Computing desk
< July 8	<< Jun \| July \| Aug >>	Current desk >

Welcome to the Wikipedia Computing Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 9[edit]

I²C over COM port[edit]

hello, does someone by chance know of a program or a library that can bitbang the I²C protocol over an RS-232 port (or, rather, an FTDI style breakout board with TTL levels), for communicating with I²C devices, preferably under Linux? I'm aware of the electrical differences between I²C and TTL Asmrulz (talk) 04:29, 9 July 2016 (UTC)[reply]

Our I2C article has some suitable source code. Tevildo (talk) 08:39, 9 July 2016 (UTC)[reply]

Wattage[edit]

Is it possible to measure an L.E.D.'s actual watt using a typical Multimeter (supposing it can measure Current, apart from Voltage, both in AC and DC) ? 210.56.110.175 (talk) 05:42, 9 July 2016 (UTC)[reply]

Not with one measurement. You need to measure the current, disconnect the multimeter, reconnect the supply, then measure the voltage. Tevildo (talk) 09:29, 9 July 2016 (UTC)[reply]

Incidentally, if you have a simple arrangement with a resistor in series with the LED, you can measure the current by measuring the voltage across the resistor, and using the equation

I={V \over R}

. You still need to make a second measurement of the voltage across the LED, though. Tevildo (talk) 09:41, 9 July 2016 (UTC)[reply]

Many thanks Tevildo. If it's not taking too much advantage of your generosity, I'd beg you to be a bit more elaborate. For instance in first instruction you say "measure the current", now shall I measure it along with LED attached or LED removed from the circuit. And is it true that current is never measured like voltage (in parallel) but in series ? As for putting resistor in series with LED that ain't difficult, why not simply jump to this step ? Please tell how voltage across resistor and across LED ultimately give us wattage. Unless I am very much wrong I think current and wattage are separate things. 124.253.145.142 (talk) 17:00, 9 July 2016 (UTC)[reply]

No problem, it's what we're here for. I'm assuming your circuit is basically the same as the one to the right ~~(from our LED article, with annotations).~~ To find the power, we need to find the voltage across the LED and the current through it, and multiply them together.

Measuring the voltage across the LED is easy:

Set your multimeter to "DC Volts".
Connect the positive lead to point B on the diagram, and the negative lead to point C.
The reading on the multimeter is the voltage across the LED. Let's call this $V_{LED}$ . For example, it might be 3 volts.

There are two ways to measure the current. The first is to measure it directly with the multimeter:

Disconnect the wire between the power source and the LED (between points A and B on the diagram).
Set your multimeter to "DC Amps".
Connect the multimeter positive lead to point A on the diagram (the supply positive output), and the negative lead to point B (the LED anode).
The LED should now be on. The reading on the multimeter is the current in the LED. Let's call this $I$ , as in the diagram.

The second way is to measure the voltage across the resistor. This assumes you know what its resistance is.

The circuit should be connected as in the diagram (with the LED on).
Set the multimeter to "DC Volts".
Connect the positive lead to point C, and the negative lead to point D.
The reading on the multimeter is the voltage across the resistor. Let's call this $V_{R}$ .
If the resistance of the resistor is $R$ , the current in the LED is $V_{R} \over R$ . For example, if $R$ is 100 ohms, and $V_{R}$ is 2 volts, the current ( $I$ ) is 20 mA (0.02 = 2 / 100).

Now we know

V_{LED}

and

I

, the power of the LED (in watts) is

W=I\times V_{LED}

. Using the numbers from above, the power is 20 mA * 3 V = 60 mW. Hope this helps! Tevildo (talk) 18:08, 9 July 2016 (UTC)[reply]

I suggest you get a device like the Kill-A-Watt meter, which does the math for you and lists the wattage directly: [1]. StuRat (talk) 17:08, 9 July 2016 (UTC)[reply]

This is for mains equipment, and the OP just wants to measure the power for a (DC) LED. Tevildo (talk) 18:08, 9 July 2016 (UTC)[reply]

To be more specific, if you try to measure the watts of an alternating current circuit by measuring volts and amperes separately and calculating the result, you end up with volt-amperes, not watts. In direct current circuits watts and volt-amperes are identical, so the technique works. The math is easy: volts times amperes equals watts, as Tevildo explained in detail above. --Guy Macon (talk) 22:09, 9 July 2016 (UTC)[reply]

Note: High power LEDs, also called compact light sources, were operated in pulse mode due more efficient power supply. The pulses may not get captured correctly by the multimeter. An oscilloscope lets you know, but note, the oscilloscope is connected to ground. Aware of shorcuts. --Hans Haase (有问题吗) 20:53, 12 July 2016 (UTC)[reply]

Thanks for reply, Hans. If you don't mind, I'd like to know what is meant by "connected to ground" ? And how does it disqualify an oscilloscope from correct and safe gauging. I think doesn't that simply mean that like many other electric appliances that run on A.C. mains, it has its outer metallic body attached to a 3rd wire that takes back any leaked current, through a cable common to whole house or complex, back to be sunk into the ground through a conductor buried considerably deep into it ? - O.P. — Preceding unsigned comment added by 150.129.198.38 (talk) 04:58, 13 July 2016 (UTC)[reply]

Unlike a portable multimeter that has two uncommitted probes (red +ve and black -ve), oscilloscope inputs are usually coaxial with the outer (shield or screen) permanently connected to the case and ground. If the LED circuit under investigation is powered by battery or some other floating power supply there is no problem. However if the LED circuit is part of mains-powered equipment whose circuit is connected to ground, the extra route to ground through the oscilloscope may cause a damaging short circuit. Some two-channel oscilloscopes offer a differential measurement mode that overcomes this limitation. It is also possible to "float" an oscilloscope without its ground connection but that strategy is risky even in the hands of experienced professionals. AllBestFaith (talk) 10:58, 13 July 2016 (UTC)[reply]

Chinese (and other exotic) Characters[edit]

Whenever a server sends traditional or simplified Chinese (same about some other languages also) as text, instead of occurring as they should, they rather appear to be rectangles confused from within, as shown in the [2] picture here. Please tell me what should be done to make them occur naturally ? 124.253.145.142 (talk) 16:17, 9 July 2016 (UTC)[reply]

You will have to upgrade to an operating system with the Unicode fonts installed. You can install more fonts on your computer as well. Graeme Bartlett (talk) 22:35, 9 July 2016 (UTC)[reply]

Thanks, Graeme Bartlett...it works ! see → 活動写真

Airlines use of continuous stationary[edit]

Why do airlines still use dot matrix printers and continuous stationery? I hear them at gates, and I assume they're printing passenger lists. The only advantage I can think of is that if you're clutching a wodge [spelling?] of continuous paper, you know you've got it all. Hayttom (talk) 20:55, 9 July 2016 (UTC)[reply]

One advantage I can think of is that if the ink runs out, you still have the indentations in the paper you can read, especially if you run a pencil lead over it. But while we are on the topic, why do most retail establishments in the US still use thermal printers for receipts ? StuRat (talk) 23:23, 9 July 2016 (UTC)[reply]

Great follow-up question. I hope we get some answers.Hayttom (talk) 14:21, 10 July 2016 (UTC)[reply]

One obvious factor is cost. The best technology to replace these old technologies, as far as readability, is likely a black-and-white laser printer (color seems like an unnecessary expense). I imagine they cost more initially (but maybe not by much, if it only prints the width of a receipt or airline ticket). However, for these high volume operations, the more important issue is what it costs per print. Can anyone provide data on this, for dot-matrix, thermal, and black-and-white laser printers ? StuRat (talk) 15:13, 10 July 2016 (UTC)[reply]

Also, (nearly) all laser printers are page printers, i.e. they print full pages. Dot matrix printers are line printers. If you need a permanent record immediately, a page printer will waste a full page on each transaction, even if you only need a single line. --Stephan Schulz (talk) 19:03, 10 July 2016 (UTC)[reply]

But is their any inherent reason why laser printers can't be designed to print smaller areas (hopefully at reduced cost) ? StuRat (talk) 22:25, 10 July 2016 (UTC)[reply]

StuRat You mean like a laser receipt printer? Probably far more economical to use a thermal printer. (See my answer below↓ too) If someone wanted to throw enough money at it, it is probably possible to shrink a laser printer down like that. I imagine you could write to thermal paper with a laser, thus avoiding the use of toner? - 220 of ^Borg 05:38, 11 July 2016 (UTC)[reply]

Those are most likely line printers, not dot-matrix. --Guy Macon (talk) 19:32, 10 July 2016 (UTC)[reply]

As to the use of thermal printers, they're very quick, but a key advantage may be that there's only one consumable (a thermal roll) rather than paper/ribbon or paper/ink or paper/toner - relatively speaking, this makes them a very much more easy to maintain than a printer which requires both. --Tagishsimon (talk) 22:34, 10 July 2016 (UTC)[reply]

re StuRats' query about Thermal Printers, I agree the big advantage of thermal printers may be speed. Where I live now they use a small thermal printer on a 'kiosk' at the library to print list of book etc borrowed. I have worked a lot (in the past) with inkjet log printers, and I was surprised that in this application were practically instant, i.e you tap the touchscreen asking for a 'receipt' and bang its there. Barely get the chance to lift my finger off the screen. Agree with Tagishsimon, in other words. I also think it's likely they can economically be made very small, likely more so than a laser. Thermals seem to be practically universal for cash register receipts.220 of ^Borg 05:38, 11 July 2016 (UTC)[reply]

Re the speed of thermal receipt printers, source here says up to 250 mm per second, so 10 inches/sec, 600 in/minute or ≈50 feet,(15 metres) per minute. That is pretty fast! 220 of ^Borg 05:49, 11 July 2016 (UTC)[reply]

Airlines were among the earliest earliest adopters of computer technology. It's entirely possible that some of what we see today just comes from the old adage of "if it ain't broke, don't fix it". If it works and fits their requirements, (including costs of consumables and break down frequency), sometimes it would cost a lot more to "re-engineer" something to work with a newer "peripheral" so you make do with the old one, until it becomes unviable. We have these old Dialogic Inc. fax cards called "brooktrout", they're old and obsolete but to replace the service will cost a fortune, so we just keep using them until they fail and then replace them with a different service. They might last 10 years or they might last 2 months but there's no point paying to change them over while they're still working. I'm not saying THAT is what's happening with line printers in airports, I'm just giving one example of why seemingly old obsolete computer hardware doesn't get replaced. Vespine (talk) 03:45, 11 July 2016 (UTC)[reply]

See If It Ain't Broke, Don't Fix It: Ancient Computers in Use Today. ---Guy Macon (talk) 04:32, 11 July 2016 (UTC)[reply]