Ask MetaFilter questions tagged with sensor

Count the pumping events

Xhris — Tue, 15 Sep 2009 06:43:17 -0800

I need to count how many times a sump pump runs in a given period. The sump pump pumps water out of a hole on the ground, it is activated by a float switch - so generally speaking it pumps the same amount of water out each time it runs (the volume of water from the top 'on' level to the lower 'off' level). I want to know how much water is being pumped per week/month/whatever. I figure that since the pumping amount is (approx) the same each activation, if I knew how many times to runs (and it's about 4-6 times per day) I can work out the total pumped amount.

Is there a sensor or method you can suggest to do this?
Or to measure the pumped water volume directly maybe?

I'm on a tight budget, so I've been thinking of 'no-cost' solutions like balancing something on the pipe (which vibrates alot) or listening for the sound with a laptop set to record, but I hoping the hive mind can come up with a brilliant idea for me.

Thanks.

Car alarm for my flat?

thewalrus — Sat, 11 Jul 2009 10:48:04 -0800

Security alarm: I'm looking for a portable, battery operated house/office alarm system which can watch the only entrance of a flat. It's a concrete building and the front door is the only way in, unless somebody scales a balcony which would definitely be noticed. This should function much like a car alarm, with a remote key fob to arm and disarm, motion sensor and a opening sensor on the door, and make a very loud noise if it's disturbed. I'll try to describe the ideal form of what I'm looking for, and see if metafilter can recommend anything similar.

It should be about the size of a large paperback book, or hardcover book, and designed to sit near the front door of a premises that has a single entrance (example, concrete condo tower). It should have a sensor to detect if the door has been opened, and/ a shock sensor, with a motion sensor that uses similar technology to the motion sensors that go in the corners of rooms. It should not be reliant on 110V/240V wall electricity, but run on its own battery, or have a backup battery for when wall power is not available. Should be armed and disarmed by a remote key fob similarly to a good car alarm. If it is disturbed when armed, it should make a ridiculously loud noise and attract attention to itself.

(BONUS): If it has the ability to put a GSM SIM in it and program it to send a SMS or dial a number and play a message when it is triggered that would be excellent. This is for use in a third world country where police response is nearly nonexistant, the wired phone network is unreliable, and third party alarm monitoring services (such as ADT) don't really exist. If something makes a really loud noise that's good enough to attract the neighbors' attention since car alarms are still very rare here. If an apartment makes a horrible screeching noise it will bring people running.

I'm thinking of something kind of like this but with connections to attach a door-opening or glass breakage sensor, and possibly an external speaker horn for a louder screech.

http://www.amazon.com/Streetwise-MA795-Strobe-Motion-Remote/dp/B000WDPPD2

Foosball, meet LED Sensor

mikeyk — Mon, 15 Jun 2009 13:22:40 -0800

How to rig up a foosball table to measure velocity of goals? Our company is foosball-obsessed, and I thought a fun side project would be to rig something up inside the goals to measure how fast the ball is moving when it enters the goal. I do mostly software, so hardware/electronics is a fun yet alien world to me. My (uninformed) thought is shining a light on one end of the goal and a light sensor on the other side, and that way measuring how long the light is obscured for. My google-fu is failing me, so any tips on getting started, or resources on where to look next would be appreciated!

Stop sneaking up on me!

teleri025 — Wed, 06 May 2009 12:02:24 -0800

How do I rig a motion-activated alert for my office door? Or just stop being startled when people walk in? I work in a lovely office, however, my computer is situated with my back to the door. Also, my co-workers are often a little on the loud side, so I almost always have on headphones and music. I am constantly being startled or ignoring people when they walk into my office. I don't see them, I don't know they there, and so they startle the daylights out of me when they touch my shoulder or say something very loudly. I even have one staff member who will stand there for a good while, waiting on me to acknowledge her. I've let everyone in the office know I'm not intentionally ignoring them, and to please speak up so that I know you're there, but it doesn't always work out.
I'd like to rig a motion sensor at the doorway to alert me via a light that someone's in the doorway. It's about 10 feet from the door to my computer and no electrical outlet by the door. I've thought about a rear-view mirror, but I'm not sure if those work well. I'm willing to take suggestions, although moving my work station is not possible. I'm kinda locked in to this location because of where the outlets and cabinets are.

interpreting a wave from clipped sensor data

compound eye — Wed, 11 Mar 2009 18:56:32 -0800

interpreting a wave from sensor data - what to do about maxed out sensors that are clipping the wave - an absurdly complex question hello hive mind,

this is the most ridiculously complex question I have ever asked here,
but the ability of metafites to slice through unreasonably convoluted quandaries with graceful ease,
has lead me to fear I have not been doing my share to feed the hive with sufficiently challenging questions

I shall now attempt to remedy this.

I am writing a small program to reads in sample data from a sensor, which is measuring the force on an object which is moving backwards and forwards.
These force values seem - to my surprise- to usually be a bit like a sawtooth wave, I just figured they would be a sine wave.

I've never done anything like this before so I've done a lot of re-inventing of the wheel along the way.

The samples from the sensor have a timestamp.

The samples are:

fairly evenly time spaced, but not perfect.
are digitised to one of 256 values, so are not perfectly accurate

the wave is a quite uneven:

it usually seems to be closer to a sawtooth that a sinewave, but its hard to tell sometimes especially when its clipped
amplitude and frequency wobble all over the place

My program tries to make sense of the wave in realtime.

The program takes particular notice of:

the high and low peaks and
the zero crossings,

From this it creates a sort of sawtooth description of the wave.
The zero isn't perfectly calibrated, but i think its good enough.

To know when the wave has reached a high peak, I wait until the size of the sample value starts to fall, and figure that the highest point was the peak.
This usually works, although sometimes there are wobbles in the signal that confuse the algorithm into thinking the wave has peaked early.
To get around this I have used a threshold "minimum fall from peak value" before I designate a peak has occurred.

Now for the tricky park:

Often the sensor gets maxed out, so my wave gets clipped, which makes it hard to work out where the peak is.

And it makes it really hard to work it out in realtime while the samples are still in the clipped zone.

If a peak is clipped, the next one is usually clipped too.

this what I am currently doing:

if a wave peak was clipped, then next time I cross zero:

I extrapolate a line from the zero crossing point before the peak, through the last good unclipped sample before the peak
I extrapolate another line from the zero crossing point after the peak, through the first good unclipped sample after the peak.

where these lines cross, is where I assume the peak was

this seems to work ok, but if I had some way of better analysing the waveform I'd use it.
This was just the best I came up with (also it all needs to happen in realtime on a not very fast chip)

When my wave moves to the opposite peak, I have a pretty good estimation of the previous peak.

If when approaching the next peak the sensor maxes out, then I assume, that peak is going to be somewhere on the trajectory I am currently on:

If I extrapolate a line from:
the zero point

through:
the last good unclipped sample before the peak.

that the peak will fall somewhere on this line

but I need to know how far along the trajectory the peak will be.
To predict this I've assumed that the peak force on the object occurs at the extremes of its motion, which also mean that the peak force occurs when the object is stationary
As the object is going from rest to motion to rest again, I assume that acceleration impulse and the deceleration impulse from peak to peak must be of equal magnitude.

using:

the time of the previous peak
the size of the previous peak
the time of the zero crossing

I can estimate the impulse which accelerates object

and that the impulse which stops the object must have the same magnitude

rate of increase of deceleration force = good sample value /(good sample time - zero time);

time object will decelerate = square root( (zero time - previous peak time) * previous peak height / rate of increase of deceleration force)

This produces an answer that's not ridiculously wrong, but not bang on either.

so I use this as a tentative value until, next time the samples fall back to zero, when I recalculate the peak size and time.

neither my predicted or later retrospectively calculated answers are perfect, but the retrospective ones 'look' better

I'm not sure where my biggest errors are:

perhaps its treating the wave as a sawtooth?
perhaps its placing too much faith in a single sample when extrapolating trends?

I've been trying to compare my predicted peaks, with the peaks I calculate later after the peak is finished, and try to adaptively adjust the predictions

Any suggestions on what I could do better?
How I could best adaptively adjust my predictions?

cheers
mathew

oh p.s. there is nothing i can do to the sensors or their placement to prevent them from clipping

Heartrate monitor for computer?

cccorlew — Mon, 02 Feb 2009 05:54:59 -0800

I'm looking USB heartrate monitor I'm doing a multimedia project and want to feed heart rate data (actually just the beat, not even the numerical rate) into my computer and read it in MAX/MSP.
I'd prefer a finger clip or ear clip model.
I can find all sorts of monitors, buy none that will let me grab the signal from it. They're all stand alone units.
Suggestions?

How do I replace the evaporative emission sensor in my car?

quin — Wed, 05 Nov 2008 09:44:19 -0800

How do I replace the evaporative emission system sensor in a Nissan Sentra? I have a new-to-me 1999 Nissan Sentra which failed its emissions test with the error P0450 (Evaporative Emission Control System Pres-
sure Sensor Malfunction). I want to replace the sensor, but I have no idea where in the car it actually lives or what it looks like.

I have an ODB-II reader, so I will be able to tell when I do get it fixed, but first, I am looking for a place to start. Suggestions on how much the part will cost or whether it will just be easier to take it in and get it fixed professionally are welcome.

I need a Magnetic and Photbeam Sensor!

kaozity — Mon, 03 Dec 2007 11:11:07 -0800

Can you show me a photo beam sensor or a magnetic sensor that is close to the size of my pinky? My partner and I are designing a cage that is meant to hold a helper monkey. The monkey too smart for a simple lock, so we need to build a system that will ensure that the door is locked when the monkey is in the cage. We decided to use both magnetic and photo beam sensors in specific places of the cage. We need a sensor that is small enough to fit is various spots of the cage. Since I do not know much about sensors, I am not terribly sure what specifics I am looking for. I do know that it needs to be programmable with the rabbit core microprocessor modules (http://www.rabbitsemiconductor.com/products/CoreModules/index.shtml) Any Ideas?

Simple, small, cheap accelerometer/LED?

Zarkonnen — Tue, 13 Nov 2007 12:41:16 -0800

I'm looking for a simple, small and cheap way of measuring acceleration. Basically, I want to make a set of LEDs light up if they are accelerated significantly, becoming brighter the faster the acceleration happens. Searching the web yields only very expensive ($300) three-axis accelerometers that are way too sophisticated for my plans. Is there some simple solution, some widget that produces voltage roughly proportional to acceleration?

Nikon goes full frame

matteo — Wed, 29 Aug 2007 09:40:59 -0800

I'm completely baffled by Nikon's DSLR strategy and I'd like to understand a few things better: first they create the DX line and seem to indicate that cropped is there to stay, now they go full frame. What should a serious film photographer who wants to go DSLR and already owns a bunch of excellent manual focus Nikon lenses do now? I also find baffling that they're not teling who manufactures their full-frame sensors -- don't potential customers have the right to know who makes the heart of the really expensive camera they may buy? Thanks.

Temperature indicators that create thermal profiles.

nickerbocker — Mon, 09 Jul 2007 07:32:07 -0800

I am looking for a temperature indicator that comes in the form of a sheet, or a film, and permanently changes color in areas that have been exposed to high temperature. Does anyone know where I could find such a device? I am running some experiments and would like to have a "thermal profile" of the maximum temperatures across a particular body.

I noticed that the food packing industry have indicators to place on shipments during transit that will tell if a temperature was met by changing color.

Does something exist that can indicate across a span or temperatures? I am particularly interested in 150-800°C. High accuracy is not needed.

Thanks for any help!

Guess it's time for a DSLR...

bjrubble — Tue, 13 Mar 2007 13:28:19 -0800

Why do point-and-shoot digital cameras have such small sensors? I'm about to return my brand new Canon A710IS, because I can't see any improvement in image quality over my nearly 3-year-old Olympus C7000Z. The electronics are much better (most noticeable in autofocus), but the noise, poor color response, and fuzziness are virtually indistinguishable. Even the image stabilization doesn't seem to help that much -- it can turn an unusable photo into a postcard-quality one, but it can't turn a postcard-quality photo into a print-quality one.

After some research, I'm convinced that the problem is the sensor size. The A710IS sensor is actually smaller than the one in the C7000Z. I don't understand this. Why is it that you can only get a larger sensor in a DSLR? Why are the P&S manufacturers putting in better lenses, better electronics, and all sorts of fancy post-processing tricks, but can't seem to improve the one part that actually generates the image itself?

May the force sensor be with you

bluefrog — Wed, 13 Dec 2006 08:50:49 -0800

What is this handheld "force sensor" Jamie Hyneman yields in the "concrete glider" episode of Mythbusters? At some point in the "concrete glider" episode of Mythbusters, Hyneman tests the strength of a sheet of concrete using is "trusty" handheld "Force sensor" (or strain gauge or probe?). Looked a bit like an oversized electronic meat thermometer. What is the exact name of that apparatus? Looking for links to manufacturers/retailers (although these should be easy to come by once I know what to call it).

What happens when, and for how long?

b1tr0t — Mon, 24 Apr 2006 10:46:48 -0800

Are there any good tools out there for analyzing binary sensor data? I would like to create something like a timeline of data. I am running a fairly large monte-carlo simulation in which the output of around 200 binary sensors is recorded. Most of the time, all of the sensors are "off." I need to better how the system goes from being all "off" to having some sensors "on."

Previously, I would visually grep through the SQL sensor logs and observe sensors turning on and off. I would like to create a visual representation of the sensors over time so that I can see the shape of the sensor clusters as they turn on and off.

It looks like it might be possible to trick Excel into doing this by explicitly computing the duration of the "off" events, and creating a stacked bar chart. Excel tends to fall over when dealing with really large data samples, so I would prefer to avoid it.

Is there a better way to do this with a tool like GnuPlot or R? I have decently powerful Windows, Linux and OSX machines at my disposal. I am capable of writing code, but would prefer to avoid doing so if this problem has already been solved.

To clarify - by "binary sensor data" I mean that the sensors are either on or off. The actual data is a SQL table with columns for sensor number, start time (in DMY/HMS plus another column for milliseconds) and end time (same format as start time) among others. The underlying data is actually not binary. Sensors take on a range of values, but I am currently examining the case where a given sensor crosses a threshold - yielding binary data.

My computer tells me it's overheating, but I know that it's not.

Tawita — Sat, 07 Jan 2006 14:25:07 -0800

The temperature sensor on my motherboard is going haywire! With an ambient temperature of around 30° Celsius, Motherboard Monitor is telling me that case temperature is fluctuating between -50°C and 50°C, with CPU temperature fluctuating between -125°C and 110°C. The sensor is obviously fried, but what can I do about it? The computer is a 2.8gHz Pentium 4, about 18 months old, built for me with off-the-shelf components. Motherboard is a Gigabyte 8S648FX-RZ, with firmware updated. OS is XP Home sp2 (updated). I suspect that the sensor problem was causing random shutdowns. I've now stopped the automatic shutdown, which seems to have resolved this issue, but I suspect that this problem is why I frequently have trouble booting up (computer thinks that the CPU is overheating so refuses to boot up?). I live on an island in the middle of the Pacific Ocean, so service options and spare parts are a long, long way away. Grateful for any suggestions.

Honda Odyssey van speed sensor cost?

wsg — Wed, 07 Dec 2005 16:45:02 -0800

What's a reasonable price to pay to replace a speed sensor in a 1995 Honda Odyssey van?

Can I get notifications of power outages by e-mail?

laze — Tue, 26 Jul 2005 12:36:21 -0800

Our dog stays at home while we're out working. She does fine and has people walking her on most days, but with the generally unreliable nature of electric power on hot Virginia days, I worry that the house may lose power and heat up to an uncomfortable temperature while my wife and I are at work. Does anyone have any experience with an inexpensive, easy-to-setup way for me to get an e-mail alert if the power goes out or the temperature gets to a certain point? This looks neat, but if the house loses power it won't do me much good.

How to clean dust off of a DSLR sensor

almostbarefoot — Tue, 19 Jul 2005 12:20:31 -0800

Has anyone successfully cleaned dust off of a digital SLR's sensor on their own? I have a Canon DRebel, and there is a dark spot that appears on every single image, no matter which lens I use, which makes me belive that it is, in fact, dust on the sensor. It's getting to be too frustrating to photoshop the spot out on every single image I take. I've tried using a blower bulb, like Canon recommends, but I haven't had any luck so far. I'd really rather not send my camera away to be cleaned unless it's absolutly nesicary, but at the same time, I'd like to avoid damaging it.

Car Temperature

AstroGuy — Sat, 15 Jan 2005 14:06:14 -0800

[CarTalkFilter] '94 Buick with electronic heat & a/c controls. External temperature reads 94 degrees F (it's currently around 10 above). What do I check out to fix this? [more inside] This is a 94 Park Avenue with the pushbutton electronic controls where you set the desired temp rather than slide levers, etc. The outside temp reading always reads WAY hotter than it really is. I bought this car last fall when it was in the 50s and at that time it read over 120 degrees as the outside temp. Not really a big deal but I think that this temperature reading MAY be used by the computer to control A/C compressor cycling. I've noticed in 20 degree temps if I have it set to the HEAT setting the A/C compressor cycles on and off (burning extra gas unnecessarily).

How would I go about troubleshooting this? A bad sensor somewhere? But where is it located? I was going to pick up a Chilton book for this car but little things like this aren't always detailed in them so I figured I'd ask here to see if anyone has any ideas.

Convincing my camera that the media card flap is closed

britain — Thu, 12 Aug 2004 05:29:04 -0800

Sensorfilter: my digital camera's media card door sensor thinks it's open, even when it's closed. Has anyone ever disabled one of these things? The door stays closed just fine, just the camera doesn't think so.