Keeping reset pin low for a while during startup?

[fubar.gr]

I implemented the 555 monostable circuit shown here, using a Intersil ICM7555 SMD CMOS chip:



It works as intended, save for a small inconvenience. When the circuit is first powered up, the output is turned on for the predetermined time interval, as if the pushbutton was pressed.

I believe the problem is that the voltage on Threshold pin takes some time to rise due to the pullup resistor (R2). I fed 12 Volts directly to the Threshlod pin, bypassing the pullup and the problem went away.

One possible solution I came up with, is to keep the Reset pin low for a while during startup, by connecting a resistor and capacitor in this fashion:




Will this work?

[danadak]

A page of applications, and one that shows another approach.

http://home.cogeco.ca/~rpaisley4/LM555.html#10

"Power-Up Reset For 555 Timers" section on page.


Regards, Dana.

[rob77]

you need to add a way of discharging the reset capacitor, otherwise it won't work in scenarios when you power off and shortly after power on the circuit.
if you have a look at the page form the post above posted by Dana - in the first schematic the D1 diode is providing the needed discharging.