Using piezoelectric to power a laser diode?

** The laser diode is attached to a mouse trap - right ?

When the mouse sets it off - the mouse gets a blast of laser light in the eye.

You will soon get yourself 3 blind mice.

Nice one - boob.

........... Phil

And for how long do you expect a sharp blow from a hammer to illuminate this led ?

Graham

--- That won't work.

Try this:

Vcc Vcc | | [10K] [100K] +-------+ | | |__ | +------[0.1µF]--+------|--O|TR OUT|O---[R]--+ |- |A | | | |A [PIEZO] [1N4148] +---|TH | [LASER] |+ | | |_ | | GND GND +--O|D | GND | +-------+ [1.5µF] 555 | GND

1.5µF will get you a 200ms pulse out of the laser and 0.15µF will get you 20ms.

Or this:

+-----+ +------|~ +|----+-----+ | | | |+ |A [PIEZO] | | [C] [LASER] | | | | | +------|~ -|----+-----+ +-----+ FWB

You'll have to determine the value of C experimentally. Start with something like 10µF and work down until the laser flashes.

It worked for me with a plain old red LED and the piezo element from a barbecue lighter, which you might be able to adapt for your use.

-- John Fields Professional Circuit Designer

--
Oops...

The 1N4148 _cathode_ should be connectedto the 10k resistor:


                      Vcc    Vcc
                       |      |
                     [10K] [100K] +-------+
                       |      |   |__     |
       +------[0.1µF]--+------|--O|TR  OUT|O---[R]--+    
       |-              |K     |   |       |         |A 
    [PIEZO]         [1N4148]  +---|TH     |      [LASER]
       |+              |      |   |_      |         |
      GND             GND     +--O|D      |        GND
                              |   +-------+
                           [1.5µF]   555
                              |
                             GND

John,

All i can say is holy cow! I didn't expect schematics!!! I'll have to give that a try!

The illumination just has to be long enough for the human eye to detect it, which i'm assuming is going to be a function of brightness, but at

5mW i'm assuming 20-50ms is plenty..(my 200 upper end range is on the long side)...lots of assuming though, eh?

I do like the mousetrap idea, though... If you can't smash em, blind em. :)

Thanks again!!!!

You're aiming a 5mW laser at a human eye?

You can easily see a 1 usec puls from a normal (high efficiency) white LED, at 100 mA. Stef

Nooooooooo. I thought i fixed that part of my post...sorry. I'm just basically building a laser pointer for a little personal project. The piezo power idea came from the desire to make it as compact as possible, with the longest shelf life i can muster. In addition, the chassis that this is mounted in will be moving kind of vigorously, so i want to capture, in essence, a 'flash' that a person can see through persistence of vision...rather than a continuous beam which would rapidly become ambigous due to aforementioned motion.

The laser will, under almost any conceivable circumstance, be aimed at a wall or 'calibrated' paper area so we can see how much the chassis has deflected.

The application would be analagous to setting a similar device in the head of a golf club. The laser is parallel with the club face, and 'flashes' upon impact with the ball. The position of the flash relative to the alignment of the tee with the player will detect how much slice or hook is in the swing.

Bad example, but it gives you an idea of what i'm trying to achieve.

That's good info, thank you. I wasn't sure how much of a duration i would need for it to be effective. I'm assuming the laser diode would essentially have the same characteristics...the shorter the pulse, the better.

John, if you're still hanging around...one quick question regarding the schematics you wrote up. In the onces with Vcc, is that an external power supply, and the piezo is just used to switch?

Thanks everyone for your replies!!!

--- Yes. Taking a look at it and adding the power and ground pins for the chip _and_ fixing an error:

Vcc Vcc Vcc Vcc | |K | | [10K] [1N4148] [100K] +-------+ | | Rt| |__ | +--[0.1µF]--+------+-------|---O|TR OUT|O---[R]--+ |- |K | | | |A [PIEZO] [1N4148] +----|TH | [LASER] |+ | | |_ | | GND GND +---O|D | GND Ct| +-------+ [1.5µF] | 555 | | GND GND

Here's what happens:

When you whack the piezo a high voltage pulse will be generated which would kill the chip, the 1N4148 diodes are in there to clamp the positive peak of the piezo voltage to roughly Vcc + 0.7V and the negative peak to about -0.7V.

The side (end?) of the piezo which generates the initial low-going pulse (which is what the 555 wants to see) is connected to the 555's TRIGGER- input, which will drive its output high for the time determined by:

t = 1.5RtCt

In this case,

t = 1.5 * 0.1E-6R * 1.5E-6F = 0.225 seconds

which about what you asked for for long pulse. You can easily make the pulse narrower by decreasing the capacitance of Ct or the resistance of Rt, but I'd leave Rt alone and go for a smaller capacitor.

One caveat about the input pulse is that it needs to be narrower than the output pulse, (a 555 peculiarity) or the output pulse will be as wide as the input pulse, which may be wider than you want. If that happens, a possible solution would be to load down the piezo with a resistor wired across it, or to diddle with the value of the

10k resistor and/or the value of the 0.1µF cap. Either way, the value would have to be determined experimentally, and if that got to be too tricky, then you could use a different one-shot and not have to worry about it. Something like an HC123 or an HC4538 would work, but you'd probably have to add a transistor to drive the laser diode.

If you're looking for a nice, sharp, short pulse, then I recommend that you _don't_ use the bridge-cap scheme since it'll smear the pulse and you wont have nice edges.

-- John Fields Professional Circuit Designer

Thank you for the explanation! I'm learning stuff every day!

I guess the only issue is that i'm trying to use the output from the piezo to directly power the laser, so i don't need an external power source...kind of like the FBR + C(ap?) diagram you had at the bottom of your previous email. I think i'm going to order up some parts for both...if the cap+bridge is too sloppy, i'll go with the other. I'm sure a few watch batteries would power the diode for years given the usage i'm going for.

Thanks again for all of your help!!! I've been surfing forums for the last few years, and forgot just how great USENET still is.

Take care!!

Bob