input on 555

Thread Starter

piniiii

Joined Oct 30, 2019
7
Im creating a timer circuit for a spotwelder and i have a problem.
i built a circuit with ne555 and it works fine, i have a button to trigger the circuit, also put a pot on it so i can regulate how strong i want the weld to be by changing the time the circuit is on, but if i keep the button open the circuit keeps on and im afraid ill burn something sometime.
 

wayneh

Joined Sep 9, 2010
17,028
Im creating a timer circuit for a spotwelder and i have a problem.
i built a circuit with ne555 and it works fine, i have a button to trigger the circuit, also put a pot on it so i can regulate how strong i want the weld to be by changing the time the circuit is on, but if i keep the button open the circuit keeps on and im afraid ill burn something sometime.
Do you have a schematic?
 

crutschow

Joined Mar 14, 2008
26,735
You likely could couple the PB signal through a capacitor with a couple resistors to the plus voltage and ground so that the 555 only sees a pulse when the PB is operated, no matter how long it is pressed.
As stated, we need a schematic to give a more detailed answer.
 

crutschow

Joined Mar 14, 2008
26,735
Here's the simulation of a 555 circuit with the added input capacitor (C2) to allow the 555 to time-out independent of the push-button closed time:
Note that the 555 pulse (yellow trace) is unaffected by the length of the PB trigger time (green trace).

I added R2 so you won't damage the 555 if you turn the pot wiper all the way to 5V.

Question: Why is the pin 5 Control pin connected to the input on your circuit?
That's a connection to the internal trigger level voltage divider and is normally either left open or bypassed with a capacitor to ground.
Connecting it as you show can give flakey operation of the 555.

1617329194515.png
 
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Thread Starter

piniiii

Joined Oct 30, 2019
7
Here's the simulation of a 555 circuit with the added input capacitor (C2) to allow the 555 to time-out independent of the push-button closed time:
Note that the 555 pulse (yellow trace) is unaffected by the length of the PB trigger time (green trace).

I added R2 so you won't damage the 555 if you turn the pot wiper all the way to 5V.

Question: Why is the pin 5 Control pin connected to the input on your circuit?
That's a connection to the internal trigger level voltage divider and is normally either left open or bypassed with a capacitor to ground.
Connecting it as you show can give flakey operation of the 555.

View attachment 234250
thank you man, it works just how i wanted, now how can i have less seconds at the pot regulation? i mean it goes from like 1 second to 15 seconds, and i want it to be from sub-seconds to a max of 3 or 4 seconds, how can i do it? by changing the cap? im new at this
 

crutschow

Joined Mar 14, 2008
26,735
i want it to be from sub-seconds to a max of 3 or 4 seconds, how can i do it? by changing the cap? im new at this
Short answer is yes.
The one-shot time is roughly equal to 1.1 times the pot resistance times the capacitance.
Try a value of 6μF or so for the cap (C1 in my schematic).

But even with a 10μF cap, the minimum should be a few tenths of a second.
Did you add the 10KΩ resistor in series with the pot to the V+ (R2 in my schematic).
 
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Thread Starter

piniiii

Joined Oct 30, 2019
7
Short answer is yes.
The one-shot time is roughly equal to 1.1 times the pot resistance times the capacitance.
Try a value of 6μF or so for the cap (C1 in my schematic).

But even with a 10μF cap, the minimum should be a few tenths of a second.
Did you add the 10KΩ resistor in series with the pot to the V+ (R2 in my schematic).
Yeah i added the resistor, i changed the 10uf cap for a 1uf cap and now it works awesome. One last question, can i drive this circuit with 12v? I need a relay on the output and i dont have any 5v ones, and where i live the shipping cost a lot, thanks you
 

crutschow

Joined Mar 14, 2008
26,735
Help! Connecting with 12 volts keeps the circuit on. I mean i always have 12v positive on pin 3
If the circuit is wired correctly it should operate for any supply voltage within the 555's spec.
So you either you have a faulty 555 or the wiring is not per my schematic.
 

Thread Starter

piniiii

Joined Oct 30, 2019
7
I was about to give up, i tryied everything all day long, unsolder everything, solder it again, protoboards and everything, and at the end it was the freaking cap, apparently the one i put was to small. Dang it. Thanks you all
 
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