frequency limiter fault

[charon1985]

Hi, i need a schematic or and idea how to make a circuit that will react if the frequency of the input signal is below a certain value. I have a motor that runs up to 40 000 rpm, it has a hall sensor that outputs a square signal 50% on 50% off only the frequency varies... if the motor is under high load the rpm will decrease. I want to have a circuit that will monitor the frequency and output an error signal if the frequency is under a set value. I have searched on internet but did not find anything. The problem is that i dont know what to search for.
I would prefer not to use uC.

Thank you. 

[ebclr]

With this

http://www.bittnar.info/en/tachometer-rcexl-alternative-firmware/

And a very simple trick on firmware you can make what you want

[charon1985]

I forgot to mention that i need a reaction speed of 2-3 cycles from the given frequency.

[ebclr]

CD 40106 and a bunch of diodes / resistor / capacitor will do what you want

[Ian.M]

Use a retriggerable monostable to stretch the tacho signal to the point that it stays high continuously if its above the limiting speed.   Use the same tacho signal to clock the monostable output into a D type flipflop so that you get a continuous level updated once per tacho signal cycle rather than a narrow low going pulse .

[Assafl]

I know you said no uC but nowadays I'd probably use an ATTiny on a breakout board (e.g. like an Adafruit Trinket). It is far too easy to use - to not use one The timers on it and hardware interrupt handler make it especially useful for this TINY circuit.... With the Arduino bootloader it could really be 10-15 lines of code or so... (use attachInterrupt on the input pin; the interrupt handler sets a volatile variable to millis(); and the main loop just compares the current millis() to the previously stored millis() - if it is too long - the output goes low... ).

It is probably cheaper too....

Were it 1990's again, I'd probably use a CMOS counter like a 4060 driven by a xtal and some jumpers (or DIP switch) to set where the minimum rate was (obviously the hall effect sensor would reset the counter every rotational cycle and the purpose would to NOT allow the counter to reach a certain value which would trigger an error latch). Would obviously need an AND gate or diodes to sense when the count was up.

Obviously the same circuit with a variable frequency source would be simpler, with the latch connected to one of the bit outputs. But also less "programmable" and accurate in nature.