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 Bigger Bigger  Smaller Smaller  Reset Reset

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!
« Last Edit: September 03, 2011, 10:50:23 am by pedrotome » Logged

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 Bigger Bigger  Smaller Smaller  Reset Reset

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.
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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.
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 Bigger Bigger  Smaller Smaller  Reset Reset

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
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 Bigger Bigger  Smaller Smaller  Reset Reset

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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