555 timer pin 4 reset not working ?

Thread Starter

Dblane79

Joined Dec 7, 2019
9
Hi guys. Beginner here.

Made my 1st 555 delay timer on my breadboard and its all good.

Just wondering about the reset pin (4). Right now i have it connected to +Vcc.

If i move it to ground then the 555 goes off but as soon as i pull the pin 4 out of the ground (not even connected to +V) the 555 carries on and the LED comes on instantly without delay. Is that normal. Operation? Or the reset pin when grounded should restart the timer?

Thanks
 

SamR

Joined Mar 19, 2019
1,835
Instead of "pulling it off" leave it connected to Vcc and put a momentary push button on it to drop it to ground when pushed. All it needs is a quick toggle to GND for reset. Yes reset "restarts" the timer and if the trigger input is low it cycles.
 

Thread Starter

Dblane79

Joined Dec 7, 2019
9
I think he should leave it *disconnected* from Vcc, and add the pushbutton to ground. You don't want a short circuit.
I tried this just now. The led goes off when i press the pushbutton (so to connect pin4 to ground momentarily) and the "delayed" led goes off but as i release the button the led comes back on straight away insteaf of the 6 second delay that it is supposed yo have.
 

SamR

Joined Mar 19, 2019
1,835
There is also something about using a capacitor to prevent autostart on power-up. Can't remember without finding and reading my old notes.
 

Chris65536

Joined Nov 11, 2019
164
I tried this just now. The led goes off when i press the pushbutton (so to connect pin4 to ground momentarily) and the "delayed" led goes off but as i release the button the led comes back on straight away insteaf of the 6 second delay that it is supposed yo have.
Can you post a schematic? There are several different ways to use a 555, so it's hard to diagnose without one.
 

ci139

Joined Jul 11, 2016
1,195
What it means is that if you attempt to reset the 555 by pulling up the pin 6 , while the pin 2 is low it will have no effect
about555.gif
 

crutschow

Joined Mar 14, 2008
24,403
When the Reset in is low, the 555 won't do anything and the output stays low.
When the Reset goes high, the 555 will then do whatever it is configured to do.
 

SamR

Joined Mar 19, 2019
1,835
Wow found it quick. I have in my notes to use a 1uF cap between pin 4 and GND to suppress auto pulse on startup. I also have to put one on pin 5 to GND instead of leaving it floating.
 
Last edited:

Chris65536

Joined Nov 11, 2019
164
Thats the circuit im using.
In that circuit, the 555 is functioning only as a comparator, and the capacitor remains charged even when you pull reset pin low. The 6 second delay will only happen once at power up. You could put the pushbutton across the capacitor to discharge it, which resets the delay. Maybe include a small resistor in series to avoid arcing.
 

crutschow

Joined Mar 14, 2008
24,403
Here's an LTspice simulation of a 555 circuit with a transistor (which could be a switch as Chris suggested) to reset the capacitor.
The delay starts when the transistor input (blue trace) goes to the supply voltage (+5V here), with the output (yellow trace) then staying high after the 6 second delay until the input goes back low, which resets it for another sequence.
Note that the Reset pin is kept high at all times.
Is that the behavior you wanted?

1580411889624.png
 
Last edited:

Thread Starter

Dblane79

Joined Dec 7, 2019
9
Guys thank you very much for the replies!

Will go through them all and try a couple of things and see how i get on. Will update in the next few days in case any1 else is interested at the results.

Thanks
 

Thread Starter

Dblane79

Joined Dec 7, 2019
9
Wow found it quick. I have in my notes to use a 1uF cap between pin 4 and GND to suppress auto pulse on startup. I also have to put one on pin 5 to GND instead of leaving it floating.
Ok I tried this on Circuit Wizard and it doesnt seem to do the trick. The cap remains charged so timer doesnt reset.
 
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