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Safety relayAuthor
April 5, 2012

Two button control relays are used in industrial safety in order to keep the operator's hands away from the dangerous parts of the machine. This feature is often required by the safety regulations.
Just for fun I designed and tested a curcuit that activates a relay by pressing two buttons at the same time, with a tolerance of 0,5 seconds. The relay is supposed to work instead of the pedal switch.
Once the circuit is supplied the input voltage (pin 2/6) is high, so the output keeps the low level.
By pressing one button the second transistor is supplied by 560Kohm, too high; so the collector keeps almost off and the input voltage is more than 8 Volts.
If the second button is not pressed within 0,5 seconds, Q1 turns on and prevents Q2 from activating. So the operation has to be repeated.
The relay turns off after few seconds, depending on the value of C2. So the operator has to release the buttons and press them again.
The switches connected in series are supposed to prevent the relay from activating in case of component failure.
For a better safety we may use two relays and a led which alerts the operator if one relay gets stuck on.
This is just an hobby project, do not use it for industrial safety.

Two hand control safety relay

Note: 1,9K and 1K resistors should be 1%.



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  • At 11 November 2014, 19:55:41 user Luiz Carlos wrote:   [reply @ Luiz Carlos]
    • Fantastic , simple and efficient . Congratulations .

  • At 14 April 2012, 12:52:53 user Andrea wrote:   [reply @ Andrea]
    • Thanks Giorgos, I'll share more here

  • At 13 April 2012, 7:07:55 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Andrea Congratulations Andrea! I really enjoy having one in my site that designs original circuits! Hope we see more from you

  • At 12 April 2012, 22:20:49 user Andrea wrote:   [reply @ Andrea]
    • Yes, I designed the circuit, but I also have to thank a simulator software (Microcap) that was quite helpful for the value of the components.

  • At 12 April 2012, 19:45:57 user Panagiotis Kalogeris wrote:   [reply @ Panagiotis Kalogeris]
    • Nice work andrea!Thanks for sharing!!!!!!!

  • At 12 April 2012, 17:05:45 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Andrea something more, did you design this circuit yourself? I haven't seen anything like this so far with the 555.

  • At 12 April 2012, 15:49:03 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Andrea SCM group, i know them of course, very big group with lots of divisions and machines.

  • At 12 April 2012, 15:30:28 user Andrea wrote:   [reply @ Andrea]
    • I worked in Makor group in the italian factory.
      Nearby my company there is another one that makes cnc woodworking machines and is controlled by SCM group.

  • At 12 April 2012, 15:14:01 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @Andrea Really??? i am a cnc technician for woodworking machines! In which company did you work?

  • At 12 April 2012, 14:27:39 user Andrea wrote:   [reply @ Andrea]
    • Thanks Giorgos, I'm glad that you find this really useful.
      At the end of the article I said "This is just a hobby project..." due to legal restrictions in my Country.
      Some years ago I was a mechanic/electrician in a woodworking (finishing and painting) machines company.

  • At 12 April 2012, 6:34:25 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • We have this sort of protection on some woodworking machines, but usually they are 2 buttons in series, so the always-bored operator can hack one button to operate it with only one. Adding this circuit is an extra protection. My company makes presses and we have 2 buttons to operate the press, but without such circuitry... which gives me an idea...

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