We all know that our precious cruisers/warships/assets can not fall into the wrong hands, and what better way to assure that information security than incorporating a self destruct button into the design? While the general premise is simple enough the only real way to make sure some crazy space-virus can’t infect your captain and force him to destroy your ship is to add a two button security system! Wait, what if it’s a computer virus?! Well [Andrea] has it covered with this two hand hand control switch. We guess you could also just use the scheme to flip on a dangerous piece of equipment, since it forces the operator to remove both hands from the machine to operate the buttons, but where is the drama in that?
The buttons are timed using a combination of voltage dividers and caps to activate transistors, one to activate the 555 timer and another to disable the button input after a half second or so. Since it’s all resistor capacitor timing your circuit may require just a bit of tuning (or precision components) to get everything right. There is a bit of an issue with using two people to trigger the output, as the second button actually operates the output relay directly. If the second button is held it will only remain active until the timer’s output is triggered, but if your second in command gets cold feet and releases the button before the core goes thermo, well, you’ll have an embarrassing jog to the escape pods.
Check the video after the jump to see [Andrea] fiddling around with the switch.
[youtube=http://www.youtube.com/watch?v=G9uHNrIsJvA&w=470]
I don’t even get the point of this. Why wouldn’t you just use a pair of switches with either their normally open contacts in series, or their normally closed contacts in parallel, depending on if you want a NC or NO double-switch?
The problem with series, or parallel is that can be easily defeated by one person. Tape one switch closed and then go press the other switch.
The whole purpose of this is that the 2 switches are far enough apart one person can’t press both, but they MUST be pressed at the exact same time. There are always methods around that too.
This is not quite the same as a safety switch on dangerous equipment, but could probably work as such.
I had a co-worker once who bypassed the two handed switch on a hydraulic press, until the Safety Officer saw it and was very unhappy…
It’s hard to tell, but why does a 555 timer with only 8 pins and a handful of other components need 3 billion link wires to make a simple circuit?
I built a Z80 on a breadboard years ago and it had less wiring than that…
I must be missing something… why is this better than two switches in series?
Because they dont have a timeout in witch the other button has to be pressed.
It isn’t really any better than two switches in series as it is shown, but this type of circuit it used where the push switches are latching types (such as locks/keys). It is then that you need to prevent a single person being able to close one switch and then go over to close the other.
not necessarily if the switches are latching types, but to prevent someone from latching a momentary switch (tape, jumper, wedge a screwdriver in, etc) to ensure that both of your hands are clear of some machinery (i.e. punch press) so they don’t get removed.
http://www.osha.gov/SLTC/etools/machineguarding/presses/twohandcont.html
Gosh, I’m glad I’m not the only one with chaotic solderless breadboard layouts.
You definitely are not alone. I used to be really bad, use all the same color wire, plan to add on to the circuit but don’t leave space. Those habits died when i TAing an intro to circuits lab… I had to trouble shoot circuits where people used the breadboard in the least efficient manor i have ever seen. Needless to say i found that trouble shooting circuits is so much nicer when you organize the bread board.
the breadboard in the video made me do a triple take.
“TURN YOUR KEY SIR!”
This is an realy old idea in the industry. Two buttons, 1relais, 1 timer-relais and voila: perfect security input.
I installed some triggers like this for large hydraulic machines.
You can’t just do this with a few logic chips and the 555?
Ever seen a printer’s guillotine?
Two buttons a yard apart, you have to pull your hands out of the danger area to press ’em. Been around since the year dot.
Simples.
Ours at my old workplace had a lever on each side of the work surface, one for the hydraulic clamp and the other for the blade. Of course, the owner had circumvented the security measure to speed certain (dangerous) tasks.
Once the blade actually drifted down while the machine was off, enough that I sliced off a 1/4 inch chunk of my right middle fingernail and nail bed.
He’s kinda lucky I am too nice. I should have reported him for all the stupid crap he pulled. Actually, he’s kinda lucky there wasn’t an unfortunate spark somewhere in his firetrap of a shop. (Open containers of sovent, open containers of solvent-soaked rags, paper, dust, oil based ink, malfunctioning equipment, faulty electrical wiring, no sprinkler system… No one would have ever known… :s)
Or do it the way industry does, two switches in series that requires both to be pushed at the same time.
But then wire is expensive and chips are cheaper. Oh wait…
The timer allows you to require that both buttons are pressed within an interval, rather than simply that both buttons are pressed.
As we can see from the video one person could press the two buttons.
You could have metal plates on the buttons and measure the resistance between the buttons.
Low resistance = 1 person (or two holding hands)
High resistance = 2 people not touching each other
Very high resistance = wearing gloves etc.