Help with a 555

Question

I need help with a 555 please!

I need to control a motor in Proteus that will simulate a water pump filling. I have a 555 that will count 5 seconds and stop the motor. That's with a button, when hold down like a switch the 555 will do infinitely if hold down. How can I convert a continuous signal like a switch to a singular pulse for the 555 count and not be obstructed while counting. And the cherry on top is that I can't use IC :( just the 555. Just analog things like transistors, capacitors, resistors, etc. Here is an example of what I have:

https://imgur.com/SjkK1r3

Answer 1

How can I convert a continuous signal like a switch to a singular pulse for the 555 count and not be obstructed while counting.

That's what a Schmitt trigger can be used for.

Since you can't use any more ICs then the a crude implementation with transistors can still be an option:

^{simulate this circuit – Schematic created using CircuitLab}

The circuit above will output a virtually-zero-width positive pulse. The pulse width can still be adjusted with the resistors but if you want to get a meaningful pulse e.g. 1 ms (I don't know actually, depends on the application) then you can use an alternative pulse generator:

^{simulate this circuit}

The pulse width can be adjusted with RT and CT. You can place this block to the 555's TR input:

^{simulate this circuit}

The good thing with this circuit is that even if the switch is left closed indefinitely the circuit will generate only one pulse. To generate another pulse, the switch must be released and then pressed again.

The bad thing is that there's no protection against switch bouncing i.e. if a bouncing happens that'll reflect to the output. But the 555 is immune anyways so shouldn't be a problem for your application.

Answer 2

The 555 is not a true monostable, because it has no positive feedback from the output to the Trigger input. Holding the Trigger input low extends the output pulse width, as you have discovered. This is documented on dozens of 555 design websites.

A trigger circuit is needed to make a pulse when the button is depressed.

^{simulate this circuit – Schematic created using CircuitLab}

Response to the button press:

For more details, see https://notesread.com/configure-555-timer-ic/