Need a simple pulse activated circuit.

[illusive]

Hello everyone!
I need a simple pulse activated circuit. I have set up an op amp comparator which output goes high just for a split second. I need a circuit that will hold that high output level for some time (minute or more). I thing something with a 555 timer will do the job, but i'm not very familiar with that IC so i can't choose the right circuit.
How about this circuit ?

[Stray Electron]

As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor. I saw a circuit like this once that was used to control a Mosfet gate and the cap would hold it on for several minutes and I think they were using a .1 uF cap or something in that range.  The most important part is that the diode have a very low reverse current flow.

[Kappes Buur]

http://home.cogeco.ca/~rpaisley4/LM555.html

Maybe 4. Basic Circuits For The LM555 Timer, circuit 3

[mikerj]

As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor.

You want a LOW output impedance to do this.  If you had high output impedance, a short pulse may not be able to charge a cap of reasonable value in time.

A monostable such as a 555, 4047, 4098 etc. would be a better idea.

[tautech]

As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor.

You want a LOW output impedance to do this.  If you had high output impedance, a short pulse may not be able to charge a cap of reasonable value in time.

A monostable such as a 555, 4047, 4098 etc. would be a better idea.

+1

If you don't know the 555 you need to.
Use it.

If low power consumption is required use the CMOS version
Plenty of calculators online for appropriate values to suit your needs.

http://www.csgnetwork.com/ne555timer1calc.html

[Stray Electron]

As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor.

You want a LOW output impedance to do this.  If you had high output impedance, a short pulse may not be able to charge a cap of reasonable value in time.

A monostable such as a 555, 4047, 4098 etc. would be a better idea.

    You need Low input impedance to charge the cap but high output impedance to prevent draining it too soon.

   I agree about the 555 probably being a better choice but the OP wanted a simple circuit. 

   And the other poster was right about getting to know the 555. It's one of the most useful and versatile ICs ever made! 'The IC Timer Cookbook' is a good place to start.  I don't know if it's still in print or not but it should be.

[michaeliv]

If you need the output to remain high for at least a period of time (you don't care if it turns off after a certain period) then you can just use a latching circuit.
If you do need the output to go back low after a period of time, using a 555 might not be ideal for your scenario as it does not play very well with long time periods.
You can use the op-amp to turn on a MOSFET that powers a circuit like this : https://www.elprocus.com/wp-content/uploads/2013/04/CD-4060-TIMER.png
And you can have output Q12 reset the power to the timing circuit ( turn it off ).

[mikerj]

As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor.

You want a LOW output impedance to do this.  If you had high output impedance, a short pulse may not be able to charge a cap of reasonable value in time.

A monostable such as a 555, 4047, 4098 etc. would be a better idea.

    You need Low input impedance to charge the cap but high output impedance to prevent draining it too soon.

Other way around.  The output is charging the cap, therefore a low impedance is needed.  A buffer following the cap would require a high input impedance to prevent discharging the cap too quickly.