As the name implies, a resistor imposes a resistance (often called impedance) on current flow. I'm sure most people reading this will have, at some stage, encountered the dangers of excess current and the damage it can do to. Thus, the resistors most common use is to limit current to safe levels. Cheap insurance....
The value of a resistor is identified by a series of at least 4 colored bands. This because on components as small as 0.25W resistors a printed value would be very difficult to read and very easy to rub off.
The code itself is quite simple; The first two bands represent the two (or three) most significant figures of the resistors value, the next band is the multiplier or number of zeros, and the last band is the tolerance. (how far off the stated value the part could actually be) The color code is given below.
This resistor is quite obviously 120KΩ.
The gold and silver multipliers will make the value smaller. The color code "Brown
equals 15 x 0.1, or 1.5Ω.
Tolerance is better described as accuracy. If you have a 1KΩ resistor with ±5% tolerance, you could have a resistor with a value anywhere between 950Ω and 1050Ω. (50Ω is 5% of 1000Ω) I've never seen a 10% tolerance resistor myself, and would probably decline to use it in most cases. If you can afford an extra 5c per piece, 1% tolerance resistors are a good rule of thumb. (in my opinion anyway)
Occasionally you will find resistors marked in a different fashion. A resistor with a single black band is a "0Ω" resistor or an expensive wire link. Some high power resistors (over 1W) may have their value printed on them explicitly or in numeric IEC code.which is the same as above but it uses numbers, not colors. This is quite rare.
Below is something a surprising number of electronic enthusiasts do not understand, or use. Yes, it's Ohms Law again. Basic, but somewhat fundamental to electronics....
Using ohms law, we can quickly calculate the required resistance to obtain a specific maximum current at a certain voltage. Say you have an high intensity LED, one of those cool electric blue ones, and you want to get the best light show you can without toasting the LED.
Somewhere on the complicated looking piece of paper often supplied with high performance LEDs will be a maximum forward current rating, usually along the lines of 40mA. To find the resistor needed, simply divide the supply voltage (which can be anything up to the LEDs breakdown voltage) by the maximum forward current. At 13.8V, (in a car) a 345Ω resistor is required. A good idea would be to take this up to the more readily available 390Ω rather than 330Ω. (which would result in more than 40mA) From ohms law again, we can find that 390Ω will allow 35.4mA to flow.
Of course, ohms law is not limited to LEDs. It can be used in any situation where two of the three variables are known. As voltage is almost always known, I mostly use ohms law to determine resistor values.
Variable resistors have a dial, knob, or screw that allows you to change their resistance. The value of a variable resistor is given as it's highest resistance value. For example, a 500 ohm variable resistor can have a resistance of anywhere between 0 ohms and 500 ohms. A variable resistor may also be called a potentiometer (pot for short).
as their name suggests, are resistors whose resistance is a function of the amount of light falling on them. Their resistance is very high when no light is present (up to millions of Ohms), and significantly lower when they are illuminated (hundreds of Ohms). These are also often called Light-dependent Resistors (LDRs) and Cadmium-Sulfide (CDS) cells.
Voltage Regulator IC's
have various uses, but are usually used for feeding 5V to anything which uses digital logic. Voltage regulators can be easily bought as an IC so you do not need to make one on your own . . . unless of course you have crazy power or
voltage requirements - such as for robots with ray guns for eyes.
Some examples below are the LM323 5amp can, LM350T, LM 723 Dip, LM7805 To3.
The Sharp IR Range Finder is probably the most powerful sensor available to the everyday robot hobbyist. It is extremely effective, easy to use, very affordable ($10-$20), very small, good range (inches to meters), and has low power consumption.
How it Works
The Sharp IR Range Finder works by the process of triangulation. A pulse of light (wavelength range of 850nm +/-70nm) is emitted and then reflected back (or not reflected at all). When the light returns it comes back at an angle that is dependent on the distance of the reflecting object. Triangulation works by detecting this reflected beam angle - by knowing the angle, distance can then be determined.
The IR range finder reciever has a special precision lens that transmits the reflected light onto an enclosed linear CCD array based on the triangulation angle. The CCD array then determines the angle and causes the rangefinder to then give a corresponding *analog value to be read by your microcontroller. Additional to this, the Sharp IR Range Finder circuitry applies a modulated frequency to the emitted IR beam. This ranging method is almost immune to interference from ambient light, and offers amazing indifference to the color of the object being detected. In other words, the sensor is capable of detecting a black wall in full sunlight with almost zero noise.
Also here in the photo are other IR LEDS, and detectors and emitters, Including a QTI line follower sensor mid right.
IC's Intergrated circuits
LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:
Example: Circuit symbol:
LEDs emit light when an electric current passes through them.
Connecting and soldering
LEDs must be connected the correct way round, the diagram may be labeled a
for anode and k
for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).
LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.
Testing an LED
Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.
LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!
Colors of LEDs
LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colors.
The color of an LED is determined by the semiconductor material, not by the coloring of the 'package' (the plastic body). LEDs of all colors are available in uncolored packages which may be diffused (milky) or clear (often described as 'water clear'). The colored packages are also available as diffused (the standard type) or transparent.
The most popular type of tri-color LED has a red and a green LED combined in one package with three leads. They are called tri-color because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.
The diagram shows the construction of a tri-color LED. Note the different lengths of the three leads. The center lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third color.
A bi-color LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-color LEDs described above.
LED's come in a variety of sizes, 3mm,5mm,10mm,20mm and a vast range of lumins [brightness].
A relay is an electrically operated switch
. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw
Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.
The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.
Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches
Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.
The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.
The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.
Relay showing coil and switch contacts
Protection diodes for relays
Signal diodes are also used with relays to protect transistors and integrated circuits from the brief high voltage produced when the relay coil is switched off. The diagram shows how a protection diode is connected across the relay coil, note that the diode is connected 'backwards' so that it will normally NOT conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.
There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labeled + and -, the two AC inputs are labeled
Example: Circuit symbol:
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.
Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop
and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage
(usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown
Signal diodes (small current)
Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.
For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.
a = anode, k = cathode
Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.
Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:
- The minimum voltage available is 2.7V.
- Power ratings of 400mW and 1.3W are common.
I hope this helps with selecting your components for your project.
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