Hi All,

I'm looking for some help.
I need to trigger a 555 timer with a positive pulse that will then trigger a relay for 20 minutes.
A SCH would be greatly received.

 

A 555 is problematic for such a long period due to the high resistances required and the leakage of the capacitors.
Better to use a CD4060 oscillator/counter circuit such as this, which doesn't require a long time-constant circuit to provide the 20 minutes.

 

Hi All,
I need to trigger a 555 timer with a positive pulse that will then trigger a relay for 20 minutes.
A SCH would be greatly received.

Using a cmos 555 like the LMC555.
C2 should be a low leakage type for best results.

 

I have a question about the circuit in post #2.

Would it not have to complete at least one cycle before being used as a one shot from the reset pin?

 

I have a question about the circuit in post #2.

Would it not have to complete at least one cycle before being used as a one shot from the reset pin?

When power is first applied, yes.

 

C2 should be a low leakage type for best results.

An electrolytic wouldn't work because of their high leakage, so that would mean a physically large film capacitor.

 

I would suggest a tantalum type.

 

I need to trigger a 555 timer

There are other timer chips better suited for this application. Does it have to be a 555?

ak