frequency in a project

[brianmay27]

so i posted a while back about a project which used a 555 switch... i finished it thanks to the help of **broken link removed**

however im useing a 10 :mu:F capasitor and both resisters are 2.7K hm:so that should be a .03 fire and a .01 down. however im looking at it through my ossiliscope and i see it being as a 30 mill sec fire and a 50 mill sec down time. but when i use it for what im doing it seems much faster then that.

can someone help me out. i dont know what it is. my ossiliscope is working and it is calabrated but i dont know whats going on.

thanks

 

[kchriste]

Post a schematic.

 

[brianmay27]

im useing the same one as in the link

 

[kchriste]

This is where the link took me:
**broken link removed**
There are LOTs of other circuits on the page and I'm sure the one above is not it. Do you mean this one:
**broken link removed**

 

[brianmay27]

oh sorry, i clicked the link i thought it would bring you to it...

its the astable timer

 

[kchriste]

Well, if you make C1 = 10uF, R1 = 2700hm: and R2 = 1500hm: then you'll get pretty close to 30ms high and 10ms low out of pin 3 of the 555 using the astable circuit I posted above. Also put a capacitor across the power supply pins of the 555. 0.1uf should do it.
Remember that electrolytics have very loose tolerances (In the -20 to +100% range) so that might be why your timing is a bit off. To get something more exact, you can use this circuit which allows you to control the ON and OFF times independently: