Circuit basics: Resistance

Last time we discussed the resistor. I promised to do a post on resistance as the resistor is quite useless if you don't know about resistance. So what is resistance?

If you imagine electrical current as a water flow, you can imagine the resistor as narrowing of the pipe as can been seen in the following image. We already learned that voltage can be seen as water pressure. At point A the water pressure could be, for example, 8 Volts. Because of the resistor, the water can not flow freely to point B. The pressure at point B would be lower, let's say 6 Volt. This means that the resistor resists 2 Volts.

Basically that's it. It's not very hard to understand resistance, but using it in your circuit is much harder. Next time we investigate resistance using resistors in a circuit.

Electronic components: The resistor

Well, I'm back from Barcelona, so it's time for a new post. This time about the electronic component: the resistor. As one of the most common components (you will find one in almost every circuit), you should be familiar with it. As the name implies the resistor has much to do with resistance. Because it's a rather big topic, today I will talk about the resistor, resistance is coming up next.

A resistor can be made of a few materials, depending on it's use. Most common is the carbon based resistor. A resistor has one big feature: resisting. So how much does it resist? That depends on the materials used. The color bands on the resistor will tell you how much resistance you can expect (with a slight tolerance).

The resistor in the previous image has four color bands: black, red, brown and gold. The first two bands will give you a number. Black is 0, red is 2, together this will be 02. That number is multiplied by the third band x 10. Brown is 1, so 1 x 10 = 10. If we multiply 02 with 10, the result would be 20 Ohm. The fourth band will give you the tolerance. Gold will tell us that the resistor has a tolerance of 5%. In other words, the resistor will have an actual resistance between 19 and 21. This is an example, you will not find any resistor with black as first band. Under normal conditions you will have no problem with the tolerance. I borrowed the following table from http://en.wikipedia.org/wiki/Resistor:

Color	1st band	2nd band	3rd band (multiplier)	4th band (tolerance)	Temp. Coefficient
Black	0	0	×100
Brown	1	1	×101	±1% (F)	100 ppm
Red	2	2	×102	±2% (G)	50 ppm
Orange	3	3	×103		15 ppm
Yellow	4	4	×104		25 ppm
Green	5	5	×105	±0.5% (D)
Blue	6	6	×106	±0.25% (C)
Violet	7	7	×107	±0.1% (B)
Gray	8	8	×108	±0.05% (A)
White	9	9	×109
Gold			×10-1	±5% (J)
Silver			×10-2	±10% (K)
None				±20% (M)

I find it easier to use a resistor calculator site like http://www.samengstrom.com/nxl/3660/4_band_resistor_color_code_page.en.html.

That concludes today's topic, thanks for reading.

Electronic components: The NE555 IC as a timer

Today we are going to build an astable flip-flop using the NE555 IC. IC stands for Integrated Circuit, which is miniaturized circuit (also called chip). The NE555 IC can be used as a timer. It will switch every X time, which makes it possible to do actions every X time. A flip-flop is a circuit that will switch states, in our example two LEDs which in turn go on and off. Astable means that will switch without external influences.

Let's talk a bit about ICs first. As said, IC stands for Integrated Circuit. You can use it by connecting the pins of the IC. You can figure out how to connect them by reading the datasheet. The following image is the NE555. An IC has a X number of pins, numbered anti-clockwise starting with the pin left of the notch or closed to a dot.

Note that the schematic symbol differs from the actual pin configuration as can been seen on the left.


For more information on what the pins mean see: http://en.wikipedia.org/wiki/555_timer_IC

The following circuit will put the NE555 to use in oscillator mode.
1 x 9V power supply
2 x 680 Ohm resistor (blue gray brown gold)
1 x 1 KOhm resistor (brown black red gold) 
1 x 10 KOhm resistor (brown black orange gold)
1 x Elco 35v 100 uF
1 x NE 555 IC
1 x red LED
1 x green LED

The Elco is a component which I will talk about later. For now it's enough to know it can store a certain amount of energy. The Elco will need to charge. While doing that, the red LED will be on. Upon the moment the energy stored in the Elco crosses 2/3 of the voltage put on pin 8 of the IC (in our case 6 V), the NE 555 will switch, discharging the Elco. This result in the green LED being lit. This continues until the voltage drops to 1/3 (3 volt in our case), in which case the charging starts and the red LED lights up again.

This is the end result, although I have to admit I blew a LED. Be careful when you are debugging the circuit, the Elco may contain voltages way more than  the LED can handle.

That's it for now. I'm on holiday for a few days, so expect new postings next week.