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# Cabwood

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1. ## Paralell resistors

I believe you are confused because you stated that two 50 Ohm resistors in parallel causes them to become 25 Ohms each. This is incorrect. They stay 50 Ohms each, but the resistance of the whole becomes 25 Ohms. A single water pipe has a certain resistance to flow, but two such pipes permit twice as much flow. The same principle applies with electrical resistance. Two identical parallel paths provide half the total resistance that one would provide. Bugger. Sorry Zeppelin.
2. ## LED dimmer

Sorry, max power will be about half supply voltage multiplied by half max current, like this: (12V / 2) * (160mA / 2) = 0.48W
3. ## LED dimmer

Yes, you can connect as many resistor/LED pairs as you like, limited only by the current and power rating of Q2. At full brightness each resistor will have about 10V across it (12V supply - 2V across diode). Use Ohm's law to work out the resistance and current for each LED. Knowing the current, multiply by the number of LEDs, and you have the total current flowing through Q2. Example: 8 LEDs, each @ 20mA. The current limiting resistor will be (12 - 2) / 0.02 = 500 Ohms. Total current = 8 * 20mA = 160mA. The power disspipated by Q2 peaks about half way between fully off and fully on, when it has 6V across it. This condition doesn't last long, of course, but you need a transistor that can handle it. To calculate the maximum power, multiply total current by half the supply voltage. Example: 160mA * 6V = 0.96W So, for these examples, choose a transistor that can handle 160mA, 1W.
4. ## LED dimmer

You could pull out Q1 and connect Q2's base where Q1's base used to be (betwen R1 and R2). This might work - though if it does the ramp probably won't be very linear, and the lighting and dimming not very smooth. Try it! As for the LEDs being always lit, they won't if Q2 isn't conducting. The whole idea of the circuit is to slowly bring Q2 into and out of conduction, thus slowly increasing and decreasing the current through the LEDs. This all has little to do with voltage across the LEDs, and is more about controlling the current the through them.
5. ## LED dimmer

Hey Theatronics. That's what I said. PWM. So that's what I did. But it's two transistors, a comparator, two caps and half a dozen resistors. I think one could wrestle a 555 into doing this, too. Slackjack wants simple, so I'll leave it at that. I was thinking of another way though, which interests me - that is to use a photodiode to feed back to some amp a measure of the light intensity from one of the diodes, and use that to straighten out the luminosity curve. Just another "way".
6. ## LED dimmer

I can't imagine a way of causing the lights to slowly dim AND slowly light without the use of a huge capacitor (or two - each 20000uF or more) and a double pole switch. Even then the initial charge/discharge rate of the capacitors will be high, dropping off with time, meaning there won't be a smooth transition from light to dark and vice versa. The use of transistors simply makes everything smooth and small.
7. ## LED dimmer

Here's one I knocked up. The trouble with LEDs is that their apparent brightness does not change linearly with current, so this design might not be as smooth as you want. But it works. It's a ramp generator - press the switch and the voltage at Q2's collector slowly and linearly drops, gradually powering up the LEDs, release the switch and it rises. Choose LED current limiting resistors as you would normally, for the supply voltage in use. As it is, this circuit's output voltage changes at about 3V/s, meaning it completely lights up after 4s with the switch pressed, and takes another 4s to completely extinguish with the switch open again. You can choose a different capacitor value to change this duration: C = 2.5e-6 * T where T is the time taken to fully illuminate or extinguish. I used a darlington pair, because I have no idea what kind of current your LEDs require. Q1 can be any small NPN signal transistor. I used a BC108. Q2 will depend on the total current you need for your LEDs. If you need more than a total of 50mA for all of the LEDs, I suggest you use a chunky transistor like a TIP31. What I'd really like to do, to make the lighting and dimming more linear, is pulse power to the LEDs and change their brightness by modifying the pulse's duty cycle, a la PWM. So I'm going away now to play with that idea.
8. ## Thinking about getting into electronics??

Fifteen years ago I was fixing PCs with an oscilloscope, logic analyser and a solder station. Fantastic! These days one almost never gets to use more than a screwdriver. Hobby electronics seems to be the only place to use the knowledge I have. So now I work with horses instead, which have an excellent track record - apparently the last major change they underwent was about 8 million years ago.
9. ## Interfacing circuits to the PC

Before you make your decision about what port to use, check this out: http://www.sparkfun.com/commerce/product_info.php?products_id=762 This "Bit Whacker" looks perfect for simple and modern interfacing with any USB equipped computer! On plug-and-play USB systems it identifies itself as a serial port, and shows up as such in Windows device manager. Linux too should automatically create a TTY device for it. This means of course that your program treats it like a serial port, but with all the speed and portability of USB. Magic! SparkFun (OK, a shameless plug, I know) has tons of other stuff for interfacing, and I encourage you to check out the whole kit and kaboodle.
10. ## Beginner's Question

Different kinds of IC technology use different kinds of transistors. TTL ICs, such as the 7400 series, use bipolar junction transistors (BJT). CMOS ICs, such as the 4000 series logic chips, use field effect transistors (FET). Generally, each stage in a logic circuit has an output section consisting of a pair of transistors in push-pull configuration - that is, one at the top, connected to the positive power rail, and one at the bottom connected to the negative power rail. BJTs have three connections: Emitter, base and collector. The 'c's in 'Vcc' refer to the collectors of all the positive-half (top) output BJTs. FETs have drain, gate and source connectors. The 'd's in 'Vdd' refer to the drains of all the positive-half (top) output FETs. The 's's in 'Vss' refer to all the sources of the negative-half (bottom) output FETs. Thus in general Vcc/Gnd are the labels used to denote the power supply connections on Bipolar TTL IC, and Vdd/Vss are the labels denoting the power connections on a CMOS IC. For bipolar logic ICs, Vcc must be about 5V higher than Gnd. For CMOS logic ICs of the 4000 series, Vdd can be up to 15V higher than Vss. For CMOS logic ICs of the 74HC series, Vdd can be 2 to 6V higher than Vss. There are of course variations on these themes, and you should check the datasheets for an IC before you power it up.
11. ## A "silly" question about a resistor and a led position

Hey Atrats. I'm no noob at electronics, and I am still confused by what people are saying here. Way too complicated. Here's my two cents: Don't think about electrons (or any charges for that matter). There are two reasons for this. The first is that in circuit diagrams and tutorials current is always shown to flow from the higher voltage towards the lower voltage. That is, in the direction opposite to electron flow! They call this diagramatic current flow from higher to lower voltage "conventional current", and if you are worrying about electrons moving around you will only end up confusing yourself. There are very few applications where you need to consider the actual charges (like electrons) that move to make an electric current. Secondly, in a simple loop circuit like the battery/LED/resistor, you can consider that all points in the circuit have exactly the same current flowing through them. Think of water flowing freely around a loop of pipe - if you squeeze the pipe in one place, the water slows down everywhere in the pipe. The same goes for electric current. You can put the resistor anywhere in the loop, and the current is reduced everywhere. As you think of water, and not of molecules, think of electric current, and not of electrons. In fact, look to water as a way of visualising electricity - the rules of electric current and water flow are so similar that you probably know a lot more about electricity already than you think. What makes a water flow? Pressure difference. What makes electric current flow? Voltage difference (mostly referred to as "potential difference). There's your first analogy, water pressure is the analogue of electrical potential, otherwise known as "voltage". To create a pressure diferrence in water, you either use a pump or gravity. In an electrical circuit, you could use a battery. Current, on the other hand is how much stuff passes a point in a circuit each second. For stuff like water, you might measure that in "litres per second". For electrons (each of which carries a charge measured in "Coulombs"), you would say "Coulombs per second". Of course, nobody says "Coulombs per second" - rather we say "Amperes" or just amps. Kirchoff pointed out that, just like water, whatever stuff flows into some point in a circuit must also come out of it. That's common sense, of course. You knew that already, and now you know that the same applies to electric current. You see it in action right here in your battery/LED/resistor circuit. It's not possible to have a different current flow at one point than at another! There are many many little "laws" like this that you probably see in daily use around you, which seem like common sense, and which apply equally to electronics. I'll continue if you want some more pointers.
12. ## Audio Selector

I assume that the cards' outputs drive an external amplifier, and do not directly drive speakers. If so, then since there is no output from the inactive sound card, why don't you simply mix the outputs of the two cards together with a summing amplifier? A single opamp would do.
13. ## Shrinking heat-shrink sleeves

I've successfully used a lighter flame before, and it was OK except that it tends to leave a black residue on the work and it's too easy to melt or burn stuff.
14. ## Pool water level stabelizer

A sealed reed switch won't be affected at all by corrosion or electrolysis. It's easy to find glass encased ones, and as for the solder contacts outside, a bit of silicon glue will seal them nicely (just be careful when soldering glass reed switches to avoid cracking them open). The magnet/switch mating is critical in door/window sensors because the magnet is weak. Stronger magnets can trip a sensitive reed switch from several centimeters away. This application does not require a very strong magnet or a very sensitive switch. As I said, with no moving parts except for the floating magnet, this solution is easy to implement, and the wiring is supremely simple - reed switch and relay coil, powered from 12V DC (or less). As for the assertion that this setup would be more erratic than the electrode idea - WHAT? Electrodes will sense a closed circuit the instant water bridges them, and any wave (even tiny ones) could potentially cause opening and closing at ridiculous rates, until the electrodes are well and truly submerged. Either the pump or relay would soon pop without some kind of timing circuit to reduce switching. The hysteresis in reed switches (or even mechanical switches) eliminates the need for complicated timing or hysteresis circuits, and would end up much, much less erratic than a basic electrode setup.
15. ## Pool water level stabelizer

Electrode corrosion would occur, almost certainly, not due to oxidisation but to electrolysis. To avoid this, the sensing current would have to be A.C., which really complicates matters from a design point of view. My money is still on the reed switch - fix it to the pipe interior, and float a magnet on the water underneath it. As the water level rises, the magnet rises too, approaching the switch. At a couple of millimetres distance or so from the switch, the switch will close, energising the relay, and switching on the pump. No fancy circuitry, no moving electrical parts, no underwater connections and no worries about corrosion.
16. ## Pool water level stabelizer

Now I think of it, mercury in a swimming pool isn't a good idea. Stick with the reed switch idea, or use a tilt switch with a ball instead. Google has all the answers you wll ever need regarding "tilt switch" and "magnetic reed switch".
17. ## Pool water level stabelizer

A mercury tilt switch is a glass tube in which mecury is free to flow from one end to the other under gravity. At one end are two contacts which the mercury flows over, closing the circuit. This has nothing to do with temerature, and everything to do with position. What's wrong with the magnetic reed switch solution? This would be my preferred option, in this situation, for its simplicity and availability, and because there are no exposed electrical contacts. You'll not get enough current to flow between two exposed electrodes (as you suggest) to trip a relay. Besides, the electrodes will corrode over time, especially with DC flowing between them.
18. ## Pool water level stabelizer

Or - perhaps even better - use a magnetic reed switch, like those used in alarm systems to detect an open door or window. I imagine you could float a magnet on the water inside the pipe, and place the reed switch in a fixed position above it. These too are sealed and exhibit hysteresis.
19. ## Pool water level stabelizer

What about a mercury tilt switch? It won't corrode since it is encased in glass, and it has inherent hysteresis which would help reduce erratic switching from small waves.
20. ## Basic Transistor Circuits questions

Perhaps your main concern might be noise. That will determine your choice of transistor right off the bat. Then you look at the noise characteristics of the transistor, and find out the collector current at which the transistor exhibits least noise. What about the input impedance of the next amplifier stage? Your choice of resistors in an amplifier will be hugely influenced by this. The source (say a microphone) will greatly influence your choice of base bias resistors. Your choice of emitter resistor will be somewhat limited by the transistor's gain, and bias current. Perhaps you have to consider battery life. This means your design should draw as little current as possible, usually implying large resistances. Any amplifier design is a compromise based on these kinds of considerations.
21. ## 'potential Difference' questions

Imagine water flowing through a pipe. It's the difference in pressure between the two ends of the pipe that causes the water to flow. If the pressure at the two ends is equal, there's nothing to push the water. The same principle can be used to explain the movement of electrical charges in a wire. Potential difference being what "pushes" those charges, causing a current flow. So, is it correct to say when there is no load across a battery, then there's no potential difference, only E.M.F.? I find that utterly confusing, especially considering that there is ALWAYS a load on the battery, even if it's only the air between the connectors! If you put a valve in the middle of that water pipe, apply a pressure difference between the ends, and switch the valve off to block weater flow, does that mean that now there is no pressure difference between the ends? Of course not. However, from a design perspective, it doesn't often help to exclusively define potential difference as the cause of current flow, or to say that current flowing will cause a potential difference to drop across whatever it flows through. If you have a constant current sink/source in series with a resistor, you imagine that the sink/source will determine the current flowing through the resistor, thus causing some calculable voltage drop across the resistor - in this case you know the current, and work out the voltage from it. That's a "current flow causes potential difference" paradigm, in spite of the above. Conversely, a there are situations where you have a known potential difference, like a battery, and the load (say a resistor/LED) is calculated from a "potential difference causes current flow" paradigm. The paradigm you settle upon(what causes what) during design will be determined by the context of the problem, rather than the nature of electric fields and charges. That's the domain of people who design transitors and such.
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