555 timer (astable, monostable, bistable and logic circuit) help

Thread Starter

freak101

Joined Aug 7, 2017
37
Hello all!

I was playing around with 555 timer and 74 series logical ICs. Untitled.jpg
The first 555 is in astable mode, middle one is in monostable and lower one is bistable mode. The digital logic circuit selects either astable mode or monostable mode and puts the final output on pin 11 (labelled as clock) with SW1, a double throw switch as select switch (yes, it is 2x1 MUX). That bistable mode 555 debounces SW1.
Now the problem is that this "selection" is not good enough. When SW1 is at 3, astable mode is selected and I see a blinking led. BUT when I throw SW1 at 2, monostable mode should be selected but the behavior of led many a times is wrong. Led keeps flashing even if monostable mode is selected.
Whats more odd is this: You select astable mode for some time ( around 10 seconds or so) , SW1 is suddenly thrown to the other position to select monostable mode and when you press and HOLD SW4 to apply trigger, it takes some time for the led to go fully bright. HOWEVER, led works normally on further "short duration" SW4 presses (to me it sounds as if led is connected across a capacitor instead of a battery and capacitor is discharged when SW4 is free).
I have connected decoupling electrolytic capacitors of 1uf as instructed by this post: https://forum.allaboutcircuits.com/threads/decoupling-or-bypass-capacitors-why.45583/
BTW this whole thing is on solderless breadboard.

Thank you so much for you help.
 

Thread Starter

freak101

Joined Aug 7, 2017
37
I forgot to add that sometimes when monostable mode is selected and after repeated presses of SW4, led suddenly starts blinking as if astable mode is selected.
 

Alec_t

Joined Sep 17, 2013
11,387
Since R1 is grounded, LED9 will flash in sympathy with Clock and should be unaffected by any SW1 or SW4 setting, unless the setting results in excessive current draw somewhere pulling down the Clock signal or the +5V supply.
 

crutschow

Joined Mar 14, 2008
25,102
It's difficult to follow the logic in U4, U5, and U6.
If you could post a schematic with the logic symbols instead of the package pinouts, it would greatly help.
 

Thread Starter

freak101

Joined Aug 7, 2017
37
Looks like the problem is cured. I was rechecking my connections and found that I was taking output of U4 from pin 4 instead of 3 and inputs were applied to pins 2 and 3 instead of pins 1 and 2 !
But I am still amazed ! I was sometimes getting correct response even with that wrong connection ! I would really like to know how it can happen.
Thanx.
 
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