TL;DR: I need to replace the button S1 by a circuit/component that could close the circuit which the button is closing by applying a 5V signal (with 0.5s duration) in the new circuit/component.

Well, I started designing some circuits without microcontrollers, so I could study and learn the functionalities of some components. In this circuit I tried to implement a toggle effect in the 555 OUT pin, I mean: when I press the button the OUT goes high, when I press it again it goes low and so on.

The circuit works with the physical button, but I need to toggle the 555 OUT with a 5 volt signal with duration of 0.5s.

I tried to figure out a solution but it's terribly expensive and complex in my opnion. So, what's the best circuit/component could I use to replace this weird solution?

Original circuit: Orignal circuit

My solution: pseudo-solution circuit

Any help would be appreciated!


  • \$\begingroup\$ You'd be much better off wiring S1 shut and using a transistor (or switch for initial tests) to pull that node down to ground when you want to disable it. Much simpler transistor switching problem. \$\endgroup\$ – Brian Drummond Dec 7 '17 at 21:20
  • \$\begingroup\$ I don't know if I understood you correctly... You mean placing a NPN transistor collector to pull the node to ground (with the emitter grounded)? What about the base? How could I toggle the 555 output with a 5V signal with this transistor? \$\endgroup\$ – KawaungaXDG Dec 7 '17 at 21:25
  • \$\begingroup\$ Yes. Connect base to (5V/0V) via a medium value resistor - 10k should be fine. Oh wait ... you're using it as a T flipflop? Maybe it won't work, I thought you were gating an oscillator (astable) on or off. I can't help thinking there's a simpler solution for that too. \$\endgroup\$ – Brian Drummond Dec 7 '17 at 21:33

Out of completeness, you were close Q3 was backwards. This way works.

(Sorry it's backwards.... 555 is on the right of this,)

enter image description here

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    \$\begingroup\$ Nice circuit! I will stay with your answer, I have all the components in your schematic. I won't have to go to the electronics shop! YAY! Thanks for the answer! \$\endgroup\$ – KawaungaXDG Dec 7 '17 at 22:20
  • \$\begingroup\$ What about using a JFET? will it work? \$\endgroup\$ – S.s. Dec 7 '17 at 22:36

In your solution, current flows the same direction in both transistors. You need a bi-directional conductor of some kind because current flows either into or out of C1 depending on the state of the latch.

One option is a small reed relay; not sexy, but it works, and C1 debounces the contacts. Another is one section of a CD4066 CMOS analog switch.

  • \$\begingroup\$ Oh, I liked that CD4066! Thanks for the answer! I will replace that Frankenstein with this IC! Just one more question: Is there any version of this IC with a single bilateral switch? Or with fewer switches? \$\endgroup\$ – KawaungaXDG Dec 7 '17 at 21:40
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    \$\begingroup\$ The ones I'm familiar with are weird little SMT parts. My guess is that Analog Devices makes a single switch in an 8 pin package. OTOH, a 2N7000 as a series gate might work. 2.5 V above the source might be enough to turn it on. Hmmm... \$\endgroup\$ – AnalogKid Dec 8 '17 at 1:59

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