how to use square wave to switch ON a device

[prash]

I want to use square wave(varying duty cycle) as input signal to switch ON a device.
When the stream of pulses stops, the circuit should give an output(high).But not(low) if the input is constantly high or low.
In other words i want to detect rising and falling edges.
prash.

 

[Externet]

And when the stream of pulses stop, what status the input remains in ?
And what time after pulses stop you want the circuit to give an output high ?

 

[microtexan]

missing pulse detector

How about this circuit:


Figure 1. Missing Pulse Detector Circuit Diagram

Missing Pulse Detector

 

[prash]

in put will be constantly high or low when pulse train stop.output should go high immediately when pulses stop.
missing pulse detector is nearly OK but it does not serve when pulses stop and input remains constantly at high status.

 

[Boncuk]

I have the strong impression that you expect others to do the thinking for you.

You even haven't posted the frequency you want to monitor.

A 74L123 is a monoflop which can be used as missing pulse detector. This chip has two outputs which you can use for your convienience.

The attached circuit monitors any frequency starting at 1Hz. It times out approximately 1 second after the last pulse received and changes output states accordingly.

The "truth table" is contained in the schematic.

If you want the circuit to switch immediately please do the math on your own.

Boncuk

 

[prash]

I have the strong impression that you expect others to do the thinking for you.
Boncuk
I regret if i put such an impression.In fact i know not much in digital (analogue is my field).I had mentioned "varying duty cycle"which i mean is varying frequency(??).
Any way, it seems you suggested the right thing.let me try it for my purpose and come back to you to thank again.
prash.

 

[Boncuk]

I had mentioned "varying duty cycle"which i mean is varying frequency(??).

prash.

No, prash,

varying duty cycle doesn't mean varying freqency. The oscillator's clock cycle doesn't change with varying duty cycle, hence the pulse rise time is always the same which is important for the missing pulse detector's proper function, since its timing circuit is triggered by the positive going pulse edge.

Boncuk

 

[prash]

thanks

thanks Boncuk for correcting me.now i realize varying duty cycle does not mean varying freq.
Prash.

 

[research.chetan]

dear sir ,
i want to reset the micro controller by hardware.i am generating square wave of 1 Hz by at mega 128 by one pin.which is a heartbeat of the controller. as whenever the LED connected to this pin will blink it means that the code is running.but with this circuit can i detect the presence of square wave and in case if software hangs it will stop generating square wave or will hang in high or low output status.can i use this circuit to detect the same and enable shutdown pin of the power supply LDO to shut down and restart the controller?

 

[colin55]

I want to use square wave(varying duty cycle) as input signal to switch ON a device.

In simple terms, put a diode on the line and an electrolytic with a high vlaue resistor across it.
You can take the output to a transistor to get inversion.

 

[SABorn]

Colin55, i first thought that too but the square wave can stop in either high or low state and if high it wont be detected to change the output to low.

 

[colin55]

use this:

 

[research.chetan]

using microprocessor supervisory circuits

dear friends ,
after so much effort i came to know about this solution for hardware reset of the micro controller.
go for tps3820,ds1232. use these chips to get three way reset controll.
1.manual reset
2.brownout detection
3.watchdog timer
in one package.

 

[research.chetan]

hi prash,

don't make complicated circuits as we cannot depend on them.use chips which are made for the same. tps3820 or ds1232.