555 astable timer?

Question

hey all! i'm trying to make a simple 555 timer in astable mode.

i want it to be on for 1/2 sec. and off for about 8 secs. i want to run it on 9v. all it will do is blink an LED.

i've seen the formulas and calculators and i will try to figure it out using them. but untill then, can anyone point me in the right direction as far as the 2 resistor and capacitor values?

thanx a million farads!!!

Answer 1

If you have the formulas, why are you asking this question?

I will mention that at long delays, make the Rs as big as possible, as this will keep the Cs down to a reasonable size.

Answer 2

Take a look at this:

http://en.wikipedia.org/wiki/555_timer_I...

Okay you have the formulas. now suppose you have a 25 microF cap.

low time is

low = 0.693 * R2 * C

0.5 sec = 0.693(25*10^-6) * R1

R1 = about 30 kiloohms

high = 0.693 \cdot (R1 + R2) \cdot C

8 sec= 0.693(25*10^-6) * (30,000 + R2)

R2 = about 430 kiloohms.

This give you duty cycle you want but it's inverted. We deal with this by grounding the load to the positive side of the power supply, or with an inverter. The inverter could be a switching transistor in the common emitter configuration or a relay.