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555 astable timer

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byron

New Member
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

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

sam_h

New Member
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

New Member
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

New Member
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

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