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Need some help.

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have not been around these forums for at least a couple of years. Anyway, I remember reading about a project this college student did where he basically built a touch sensor onto his laser so that there were not moving parts on it. It used some really low voltage jfet or something like that. The voltage to switch it on came from the potential difference of him putting his finger across 2 different types of metals(i think he got it to work with coper and aluminum).

If anyone knows what I am talking about and happens to know where I can find his writeup of his project I would love to read over it again and hopefully duplicate his work. I thought this was an awesome idea from the first time I read it.

Other than that, let me take a moment to say "Hi I'm back."

My life situation has changed a bit and I want to get back into playing with lasers. I am probably going to be getting the stuff together to build a few lasers over the next few days/weeks. I am interested in getting a nice 100+mw green and i have always wanted a 445nm blue. (and no I am not forgetting about goggles)

Any help that can be offered will be appreciated.
 

123splat

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It sounds like a capacitive sense switch, using capacitance on the junction FET's gate to fire the JFET. Most texts on FETs have something similar in applications examples. I, personally, don't think that it is such a good idea to have that as the switching mechanism to activate a laser (safety/liability issue) and would advise against it. But,,, Shouldn't be too hard to find something similar and adapt it though, if youare seriously considering it.
 
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You are probably right it isn't the greatest idea for a higher powered laser. I would still love to do it even if it was only on a 5mw.

EDIT: oh yeah, last night before I fell asleep I was reading exactly what you are saying about the capacitive touch sensors. Actually i found several different circuits. From my understanding though, The way he was doing it was through voltage generated through the connection of two dissimilar metals causing a voltage kind of like a thermocouple. I am not sure about the specs on jfets, but last night before i went to sleep i found one that could be triggered off less than .3 volts and that is for complete cutoff. Since it will be chained with two of them, I think maybe I would be able to trigger something that is more digital than a jfet with it so that is more like a switch and less like a variable resistor. I wouldn't want a continueous battery drain.
 
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123splat

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Remember that capacitive switches usually act like antennas and pass stray AC in your body to the FET's gate (in the form of a voltage spike). If you go digital, you may be looking at more batt drain (maybe less with CMOS) Look at some of the older JFET OP Amps. The off state leakage there will probablly be in the uA to pA range, so batt drain will not really be an issue.

But, I would still suggest a full cut off with a regular S.P.S.T (or the like) switch.

Be waiting to see what you come up with. If you get it to work, be prepared to get some flack from the safety zelots here. They are really only looking out for your safety, but they sometimes get a little harsh about it. Good Luck!
 

strik3

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I wouldn't see any problems with this if there was a safety interlock. Such as a key to power the circuit for the capacitive switch, so that it can't be accidentally activated while in storage or by holding it wrong.
 

123splat

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Please explain how a key interlock switch will prevent the laser from being fired if the laser is "being handled wrong" ( read: the capacitive pad/s are touched). All an interlock is gonna do is helpensure that the right person can fire the laser... Not keep anyone from having a momentary lapse and accidentally touching the pad(s) when the interlock is closed.

Still a bad idea for a laser, in my book.
 




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