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.
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.
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...