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Turning on a light through glass

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JoeDarkon

Thu Dec 20 2007, 11:22AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

Hi I am a student at a university and our mail arrives in boxes kind of like a p.o.box. On the front there is a combo lock and a small window so we can see if there is anything in there without opening it. The problem is that there is bad lighting so its hard to peer in without a flashlight. So as a matter of convenience I wanted to put a little led in there so I can see in and save me the extra minute of having to open the lock. I know I can just throw a led in there and it will last a long time but I wanted to make it switchable from the outside. I dont want to spend more than a few bucks and I may even have most of the required parts to build something that can switch it on through the glass. The best idea I have is to put a magnet up to the glass which would pull a piece of metal to make contact with another to complete the circuit this would be pretty easy to set up but I wanted to know if any of you had other ideas because this would mean I would have to bring a magnet around with me a lot and I just know that will end up doing damage to either my cell or my credit cards or something.
thanx for any help

Zum Beispiel

Thu Dec 20 2007, 11:27AM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

IR led + phototransistor? Or magnet + hall-effect sensor?

Capper

Thu Dec 20 2007, 12:26PM

Registered Member #914 Joined: Fri Jul 20 2007, 06:22PM
Location: South Bend, IN
Posts: 85

CDS photocell circuit + keychain LED flashlight.

or...

Capacitive sensor attached to the glass.

or...

Get some hot college babe to get your mail for you...

Dr. Slack

Thu Dec 20 2007, 12:51PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It must presumably be zero or low power.

A reed switch inside and a magnet outside would be a good zero-power choice, but if you can't be bothered to carry a torch, then will you be carrying a magnet?

A very low power way, which requires you to carry nothing, would be a through-glass capacitive sensor. There are several ways to do it. All work by sensing the change of capacitance to ground of a sensor patch inside the glass, as a result of you touching the outside of the glass. You could sense the change in frequency of an oscillator built with this capacitor, you could sense the unbalance output of a bridge using this capacitor as part of a Wheatstone arrangement. The trick is to do it at very low power. My suggestion is to sense the change in delay when the sensor is part of an RC network.

CMOS consumes practically nothing when static. Build a low frequency oscillator (in the high audio range) using a small cap (to minimise active power consumption) and huge resistor round an HC14 hex schmidt, then use it to drive *two* RC delay networks in parallel, one using your sense capacitor, the other using a reference capacitor. The two delayed waveforms are then squared up by two more of the schmidt gates, and applied to the clock and data of a HC74 D latch. This will measure the *differential* delay between the two edges, compensating for voltage and temperature variation. Adjust your reference delay so that the D latch switches with a 50% touch. HC is useable from 2v to 6v, so no need for a power-wasting regulator, just 3 or 4 AA batteries in series.

Caution - I've not built this, it just sounds like a reasonable idea. I'm not certain how practical things like power-line pickup to the sense electrode would affect it. You can sreen the back of the electrode, if it's a metal box then it is automatically screened, but the front needs to be accessible.

Other ideas. Glue a contact mike on the back of the glass, build a low power amplifier round a transistor biassed at 1uA or so to get it to detectable levels, rap on the glass with a coin to active a monostable for a few seconds.

The best solution however would be to have a torch on your key-chain, and use that to look through the window!

Bored Chemist

Thu Dec 20 2007, 01:13PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

A reed switch and a magnet tied to the post box with a bit of string would work.
Selling the idea to the others who use the post boxes might not just make you a bit of beer money but it would mean they were not likely to pinch the magnet.

Ken M.

Thu Dec 20 2007, 01:54PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I wouldn't worry about a standard or even neo magnet harming ur credit cards, Considering it takes I think 50K Gauss of magnetic power to even start to effect the cards data. (sorry if i misspelled the unit of measure for magnets) Also I'd say just use an Alu switch that will activate the led when mail is inside ur mail box and then just have a low power oscillator to turn it on and off to reduce power consumption.

HV Enthusiast

Thu Dec 20 2007, 02:22PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Just a word of warning . . .

You DO NOT want to put anything in that PO box with wires / electronic circuits / etc... especially in this day of age. Sure, it may only be an LED and battery, but to the postman / postmaster, they have no clue and being federal property, that can be very bad.

As a moderator of 4hV, i want to put on record that we do not advise doing this unless you get permission from your university post office.

Also, if you were planning on "modifying" the PO box in anyway, that that in itself is a federal offense.

Of coures, this assumes your PO Box is actually a US Post Office box. Otherwise, use common sense and whatever rules your university has regarding their mail boxes.

JoeDarkon

Thu Dec 20 2007, 08:41PM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

Thanx for all the ideas I know its a bit of work when I can just carry a torch or magnet but what about all those times I will forget them? you know the whole absent minded thing. So I like the sound activated one and the capacitive touch one. Can I get a little more help with whats involved in those? Im used to dealing with higher power things and haven't really been so successful with smaller circuits.

oh and its not actually a PO box its serviced by students I just want it cheap in case the mail person swipes it

Dr. Slack

Sat Dec 22 2007, 10:41PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I reckon your best bet would be to go for the sound activated one. If you don't have a contact mike, then an ear-bud would do. See if you can score a sample MAX4038 from Maxim, quiescent current < 1uA per each of the dual amplifiers, use one as an amp and the other as a monostable. Google for amplifier and monostable circuits, and you'll have to do some sensitivity experiments.

Simon

Mon Dec 24 2007, 02:45AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

WhiteArc wrote ...

I wouldn't worry about a standard or even neo magnet harming ur credit cards, Considering it takes I think 50K Gauss of magnetic power to even start to effect the cards data. (sorry if i misspelled the unit of measure for magnets)

I can't say I've had a bad experience wiping my cards with a NIB but I wouldn't recommend keeping one near a credit card . (It's not the same, I know, but a NIB from a HDD can wipe a cassette's tape with a single close swipe.)

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