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After venturing to the computer store to buy our kits, Chris and I came back to the lab with two breadboards and nothing to put into them (next shipment of kits arrives in 1.5 weeks). We went scrounging for supplies and managed to do Step 1 without much trouble (at least, it looks pretty easy now that I've actually done it). At first there was some confusion when the third LED was inserted into our circuits, but then we remembered one of the demos from class. It's interesting that two LEDs can be lit up pretty well--they're not exactly dim--but add a third one, and they can't muster anything. Chris found a photoresistor and played with it. Pretty cool. We think it's amazing that this thing probably costs half a cent. There's still an issue that we're struggling with. Debating it with electrical engineers over napkin drawings at lunch got us nowhere: why is an LED that comes after the resistor just as bright as one that comes before the resistor? If current has a directional flow, shouldn't a pre-resistor LED suck up everything before it can get to the resistor? It seems the purpose of a resistor is to soak up excess voltage. So putting it after the LED seems like putting a water filter underneath your glass as you hold it under the tap. A few people have told us the circuit fills up with X volts so quickly, the difference quickly balances out. This only frustrates us more. Electricity looks instantaneous to us, but we aren't electrical components. Can someone enlighten us? Each component in series converts some of the voltage to another form of energy. When there are other components in series before or after it to convert the rest of the voltage, it only converts what it needs. When there aren't, it has to use all the energy, and explodes. Think of it like this: If I jump from 90 feet to the ground, I convert all the potential energy of the height into kinetic energy, and I die. If I jump from 10 feet, there's a smaller amount of energy to convert, and I live. If I jump from 90 feet onto a pile of pillows that's 80 feet tall, I only fall 10 feet, and I live. Just because the difference between me and the ground is 90 feet, doesn't mean the difference between me and the thing I actually fall onto is 90 feet. You're measuring relative voltage, not absolute. I like the "each component takes only what it needs" explanation. It helps. For my project I made the power go from power regulator --> a resistor --> a push button switch --> a green diode --> a variable resistor --> 3 red diodes. So to light up the red diodes, you need to first press the push-button switch until the green diode lights up and then, while holding down that push-button switch, you can turn the variable resistor to dim/brighten the red diodes. But when the red diodes brighten, so does the green, which I don't understand. Doesn't the current only flow in one direction?
Also, when would a variable resistor need to be grounded? Fiona, did all LEDs (including green) go out when you turned the pot to minimum resistance? Based on observations from step 2, it seems they would, unless you upped the power. Fiona, your answer lies in the nature of electricity, when you drop the resistance in your circuit, your current will go up according to ohms law V=IR thus causing your green diode to brighten up assuming you connected your LEDs in series. Another question rise from this point though, how can we prevent our green diode to get brighter, what comes to my mind is capacitors although I am not hundred percent positive. Maybe someone more experienced enlighten us at this point. hth. Matt, actually as you turn the pot towards minimum resistance, the green diode dims with the red ones, but then when it reaches minimum resistance, the green gets bright again (even though the red ones are not lit). My Step 1 board came together pretty easily. The picture in the lab has the black above the red going into the regulator but I decided to stick with my notes from class and the handy "IGO" neumonic device. When I got to Step 2, I figured out that I actually had my power coming in backwards. As a result, the regulator had gotten very, very hot. I fixed the power issue and threw out the regulator, replacing it with a new one. Three lights offered too much resistance to light up, but two worked OK. Step 3 went fine. For Step 4, I ran with the photocell theme and set up 4 photocells physically close together on the breadboard. Only two of them were connected to the circuit, however. They were connected in parallel to eachother and then serially to a red light. Only by covering the correct two photocells could the red light be turned all the way off. It was basically a very primitive combination lock. My status at this moment is I am still gathering gear. I found a few good sites online for parts, though the bookstore might turn out to be advantageous if the shipping delay causes me to be behind, which I will be for tomorrow. I do have my power chord, but my AC is en-route. I understand the lab pretty well, however I'm still waiting. I make daily trips to the computer store. . . . . Steps 1-3 all went smoothly. Putting 3 LEDs in series meant that the circuit had a total resistance that when multiplied with current is > 5V… right? But 3 LEDs in parallel allowed enough current to flow so that each LED was dimly lit. Being a dimwit, I didn’t know how great my current was, or what the resistance of each LED was, so I couldn’t do the math to work out exactly what was going on. I then went ahead and made my own switch using wires with stripped ends. Was it the low voltage, the low current (or both?), that means I didn’t electrocute myself? I played around with variable resistors – both photoresistor and potentiometer. Why does the potentiometer need to be connected with a third wire, when the photocell does not? The bookstore ran out of kits last week and have been short on many supplies. I had a bit of my own personal stash to finish the first lab. For the second lab, I was missing the clock crystal and the capacitors, but luckily a second year student gave me a resource for a vendor in china town (259 canal street between Lafayette and Broadway) and I was able to pick up my missing parts yesterday. I can't speak for the quality of the merchandise only because I'm too much of a newbie to be able to spot quality from not. But if you are in a jam and need a local place ASAP try this out. Retroactive post here. It's taken me a while to set up my blog properly. I guess better late than never. I was on the class notes section before and saw hardly anyone had posted. Today in the firmware lab, Mike told me about the lab section. Anyway, what I made technically was neither a game or a combination lock, but it had implications for both and seemed to have the right mixture between theater and function. I made switches out of clothespins by putting conductors on the tips that would touch when the spring loaded pin closed. Two of these switches were clamped onto a cd with slots cut into them. the cd was mounted onto a spindle so it would spin. (Imagine a turntable, but the stylus is pinching the record from both sides. The record has holes in it, so periodically, the stylus slips off the edge into the hole. Come find me if you can't visualize this!] As it spun, the clothespins would make an electrical contact when ever the tip slid into a slot. The switches were each hooked up to an led that lit when the switch made contact. By varying the length and and depth of the holes, the pattern would determine the duration and timing of the flashing leds. I call the contraption a "two bit hard drive." I think of it as a rotating punch card storing binary info (albeit inefficiently), in the configuration of current storage media. Documented my lab a while ago but never posted. Here it is. Jet lab #1 |