I have a problem with the 555 delay before turned-off. The problem is that I want the time cycle to start after the switch is released if it was held for a period of time.


I found this circuit at "learningaboutelectronics" and I got 11 seconds delay with 100UF capacitor and 100KΩ resistor, but the problem is if I held the push button "Momentary switch" for more than 11 seconds there will be no delay before turned-off because the time cycle begins immediately when I push the switch.

My question: Is there a way to make the time cycle starts after I release the switch when it was held for a period of time?

Because I'm planning to make a homemade intercom, the delay will be useful when I hold the switch to talk on the intercom" For a period of time the switch will be held to activate the audio amplifier chip" and then when I release the switch the camera and the monitor both will be working for 11 seconds to be sure that the person who I was talking to is left or still there.

  • \$\begingroup\$ I would need to think about this more but I have an off-the-hat solution that would work. For the 555 monostable circuit, it starts when the trigger is low. Since your switch probably goes low when pressing, this is your issue. There should be a way to devise it so that the trigger pin is always high, when you press the switch, it charges a RC combination with a time constant such that it takes 11s for the voltage to go from 5V to 1/3Vcc (I believe). Downside to this is that a fairly large RC will probably be needed (hundreds of KOhm and hundreds of uF). \$\endgroup\$ – Addison Apr 17 '18 at 19:47
  • \$\begingroup\$ I think I found a way I will try it, I hope it will work. i.imgur.com/j0sZsWc.png I will use a JFET or a circuit I found here in this forum which will make a transistor to act as a JFET N-Channel. If the switch is held the transistor will turn-off the 555 or a LED or anything and when the switch is released the transistor will power-on the 555 and monostable will happen for 11 seconds and then turn off, and if the switch is held again the circle will happen again. \$\endgroup\$ – James Kan Apr 17 '18 at 20:39
  • \$\begingroup\$ I've done this with a PNP. m8051.blogspot.com/2013/02/… \$\endgroup\$ – Zekhariah Apr 18 '18 at 3:08

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