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How to make push button switch with auto off after about 5 seconds. Output circuit is high (active) for max 5 seconds when I keep pressing push button. Output will be low after this time or when I release push button. To reactivate, I have to release push button and press again. I think that it's using a capacitor charging with some transistors. But how to make it work?

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  • \$\begingroup\$ Monostable circuit (555 or transistor version). \$\endgroup\$
    – user208862
    Mar 18, 2021 at 5:27
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    \$\begingroup\$ Btw, if you choose a transistor version a good approach is to trigger it with RC differentiator connected to signal diode to transistor base to avoid holding output high while pressing button too long. \$\endgroup\$
    – user208862
    Mar 18, 2021 at 5:56
  • \$\begingroup\$ Yes I looking for transistor version. And as simple as possible. Can you show your schematic in answer? \$\endgroup\$
    – Hida
    Mar 18, 2021 at 6:23
  • \$\begingroup\$ It can be done with transistors BUT it is much easier to use eg a single IC such as a 74xx14 / CD 40206 / .... hex Schmitt trigger. I can provide a circuit IF it would be of interest. \$\endgroup\$
    – Russell McMahon
    Mar 18, 2021 at 6:56
  • \$\begingroup\$ The tricky part is to have the count starting when you push the button instead of when you release it. A normal monostable would lengthen the pulse but this can also shorten it. The trick is AC coupling the button so it only gives a pulse. In IC form the 4047 is the historical implementation. Doing it with transistor is feasible but timing tend to be inconsistent unless you use a somewhat complex circuit (the problem historically solved by the 555 which is sold as a 'precision' timer) \$\endgroup\$ Mar 18, 2021 at 7:21

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This is the basic idea (functional), but it may need to be tuned like R5/C2 time constant, switching spikes on transistors (add little capacity to slow it), width of trigger pulse, etc...

Edit: You get better result with R3,R8=1k , and C2= 68uF (almost exact 5sec)

enter image description here

enter image description here

(green is button press, blue is output voltage)

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  • \$\begingroup\$ The timing looks good, although fall time not so quick. But seem that at the next button pressing we have to wait button open for a moment to let C2 fully charged before pressing again. Without C2 fully charged, on time will reduced. CMIW. \$\endgroup\$
    – Hida
    Mar 18, 2021 at 12:25
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    \$\begingroup\$ You are right, all you can to speed up charging is to change R3 and R8 to 1k as I mention in Edit or use smaller C2. \$\endgroup\$
    – user208862
    Mar 18, 2021 at 12:29
  • \$\begingroup\$ @reverse diode across R6 should improve retrigger time. \$\endgroup\$
    – Russell McMahon
    Mar 19, 2021 at 20:57

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