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Say for instance, the goal is to light an incandescent bulb for x amount of seconds (let's say 20).
What is the simplest configuration possible?

Of course, simple timer circuits aren't rare, and can be found easily online. A simple search engine query will show this. The thing is; they often vary in the number of components, and their configuration. As far as I can tell, the extra complications often seem superfluous and unnecessary, if the same goal can be achieved without them.

If you'd like an example, here's one that I came across on this site, for a 30 second timer.

Okay, so everyone seems more focused on my commentary than the actual question. I'm happy to discuss things, etc. But my priority is answering the question.

So to bring the focus back to the initial goal, I'm sharing my first attempt at a 20 second timer. I tried to make it as simple as possible. And the values are totally wrong. If you couldn't tell, I'm new to this, and don't fully understand everything yet, so please be kind.

schematic
This is my schematic.
I think it was originally an attempt to make sense of this page, via chance & data; trial & error.

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    \$\begingroup\$ extra components, like decoupling caps, are added for a reason, usually stability. \$\endgroup\$ Commented Aug 28, 2017 at 19:31
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    \$\begingroup\$ Potentiometers are used to adjust timing. If you don't want to adjust things (or already know the right resistance) a pot can be replaced by a resistor. Often, the pot will be in series with a resistor -then the pot+resistor can usually be replaced with a single resistor. \$\endgroup\$ Commented Aug 28, 2017 at 19:35
  • \$\begingroup\$ Sure, no doubt. \$\endgroup\$ Commented Aug 28, 2017 at 19:37
  • \$\begingroup\$ 555 is the last part I would consider. Use a CD4060 with a ZCS optocoupler & Triac \$\endgroup\$ Commented Aug 28, 2017 at 19:37
  • \$\begingroup\$ What do you mean by that? I'm studying them at the moment. That's what led me to ask about it. \$\endgroup\$ Commented Aug 28, 2017 at 19:39

2 Answers 2

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Don't confuse the circuitry for the timer part with the remainder of the circuit.

In the example you gave the components shown left of R3 are actually there to de-bounce the push button switch which presumably was a critical issue for this designers application.

The remainder is your typical 555 one shot circuit, with decoupling, a power on reset delay, and of course a load LED.

Designers seldom add components for no reasons, in fact, usually we strive to eliminate components.

Sometimes the wisdom of what is added to perform some function can be a bit suspect however.

Update

Your schematic for a simple timer is correct, but the values are way off for a 20s timer.

The formula for the time period of a one shot 555 timer is the standard equation for the amount of time it takes an RC circuit to charge up the voltage on a capacitor to 66.666 % of the final voltage.

\$T = 1.1 * R * C\$ or more accurately \$T = 1.0986 * R * C\$

Since capacitors come in more limited sizes, pick a reasonable capacitor value first, based on cost and dimensions. Since you want a fairly lengthy pulse, lets pick \$100uF\$.

So now we need to calculate a value for \$R\$. Rearranging the formula above you get.

\$R = T / (1.1 * C) = 20/(1.1 * .0001) = 181,818.818...\Omega\$

Round that to the nearest E-Series resistor 180K. (quick check T=19.8 Seconds).

At this point we normally sanitize the answer. That is, if R is ridiculously high, eg > 10M, or far too low, we would chose a bigger or smaller capacitor and redo the math.

TIP: A shortcut to do this quickly is simply to multiplying the cap by 10 while also dividing the resistor by 10, or vica-versa.

So here is your 20s timer circuit..

schematic

simulate this circuit – Schematic created using CircuitLab

Remember there are part tolerances involved so, with 5% parts your timer will be anywhere in the range of 17.85s to 21.8s.

If you need it more accurate than that, use 1% parts and possibly add a trim-pot in series with the charge resistor. However, be aware, the period will change with temperature.

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  • \$\begingroup\$ FYI: I'm not criticising that circuit in particular, it's just one of many variations I've seen. I can provide more examples if that helps illustrate the gist of my question. \$\endgroup\$ Commented Aug 29, 2017 at 2:28
  • \$\begingroup\$ @tjt263 yes I realize that, I was just highlighting that every 555 circuit usually contains the same basic 3 or four components, though their arrangement can be misleading. The "other" stuff is not really timer related. \$\endgroup\$ Commented Aug 29, 2017 at 11:51
  • \$\begingroup\$ Yeah, forget I mentioned any of that stuff, I might even remove it from the question because people can't seem to get past it. You mentioned the same basic 3 or 4 components. How many exactly? What exactly are they? What is their simplest possible configuration, while achieving the goal of a one shot 20 second timer? Bare bones. No bells & whistles. Can you show a schematic? If you can do that, I'll vote Δ, hit ✔, say thanks, and the question will be resolved. \$\endgroup\$ Commented Aug 29, 2017 at 14:17
  • \$\begingroup\$ If that comes across rude, it's not my intention. Just saying. \$\endgroup\$ Commented Aug 29, 2017 at 14:19
  • \$\begingroup\$ @tjt263 you already did show the minimum in your updated question.... \$\endgroup\$ Commented Aug 29, 2017 at 14:20
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Similar Circuit

I think this circuit should probably be the simplest in terms of a monostable circuit. If you are interested in learning more about 555 timer circuits, I would highly Forrest M. Mims III timer and op-amp notebook: Amazon Link. I think his notebook is a great starter for the device, and it will give you a little better idea of the different types of 555 configurations out there.

To give you a quick rule for this circuit though, the time for the on pulse is completely dependent on the 200k resistor, and the 100 uF capacitor. The duration of the pulse is given by T = C * R (the two components just mentioned). This time is completely independent of the 1 k resistor (which is used as a pull-down resistor, and can also be kept as a 10 k resistor just as well), and it is also completely independent of how long you press the switch (assuming you don't hold it for longer than 20 seconds).

The short answer to your question, this is likely the simplest monostable 555 timer circuit out there

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  • \$\begingroup\$ T = C * R, not quite... Its actually 1.1 times that. \$\endgroup\$ Commented Oct 30, 2017 at 12:09

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