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555 timer question

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rayclark

New Member
Hello,

I have built an astable timer using the 555 chip. It is set up to be on for 70 seconds and then off for 2 seconds.

The problem is that when I first power the circuit up the on time is about 90 - 100 seconds (20 - 30 seconds longer than it's suppose to be). Once it has completed the first cycle it operates as it should with an on time of 70 seconds.

What can I do to get it to be correct when power is first applied?

Any info would be greatly appreciated.

Thanks

RC
 

kinjalgp

Active Member
This happens because the time you have calculated (70s) is the time taken by capacitor to reach from 1/3 Vcc to 2/3 Vcc. When you first power on the system, there is no voltage across the capacitor. That means it has to charge from 0V to 2/3 Vcc with the same time constant during the first cycle. And thats why it take more than 70s.

To avoid this, you should set the initial conditions on the capacitor i.e. before starting, the capacitor should have voltage just below 1/3 Vcc .
 

Roff

Well-Known Member
The problem is that the cap starts with zero volts across it, and thereafter oscillates between 1/3 VCC and 2/3 VCC. You need the cap to start out with 1/3 VCC on it. The circuit below should get you very close.
 

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rayclark

New Member
Thank you for the help. It helped alot . The difference is down to about 6 seconds. Ultimately it would be great to get it down to a second or two but maybe I can't do that without a different type of circuit.

All I need is a buzzer to sound every 70 seconds for about 2 seconds and keep repeating the cycle until I turn off the power. It's important for the time not to vary more than a couple of seconds.

I figured the 555 would be good because it's simple and cheap. Any other suggestions for a circuit. Should I consider using a counter IC?

Thanks again for the info.

RC
 

Roff

Well-Known Member
Here's a version with an adjustment (RV1) for the first cycle. If it doesn't have enough range, change the pot to 10k and the 7.5k resistor to 4.7k.
 

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Screech

New Member
A smaller cap takes a shorter time to charge. Yeah?

Cant you simply use a smaller cap(eg.100 times smaller) and higher resistance resitors (eg.100 times higher)?

Please correct me if I'm wrong.
 

Roff

Well-Known Member
Screech said:
A smaller cap takes a shorter time to charge. Yeah?

Cant you simply use a smaller cap(eg.100 times smaller) and higher resistance resitors (eg.100 times higher)?

Please correct me if I'm wrong.
Go back and read the second and third posts in this topic. Below is a plot of the cap voltage for a 555 astable multivibrator, illustrating the problem with the first cycle.
 

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