How fast does it lose it's charge just sitting there?
Metal film caps are all low leakage compared to electrolytics. There's something else call "dissipation factor" that only comes into play at some AC frequencies (how much power the dielectric wastes as heat).
What you are doing is low frequency and the capacitors aren't critical.
For a 'laser alarm', i.e. detecting a light beam, the presence of anbient light (generally white-ish and not modulated) is a guaranteed interfering signal.
If you use a photodiode (instead of LDR) your receiver can pick up any frequency from zero (direct current output) to 1 MHz, as opposed to zero to 1 kHz (for CdS, which has a storage time).
If you start with a laser, you can use a long black-painted tube to aim the receiver in one and only one direction. Alignment will be fiddly, though.
Laser or LED light also has a characteristic color, not common in nature, so some kinds of filtering will reject interference while passing the whole intended signal.
The best ( easiest) way to make it all work, might be to make the laser flicker at some frequency, and use a LM567 'tone decoder' that can pick up on one-and-only-one frequency of light-brightness-oscillation. Because both the laser and receiver require power, it could be good to package them together, and use retroreflective tape or other backscatter optics at the 'target'.
Put the LDR in a 1 inch plastic tube,20 inch long and paint the tube inside with black flat paint(used for school boards). That will keep the ambient light out, and does not inhibit the laser beam.
LDR is too slow. He wants something that *briefly* interrupts the beam to produce a 'meaningful' output from the detector. The 'burglar' is not going to stand in front of the beam until the LDR has time to change resistance.
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You might try adding a potentiometer so you can adjust the sensitivity, then take some time and effort and install the detector in a light-tight tube with a non-reflective black inside coating, and position the detector so the only light it sees is the emitter (laser in this case).
6' should be doable even with your simple system.
You are using a laser for a light source. It should work the way it is, but if you think about it, wouldn't it make more sense to use a narrow very bright laser beam on a small area like a photodiode or photo-transistor? Concentrate the light on a small target then any extraneous light will be much weaker by comparison.
LDRs are used at audio frequencies and work very well. They aren't fast if you are talking about data transmission rates, but in human terms they are very very fast.
His circuit just needs to drop below a reference voltage to trigger, speed isn't important.
I'm suggesting you keep the beam width narrow - unless it becomes impossible to aim. The sensitive area for a photo transistor or diode is less than a millimeter square. So keeping the beam narrow and bright will help.
But go ahead with the photocell I believe it can work too. The photo alarm I built for my motorcycle parking area was easily 6 feet. Across the whole carport ~8 feet and into a small laundry alcove. (down in Puerto Rico where pipes never freeze the laundry can be done outdoors) The carport provided shade from the sun so I didn't need a baffle to keep sunlight out of the detector.
Cap leakage is sometimes defined as a time, the self-discharge time constant. That's how long a charged cap, connected to nothing, takes to discharge itself to 37% of its original voltage. It can range from minutes to years. Good film caps can be many years. Electrolytics vary a lot, minutes to weeks maybe.
Sometimes caps are rated for megaohm-microfarads, which is exactly this same time constant. Namely Rl * C, where Rl is the equivalent leakage resistance.
Film caps are usually way, way better than the circuit they are used in.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
Big oil or film metal-can energy-storage caps are usually shipped with a bunch of bare wire shorting the screw terminals. This prevents killing customers.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
Those are typically quite low F value caps, though. Not much energy can be stored in them. In any case, why not just send them out uncharged? Seems the obvious thing. :-/
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Basically a parasistic parallel resistance that drive some current inside the cap. For electrolytics, there are ionic recombination too. Also thermal shocks can affect the auto-discharge rate. For electrolytics,, those parameters merely affect the nominal value. The tolerance (+/10...20%) has no relation with auto-decharge.
The more you put caps in //, the more you put these resistor in // and
With caps in series, it is the opposite.
I think tantalum ones are the worst. And as already written above, mica and plastic films the best, and of course, air ones for low values. Oil and glass caps are rather good too but uncommon (UHT voltages). The auto-healing once "holed" lower their // resistance.
electrons and the // resistance is a matrimonial agency !
There are some effects that cause capacitors to develop bothersome charge even after having been discharged. One is dielectric absorption, sort of like charge hiding away in nooks and crannies for a while, before coming out again when you least want it. Another is the slow accumulation of stray free charge. That's probably self- limiting.
Anyway, it's safer to ship big capacitors shorted. I've had some nasty surprises in cases where this wasn't done.
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