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Topic: Resetting a 555 Astable circuit (Read 613 times) previous topic - next topic

pedrotome

Sep 03, 2011, 05:47 pm Last Edit: Sep 03, 2011, 05:50 pm by pedrotome Reason: 1
Greetings.
I have a simple 555 astable circuit with a 16 seconds Mark (HIGH) and a 0.3 seconds Space (LOW): Pic.

I want to reset the circuit every time I feel like resetting it. This means that every time I press a button or send an electric signal, I want the 16 second HIGH countdown to go back to 0 and start again.
This means that if the output has been HIGH for 10 seconds and I press the button, the output will be LOW in 16 seconds and, when that happens, I can say that the output was HIGH for 26 seconds.

According to the first image in this page, the way this can be done is by fully discharging the timing capacitor (C1 in that page's image and C2 in the image I linked to above).
Using the Reset pin makes the output go LOW, which is definitely not what I want.


So here is my question: how do I discharge that capacitor?
Will this work (the point is: I send a 5V signal from an Arduino, or something else)?
I am effectively short-circuiting the capacitor - is it... well... bad?


Thank you!

CrossRoads

Nope, not bad to discharge the cap.
You might try just controlling pin 5 instead.
Internally it sits at 2/3 of Vcc using a resistor/voltage divider.
Try pulsing it hi, or low, you may not need the external transister.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

pedrotome

I don't see how that could work, CrossRoads.
As a beginner, I'm too narrow-minded, so I'll follow the advice of a member of another Forum: add a 100ohm resistor in series with the transistor to protect it from the capacitor and call it a day.

Thank you, CrossRoads.

floresta

Can't you just implement the 'reset' pin, pin 4?  Pull it high with a 10K resistor and use an Arduino pin to bring it low when desired.

Don

pedrotome


Can't you just implement the 'reset' pin, pin 4?  Pull it high with a 10K resistor and use an Arduino pin to bring it low when desired.

Don


Quote from: OP
Using the Reset pin makes the output go LOW, which is definitely not what I want.

The problem's solved. I just added a 1K resistor in series with the transistor's base (last link in the OP) to manage the current on the collector.

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