# input on 555

#### 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?

#### SamR

Joined Mar 19, 2019
3,180
if i keep the button open the circuit keeps on
This needs some clarification. To paraphrase, A schematic is worth a thousand words!

#### 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.

#### piniiii

Joined Oct 30, 2019
7

#### 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.

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#### AnalogKid

Joined Aug 1, 2013
9,042
To paraphrase, A schematic is worth a thousand words!
To paraphrase, "Engineers don't take a dump, son, without a schematic."

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#### 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

#### Yaakov

Joined Jan 27, 2019
2,361
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
The datasheet (https://www.ti.com/lit/ds/symlink/lm555.pdf) for the 555 explains timing, including this figure:

#### 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
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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#### piniiii

Joined Oct 30, 2019
7
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

#### wayneh

Joined Sep 9, 2010
17,028
One last question, can i drive this circuit with 12v?
Yes. You’ll need a different resistor to limit the LED current. I’d use 1K but a 470 ohm is OK if you need more brightness.

#### piniiii

Joined Oct 30, 2019
7
Help! Connecting with 12 volts keeps the circuit on. I mean i always have 12v positive on pin 3

#### wayneh

Joined Sep 9, 2010
17,028
Help! Connecting with 12 volts keeps the circuit on. I mean i always have 12v positive on pin 3
Which schematic are we talking about?

#### piniiii

Joined Oct 30, 2019
7
The one crutschow posted

#### wayneh

Joined Sep 9, 2010
17,028
The one crutschow posted
Try pulling the trigger (pin 2) high via the 100K resistor, no other connections to that pin. Does the output go low?

#### 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.