555 astable timer

[byron]

I built a simple astable timer, but since I'm a novice at electronics and am just starting out, I have a problem. I am trying to have a timer to turn something on for 5-10 seconds and then turn off for about 20-30 seconds. To test this out I put an LED on pin 3 (output), but the LED is flashing at the same rate about every 1/2 second. I have a 2.2M OHM resistor between pins 7-8. I have a 1M OHM resistor between pins 7-6. The Cap. is 10 uF. Am I correct that the Output High is the time that the circuit is off and that the Output Low is when the circuit should switch on. This calculated if I am correct to be 22 seconds off time and almost 7 seconds on time. Why is my LED just blinking on/off the way it is?

 

[nettron1000]

Heres a tutorial on the 555 Timer that will tell you everything you need to know, including all calculations.

 

[sam_h]

The 555 has a frequency output from pin 3 when setup for astable purposes, so the time the out put is on is the same amount of time the output is 'off'.

 

[nettron1000]

Sorry Sam but thats not right. You should take a look at that link as well.

Byron, i just bread boarded your 555 timer circuit and it works just fine. The LED is 'off' for about 22 seconds and 'on' for about 7 seconds just like you wanted. You either have something connected wrong or you are reading your component values wrong.

 

[nettron1000]

Follow-up:

I have my LED connected in series with a 220 ohm resistor which is connected to ground (-V), this will tell you that the output at pin 3 is 'high' when the LED is 'on' and 'low' when the LED is 'off'.

If you have them connected to the positive supply you will get the opposite effect.

 

[sam_h]

Sorry for the mis-information, I'll have to have wods with my electronics tutor.