Hi,
does anybody have a small circuit that would change the state of a 4013 flip flop at power on?

I need to supply +5v (from a 5v rail) for about half a second (after which it turns off) at initial power on to reset the state of the toggle at first power on.

I would appreciate any help or advice!

 

Connect a 10uf capacitor from 5V+ to the reset pin, and a 47k resistor from the reset pin to gnd.

 

wont that hold it high indefinitely ?

 

Initially with no power applied the capacitor will be discharged at zero volts.
When 5 vdc is first applied, the capacitor will pull up the reset to 5 volts for a short period of time, then charge to zero vplts on the reset pin through the 47k resistor.

 

Better put another resistor across the cap in case you want this to work the second time you use it.

With CMOS you need to make a path for the cap charge to go.

There are also chips that do this for you, Microchip among many others make these. The one I used was a SOT, but I believe they have some leaded styles too.

 

Better put another resistor across the cap in case you want this to work the second time you use it.

With CMOS you need to make a path for the cap charge to go.

.....................

Not necessarily. When the power goes to zero, the capacitor will discharge through the resistor and also through the CMOS input protection diodes which go to both ground and the V+.

 

Not necessarily. When the power goes to zero, the capacitor will discharge through the resistor and also through the CMOS input protection diodes which go to both ground and the V+.

OK, point taken... can you predict how much current will flow thru the protection diode?

I can't... so I'd put a resistor in series with the pin.

On second thought... maybe not...

 

Thanks guys, such a simple but elegant solution!