Trigger Circuit Help

[Suraj143]

I'm getting a relay contact from a PIR sensor (12V).When triggered its relay will close for 6 seconds.This time is too much.
What I need is another circuit with a relay which the relay operates only for 2seconds when the PIR activates.How to do that?Any descrete method?

 

[Nigel Goodwin]

555 timer.

 

[Suraj143]

The problem is in NE555 monostable mode, if the trigger pulse is longer than the output pulse, then the output it will never turn off

I have 6 seconds trigger pulse, but need only 2sec output.

 

[Grossel]

You could always use a schmitt-trigger and a logic gate to make sure the pulse length is always short.

 

[Nigel Goodwin]

The problem is in NE555 monostable mode, if the trigger pulse is longer than the output pulse, then the output it will never turn off

I have 6 seconds trigger pulse, but need only 2sec output.

You can persuade 555's to do it, with a little extra circuitry - or even better program an 8 pin PIC to do the job, it's a pretty simple task - you can make it do exactly what you want then.

 

[dr pepper]

Put a 0.1uf cap in series with the 555's pin 2 trigger input, then a 22k resistor from pin 2 to pin 8 (assuming the supply is around 12v).
Then the timer will receive a short trigger pulse & you can shorten the period of the 555 to less that 6 seconds.

 

[AnalogKid]

The problem is in NE555 monostable mode, if the trigger pulse is longer than the output pulse, then the output it will never turn off.

As above, adding one small capacitor to the standard monostable circuit will make it edge-triggered. Once the Trigger input is pulled low through a capacitor, the 555 will time out even if the Trigger signal source still is low.

ak

 

[gophert]

The problem is in NE555 monostable mode, if the trigger pulse is longer than the output pulse, then the output it will never turn off

I have 6 seconds trigger pulse, but need only 2sec output.

then use a super-simple cd4047. You can select to configure inputs as retriggerable or not, mono stable or astable, rising edge or falling edge trigger. needs one resistor and one cap. Supper low current in off-state.

it cannot directly drive a relay like the 555 so you'llneed a pass transistor to handle the coil current. Or, justlet the cd4047 drive a beefy MOSFET to drive your device and forget the relay (if your device is 12VDC).