin 1) from your Hall sensor, through a DC blocking capacitor (between the Hall sensor signal and pin 1) with a resistor of around 100k from pin 1 to 0V.

There's information in the data sheet to help you calculate the timing resistor and capacitors on pins 2 and 3, and the speed threshold is set by the voltage on pin 7; you can replace the two fixed resistors in the diagram with a potentiometer or preset potentiometer (trimpot) to make the threshold adjustable.

The only weakness of that circuit is that there is no hysteresis, which means that the output may witter around the switching point.

If you're using this circuit to disable the power to whatever is creating the rotation that activates the Hall sensor, and you want the rotation to stop once overspeed is detected, you will need a latch, which is activated when the LM2917 output goes active, to kill the power, and can only be deactivated manually. Without the latch, when the LM2917 detects overspeed and cuts the power, the rotation will slow down, so the LM2917 will detect that the speed is OK, and reconnect power. This will cause rapid cycling of the power, as the speed varies from below the overspeed limit to above the overspeed limit.

Any questions feel free to ask.