555 Bisable -Latch pushbutton

Dear all,
i got this Bistable circuit,
the purpose of it to use one pushbutton to turn the LED ON\OFF
but once its on, it never turns off!!!
what could be the problem?>!

What is forward voltage of the LED? What voltage is at 555 output when the LED is ON?

Well, it certainly looks similar to the 555 toggle circuit here:
http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html#31A

How are you closing and opening that switch in your simulation ?

In real life, you need a capacitor from pin 5 to ground especially if you use a CMOS version of the 555.

Smajdalf:
What is forward voltage of the LED? What voltage is at 555 output when the LED is ON?

LED 1.7V … voltage at 555 ouput is 3.55v

6v6gt:
Well, it certainly looks similar to the 555 toggle circuit here:
50 - 555 Circuits

How are you closing and opening that switch in your simulation ?

In real life, you need a capacitor from pin 5 to ground especially if you use a CMOS version of the 555.

whats the rule of this capacitor attached to pin 5 ?

I just built the circuit with the exact values as in the schematic above, and it works as expected. Of course you have to wait a few seconds between switches. The only (but important) difference: I'm powering it at 5V as that's within spec for the NE555 (4.5-16V). That 3.7V is too low a voltage for the NE555, though CMOS versions like the TLC555 will do just fine at that voltage.

If it doesn't work for you: check your connections.

adolfhack:
whats the rule of this capacitor attached to pin 5 ?

It's to keep the internal reference stable. Normally a 10 nF ceramic cap is connected to it. The pin can be used to offset the trigger and threshold voltage, but applications for that are really rare.

Since pin 5 is the comparator voltage, it prevents transients being coupled to the comparators and causing them to misbehave when faced with sudden transients such as encountered when you press a switch. :sunglasses:

The 555 expects to see mainly smoothly changing capacitor voltages - this circuit works the other way round.

Thank you all for your kind help..
i did few tweaks and it worked.. i dont know exactly how but with few experimentations based on your suggestions it worked

i did few tweaks and it worked..

Define 'tweak' . Either it was wired correctly or it wasn't and now it is.
The circuit has no pots to 'tweak' (which, FYI is what tweak means)
Glad to hear it works, although I don't think 'tweak' is the correct term for what you did .
We have seen a lot here on the forum, but so far no magic.

raschemmel:
Define 'tweak' . Either it was wired correctly or it wasn't and now it is.
The circuit has no pots to 'tweak' (which, FYI is what tweak means)
Glad to hear it works, although I don't think 'tweak' is the correct term for what you did .
We have seen a lot here on the forum, but so far no magic.

i changed capacitor value and resistors as well, added a capacitor to pin 5 .. it worked

i changed capacitor value and resistors as well, added a capacitor to pin 5

FYI, the 01uF (100nF) cap on pin 5 is almost mandatory.
The timing cap value should have been correct as shown in the linked schematic. (10uF)
The value in your schematic (47uF) is almost 5 times larger than it needs to be. The correct
value is 10uF.
4017 toggle circuit

[555 Timer Flip Flop](http://www.elo.jmc.utfsm.cl/sriquelme/apuntes/555/El timer 555.pdf)

(don’t tell anyone but it’s bisTable, not bisable)