Hello,

I just started to play with electronics but quickly found some practical circuits that I would like to enhance to meet my needs.
Here I have simple timer built on monostable 555 IC:

As you can see there are two push buttons - one to start timer, second to stop already running timer.
I would like to enhance this circuit and leave only one push button: it should work as timer start button when timer is not started (pin 3 is low) and as timer reset button when timer is started (pin 3 is high):

I would prefer simple solution (maybe second 555 or pair of transistors), but it should keep working when reset/start button is kept pushed for more than some microseconds (with current R3 and C2 values timer keeps output high after start for 5 minutes and push button never will be pushed for that long).
Thanks for your advises!

 

Why not try a soft latch power switch? See here:
http://www.youtube.com/watch?v=Foc9R0dC2iI&feature=player_embedded
That assembly with 555 has the disadvantage of weak immunity to external interference signals.

 

Did you try to design the "feedback magic" black box yourself?

I've tried but it seems it would take more than just a couple of transistors to accomplish just to remove one push button switch. So just KIV this project, and try to solve it yourself as an exercise when you've gain enough knowledge.

Allen

 

Hi all,
Thanks donpetru, I looked how to increase triggering reliability and made some changes to 555 TRIG input.
Allens answer/suggestion made me to try harder and I could say that I succeeded without reading more chapters of "Make: Electronics" by Charles Platt (before this project I finished second chapter)
So because I am software engineer I looked to the problem from the logic side. Designed simple truth table how push button, timer output, reset input and trigger input depends on each other, read chapter about logic gates @allaboutcircuits. So I got two equations:
RESET=OUTPUT'+INPUT and TRIGGER=OUTPUT+INPUT
and found how to implemented this logic with 4 NAND gates (I had in my stash some 74HC00).
But I already knew that INPUT pulse should be very short to not allow same INPUT pulse to interact with changed OUTPUT value. So I downloaded Tina-TI and tried several simulations by putting capacitors and resistors at some connections (because I am new to electronics I did not knew what I was doing but quickly found correlations). After simulation worked I put everything on breadboard and after increasing capacitor C5 value in OUTPUT line and decreasing capacitor C4 value in INPUT line I got single push button working.

I am attaching Tina-TI schema file with probes if someone would like to simulate similar circuit.
I don't have oscilloscope myself so I don't know how far this single button schema is from some unstable behavior (I decreased value of C4 because when timer was running and I pushed the button timer was reset but it started again from the same input signal).

So this is my first success on DESIGNING something by myself

 

Congrats! You finally did it.

But if you want to use 15V as your Vcc, you have to change 74HC00 to 4011 CMOS NAND. The Vcc for 74HCXX is 2V-6V only.

Allen