555 push pbutton

[Donald Moreau]

Hi everyone,

I look for a schematic for a button that I use with a 555. This button must doesn't stay turn on if I hold it, for my 555 turn off the led or relay or anything, by itself. I use a monostable circuit and now when I hold the push button the led stay on even time is pass.
Thanks,

 

[HarryA1]

Try putting a resistor and capacitor in parallel after the switch. The capacitor will pull pin 2 down until it charges through the upper  10k resistor. The resistor in parallel   with the capacitor will discharge the capacitor during the period the switch is open.

Do you need help with the calculations? As a quick test make the new resistor 50k and the capacitor perhaps 0.1ufd - I am just guessing. You want the capacitor to discharge before the output ends.

 

[HarryA1]

In the photo the yellow trace is pin 2 (trigger) and the blue trace is pin 3 (output). Starting on the left pin 2 is at the Vcc voltage of 12 volts (0 is at the bottom of the screen) . Pin 3 is at 0 volts. On switch closure pin 2 drops to  near 0. And pin 3 rises to near 12 volts. The switch is held on until the later part of the trace on the right most side of the screen.

 

[Donald Moreau]

Hi Harry,

 I put a 56k resistor (that's what I have) and the capacitor 0.1ufd; and it's work great. I'm very happy.

Please give me how to calculate resistor and capacitor. 

I appreciate help like yours it is something to make my life happier.

 

[HarryA1]

I am lazy and use one of the numerous online calculators like:

http://referencedesigner.com/rfcal/cal_05.php

 

[Donald Moreau]

Harry, I thought you talked of the capacitor and resistor to put on the button like you told, instead to make a guess with a 50k and a 0.1ufd.

For the time I was OK to calculate it. But I would like to test it, but I don't know how !!!!!

 

[HarryA1]

What you want to know is will the capacitor charge to above 1/3 of Vcc within the period of the 555 pulse; thus it will not get re-fired.

Using:



You get 405 microseconds,  See:

http://mustcalculate.com/electronics/capacitorchargeanddischarge.php

And will discharge to 4 volts from 12 volts through the 50k resistor in 5.6milliseconds.

 

[Donald Moreau]

Harry, I don't understand what is : "From voltage, to voltage," if you can do the example with the example that you gave me:

Do you need help with the calculations? As a quick test make the new resistor 50k and the capacitor perhaps 0.1ufd - I am just guessing. You want the capacitor to discharge before the output ends.

How the capacitor and resistor is chosen?

Thanks

P.S. My electronic classes are far, 40 years.

 

[HarryA1]

Two things from your circuit are the 20k ohm potentiometer and the 10k resistor at pin 2.

The potentiometer going from 0 to 20k ohms can produce a wide range of outputs in time/widths. So the RC circuit must charge above 4 volts fairly fast to accommodate the shortest output and while charging through the 10k resistor. A 1.0 ufd would require 20 ms, perhaps to slow. So trying a 0.1 ufd gives 400 microsecond which would seems fast enough(?).

To discharge the capacitor before the next switch closure the  parallel resistor must set the voltage at pin 2 above the required 4 volts while the switch is closed. A 50k sets the the voltage at: 50K/(50k+10K) * 12volts or 10 volts a bit higher then necessary. A 10k resistor would set the voltage at 10k/(10k+10K) * 12 volts or 6 volts which would be adequate. Discharging from  6 volts to 4 volts in 2 ms after the switch is opened.