I want to use the 555 time to control the timing on the power pulse of a spot-welder using a foot switch.

I have seen a few 555 mono-stable circuits, but am having trouble finding the values of the resistors and capacitors in order to get times below 1 second.

Also, I want to connect this circuit to a spot welder in a safe way, because of power difference. My spot welder is made of a MOT, and have a pedal switch to do the switching.

How should I connect these parts together? What kind of pot/cap combo for sub 1 second timing with a 555 timer?

enter image description here

I wanted to add the image at the bottom of the page, but didn't know how. So, i have to put over here. Please let me know any modifications that i should do with the connections. Also, i would like to add a POT in the 555 timer circuit in order to adjust the timing. Where should i put it? Thank you for any help...enter image description here

  • \$\begingroup\$ As you correctly observed, the timer circuit is just a small part of your problems. You need something to switch large currents. That's where relays come in. There's timer relays. As usual, when someone tries to actually build something useful from the 555: for most its applications that ic has long be superseded with cleverer contraptions. \$\endgroup\$ Apr 8, 2018 at 1:40
  • \$\begingroup\$ And the fact that you know neither how to design that 555 circuit nor what kind of switch you might want nor how to wire these up: I'm afraid the by far shortest possible answer to your question would be to build that whole thing for you; an indication that your question is too broad. \$\endgroup\$ Apr 8, 2018 at 1:43
  • 1
    \$\begingroup\$ One option is to buy "relay timers". They are 6 to 8 pin and plug into a socket with screw terminals. The timer is built in and adjustable. \$\endgroup\$
    – user105652
    Apr 8, 2018 at 4:16
  • 1
    \$\begingroup\$ 555s have datasheets. Datasheets contain example circuits and formulas for calculating resistor and capacitor values. All that is easy compared with the rest of the welder. \$\endgroup\$
    – user16324
    Apr 8, 2018 at 10:25
  • \$\begingroup\$ Thank you very much for your comments. I didn't know that there's timer relays, i will look into it. I have an ssr 80amps to isolate the 555 circuit. The problem i have is connecting properly the timer circuit to the ssr and spotwelder and the foot switch to the timer circuit... also trying to adjust the timing to control the amount of power (which is fixed) send to the welding spot. Because i will be spot welding different thickness i would like to be able to adjust the amount of power controlling the timing in miliseconds. Thank you very much for your comments \$\endgroup\$ Apr 10, 2018 at 0:47

3 Answers 3


555 MonStable

MonoStable with switch

Sorry fella, I should read it a bit clearer. I had it in my mind with the last post it was a MIG type thing with pulsed outputs...Hopefully the above will make it a bit clearer for you.

  • \$\begingroup\$ No problem, i appreciate your input . Because i would like to dynamically change the timing, which resistor should i change with a potentiometer. Do any pot will do? Or it is a specific kind of pot that I should used? \$\endgroup\$ May 3, 2018 at 2:05
  • \$\begingroup\$ If you had say 20uF cap for C1 then a 100K pot for R1 this would give you a theoretical range of around 0 to 2.2 seconds.. No special requirements for the type of pot.. \$\endgroup\$
    – perfo
    May 4, 2018 at 2:42
  • \$\begingroup\$ Thank you very much for your help. .... will be possible to let me know if my connection and wiring are correct in my crude drawing ? Just want to make sure, thanks again \$\endgroup\$ May 5, 2018 at 15:06

enter image description here

something like 82Uf for C R1= 10 K and R2 maybe a 0 to 25 k pot with a 5k in series would get what you asked for. But you may want to alter the mark space ration rather than just the on period so maybe 1 second on and five seconds off. Has the spot welder already got a small switch on the handle or something ? If so then it may already have the facility to use a low voltage switch to trigger the large current welding supply. Hope this helps.

  • \$\begingroup\$ Thank you for the sketch and the cap/resistor values. I will try them. \$\endgroup\$ Apr 10, 2018 at 0:52
  • \$\begingroup\$ Would that be an answer then ? or maybe a point ? \$\endgroup\$
    – perfo
    Apr 11, 2018 at 8:17
  • \$\begingroup\$ Yes, partially. It's good to have those values, good start point. \$\endgroup\$ Apr 15, 2018 at 15:08
  • \$\begingroup\$ Yes, partially. It's good to have those values, good start point. Having an sketch is a lot easier for me to understand. Would you please tell me based in ur sketch how/where i connect the relay with the spotwelder and the food pedal switch in order to trigger the timer. I appreciate very much your time and help. Thank you \$\endgroup\$ Apr 15, 2018 at 15:18
  • \$\begingroup\$ I can't really answer that unless I knew the lay out of yoru particular system. \$\endgroup\$
    – perfo
    Apr 17, 2018 at 10:44

You don't want to pull down pins 2 and 6 with the switch you want to do it as I have shown in the diagram above. Also the output will go high (ie + WRT 0V) so your SSR would have to be around the other way but still tied to pin 3. You'd have to check with the data sheet of the SSR to see if it is happy to be connected directly (Volts and current wise) to the output of the 555. If you already have these components then the 555 stuff is so cheap you can set it up on a breadboard utilising maybe 12v and make sure everything is doing what you want before going anywhere near the lethal stuff. Remember these are only hypothetical ideas you take full responsibility for your own actions..

  • \$\begingroup\$ Hello... and thanks again for your comments. I uploaded a revision of the drawing (are my connection OK?). I don't understand by putting the SSR around the other way, could you clarify that please. Will be great if can draw any changes and suggestions, that way I don't have to guess. if is not too much trouble of course. Thank you. \$\endgroup\$ May 6, 2018 at 17:26
  • \$\begingroup\$ If you look at the diagram below you will see that the output of the 555 sits at 0v then pulses to near the supply voltage for the duration of the pulse then returns to 0v again. The way you have it connected to the SSR is the SSR would be on all the time the output of the 555 was at 0V thus all the time the pedal isn't pressed. The output of the 555 needs to go to the + of the SSR. As said in my last post check the fan out of the 555 is higher than the fan in of the SSR. IE make sure the 555 can supply enough current to turn on the SSR. Other than that your diagram looks ok. \$\endgroup\$
    – perfo
    May 8, 2018 at 14:44
  • \$\begingroup\$ Hello and thank you for the clarification. ... many years ago i took basic clases of electronics, i enjoy fixing and doing projects. Many times i get into trouble figuring out how a circuit works cause i learned that current flow from negative to positive. In the 555 timer circuit, i assume that output 3 in negative. Anyway , i really appreciate your help. Once i put it all together will let you know how it went. Thanks \$\endgroup\$ May 11, 2018 at 16:45
  • \$\begingroup\$ Today i was testing the circuitry. It works, with one inconvenience. When i try to fire up repeatedly, it doesn't work, i think it needs a few seconds to recharge the capacitor, if i wait a few seconds like 4 or 5 seconds, it does work ok. Is there a way to modify the circuit in order to make it fire up repeatedly with less than a second between firing? Hope there is!! Thanks \$\endgroup\$ May 12, 2018 at 21:48
  • \$\begingroup\$ Well ok you could multi fire it but it wouldn't then fit your original request. If you want a one second (or whatever) pulse no matter how many times or how long the foot pedal is pressed then the above circuit will do it. In your rapid pedal pressing idea what do you want to happen if you press the pedal multi times during the on period ? \$\endgroup\$
    – perfo
    May 13, 2018 at 8:51

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