Presented
here is a simple circuit that can drive two motors for a small robot,
allowing the robot to negotiate an obstacle course. Two light-dependent
resistors (LDRs) are used to detect the obstacle and the motors are
driven correspondingly to avoid the obstacles automatically. Two
H-bridge motor circuits are used that can drive each motor forward or
backward, or stop it, independently.
Circuit and working
Fig. 1
shows the circuit of dual motor control. The circuit is built around
four-channel multiplexer CD4052 (IC1), light-dependent resistors (LDR1
and LDR2), four BC547 npn transistors (T1 through T4), four BC338
transistors (T7, T8, T11 and T12), four BC327 pnp transistors (T5, T6,
T9 and T10) and a few other components.
Fig. 1: Circuit of the dual motor control
As
mentioned earlier, there are two H-bridge circuits to drive the two
motors. Motor M1 drives the left side, while motor M2 drives the right
side. Each H-bridge circuit is built around a pair of npn and pnp
transistors as shown in Fig. 1. Each driving transistor has a diode
connected between
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The
semiconductor Signal Diode is a small non-linear semiconductor devices
generally used in electronic circuits, where small currents or high
frequencies are involved such as in radio, television and digital logic
circuits. The signal diode which is also sometimes known by its older
name of the Point Contact Diode or the Glass Passivated Diode, are
physically very small in size compared to their larger Power Diode
cousins.
Generally,
the PN junction of a small Signal Diode is encapsulated in glass to
protect the PN junction, and usually have a red or black band at one end
of their body to help identify which end is the cathode terminal. The
most widely used of all the glass encapsulated signal diodes is the very
common 1N4148 and its equivalent 1N914 signal diode. Small signal
and switching diodes have much lower power and current ratings, around
150mA, 500mW maximum compared to rectifier diodes, but they can function
better in
If Resistors are the most basic passive component in electrical or electronic circuits, then we have to consider the Signal Diode
as being the most basic “Active” component. However, unlike a resistor,
a diode does not behave linearly with respect to the applied voltage as
it has an exponential I-V relationship and therefore can not be
described simply by using Ohm’s law as we do for resistors.
Diodes
are basic unidirectional Semiconductor Devices that will only allow
current to flow through them in one direction only, acting more like a
one way electrical valve, (Forward Biased Condition). But, before we
have a look at how signal or
An
electron traveling through the wires and loads of the external circuit
encounters resistance. Resistance is the hindrance to the flow of
charge. For an electron, the journey from terminal to terminal is not a
direct route. Rather, it is a zigzag path that results from countless
collisions with fixed atoms within the conducting material. The
electrons encounter resistance - a hindrance to their movement. While
the electric potential difference established between the two terminals
encourages the movement of charge, it is resistance that discourages it.
The rate at which charge flows from terminal to terminal is the result
of the combined effect of these two quantities.
The flow
of charge through wires is often compared to the flow of water through
pipes. The resistance to the flow of charge in an electric circuit is
analogous to the frictional effects between water and the pipe surfaces
as well as the resistance offered by obstacles that are present in its
path. It is this resistance that hinders the water flow and reduces both
its flow rate and its drift speed. Like the resistance to
water flow, the total amount of resistance to charge flow within a wire
of an electric circuit is affected by some clearly identifiable
variables.
First,
the total length of the wires will affect the amount of resistance. The
longer the wire, the more resistance that there will be. There is a
direct relationship between the amount of resistance encountered by
charge and the length of wire it must traverse. After all, if resistance
occurs as the result of collisions between charge carriers and the
atoms of the wire, then there is likely to be more collisions in a
longer wire. More collisions mean more resistance.
Second,
the cross-sectional area of the wires will affect the amount of
resistance. Wider wires have a greater cross-sectional area. Water will
flow through a wider pipe at
A
resistor is a passive two-terminal electrical component that implements
electrical resistance as a circuit element. Resistors act to reduce
current flow, and, at the same time, act to lower voltage levels within
circuits.
A number
of different resistors are shown in the photos. (The resistors are on
millimeter paper, with 1cm spacing to give some idea of the
dimensions). Photo below shows some low-power resistors, with power
dissipation below 5 watt (most commonly used types) are cylindrical in
shape, with a wire protruding from each end for connecting to a circuit.
This photo shows some higher-power resistors, with power dissipation above 5 watt are shown below.
The symbol for a resistor is shown in the following diagram (upper: American symbol, lower: European symbol.)
Resistor symbols
The unit for measuring resistance is the OHM. (the Greek letter Ω - called Omega). Higher resistance values are represented by "k" (kilo-ohms) and M (meg ohms). For example, 120 000 Ω is represented as 120k, while 1 200 000 Ω
is represented as 1M2. The dot is generally omitted as it can easily be
lost in the printing process. In some circuit diagrams, a value such as
8 or 120 represents a resistance in ohms. Another common practice is to
use the letter E for resistance in ohms. The letter R can also be used.
For example, 120E (120R) stands for 120
Resistor Markings
Resistance
value is marked on the resistor body. Most resistors have 4 bands. The
first two bands provide the numbers for the resistance and the third
band provides the number of zeros. The fourth band indicates the
tolerance. Tolerance values of 5%, 2%, and 1% are most commonly
available.
The following table shows the colors used to identify resistor values:
a. Four-band resistor, b. Five-band resistor, c. Cylindrical SMD resistor, d. Flat SMD resistor
RESISTORS LESS THAN 10 OHMS
When the third band is gold, it indicates the value of the "colors" must be divided by 10. Gold = "divide by 10" to get values 1R0 to 8R2 See 1st Column above for examples.
When the third band is silver, it indicates the value of the "colors" must be divided by 100. (Remember: more letters in the word "silver" thus the divisor is "larger.") Silver = "divide by 100" to get values 0R1 (one tenth of an ohm) to 0R82 e.g: 0R1 = 0.1 ohm 0R22 = point 22 ohms See 4th Column above for examples.
The letters "R, k and M" take the place of a decimal point. The letter "E" is also used to indicate the word "ohm." e.g: 1R0 = 1 ohm 2R2 = 2 point 2 ohms 22R = 22 ohms 2k2 = 2,200 ohms 100k = 100,000 ohms 2M2 = 2,200,000 ohms
Common
resistors have 4 bands. These are shown above. First two bands indicate
the first two digits of the resistance, third band is the multiplier
(number of zeros that are to be added to the number derived from first
two bands) and fourth represents the tolerance. Marking
the resistance with five bands is used for resistors with tolerance of
2%, 1% and other high-accuracy resistors. First three bands determine
the first three digits, fourth is the multiplier and fifth represents
the tolerance.
For
SMD (Surface Mounted Device) the available space on the resistor is very
small. 5% resistors use a 3 digit code, while 1% resistors use a 4
digit code. Some SMD resistors
are made in the shape of small cylinder while the most common type is
flat. Cylindrical SMD resistors are marked with six bands - the first
five are "read" as with common five-band resistors, while the sixth band
determines the Temperature Coefficient (TC), which gives us a value of
resistance change upon 1-degree temperature change. The
resistance of flat SMD resistors is marked with digits printed on their
upper side. First two digits are the resistance value, while the third
digit represents the number of zeros. For example, the printed number
683 stands for 68000W , that is 68k. It
is self-obvious that there is mass production of all types of
resistors. Most commonly used are the resistors of the E12 series, and
have a tolerance value of 5%. Common values for the first two digits
are: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. The
E24 series includes all the values above, as well as: 11, 13, 16, 20,
24, 30, 36, 43, 51, 62, 75 and 91. What do these numbers mean? It means
that resistors with values for digits "39" are: 0.39W, 3.9W, 39W, 390W,
3.9kW, 39kW, etc are manufactured. (0R39, 3R9, 39R, 390R, 3k9, 39k) For
some electrical circuits, the resistor tolerance is not important and
it is not specified. In that case, resistors with 5% tolerance can be
used. However, devices which require resistors to have a certain amount
of accuracy, need a specified tolerance.
Resistor Power Dissipation
If the flow of current through a
resistor increases, it heats up, and if the temperature exceeds a
certain critical value, it can be damaged. The wattage rating of a
resistor is the power it can dissipate over a long period of time. Wattage rating is not identified on small resistors. The following diagrams show the size and wattage rating:
Resistor dimensions
Most
commonly used resistors in electronic circuits have a wattage rating of
1/2W or 1/4W. There are smaller resistors (1/8W and 1/16W) and higher
(1W, 2W, 5W, etc). In place of a single resistor with specified
dissipation, another one with the same resistance and higher rating may
be used, but its larger dimensions increase the space taken on a printed
circuit board as well as the added cost. Power (in watts) can be
calculated according to one of the following formulae, where U is the
symbol for Voltage across the resistor (and is in Volts), I is the
symbol for Current in Amps and R is the resistance in ohms:
For example, if the voltage across an 820 Ω resistor is 12V, the wattage dissipated by the resistors is:
A 1/4W resistor can be used.
In many
cases, it is not easy to determine the current or voltage across a
resistor. In this case the wattage dissipated by the resistor is
determined for the "worst" case. We should assume the highest possible
voltage across a resistor, i.e. the full voltage of the power supply
(battery, etc). If we mark this voltage as UB, the highest dissipation is:
For example, if UB=9V, the dissipation of a 220 Ω resistor is:
The
relationship between Voltage, Current and Resistance in any DC
electrical circuit was firstly discovered by the German physicist Georg
Ohm. Ohm found that, at a constant temperature, the electrical current
flowing through a fixed linear resistance is directly proportional to
the voltage applied across it, and also inversely proportional to the
resistance.
This relationship between the Voltage, Current and
Resistance forms the bases of Ohms Law and is shown below.
Ohm's Law Relationship
By
knowing any two values of the Voltage, Current or Resistance quantities
we can use Ohm's Law to find the third missing value. Ohm's Law
is used extensively in electronics formulas and calculations so it is
“very important to understand and accurately remember these formulas”.
To find the Voltage ( V )
[ V = I x R ] V (volts) = I (amps) x R (Ω)
To find the Current ( I )
[ I = V ÷ R ] I (amps) = V (volts) ÷ R (Ω)
To find the Resistance ( R )
[ R = V ÷ I ] R (Ω) = V (volts) ÷ I (amps)
It is
sometimes easier to remember Ohms law relationship by using pictures.
Here the three quantities of V, I and R have been superimposed into a
triangle (affectionately called the Ohm's Law Triangle) giving voltage
at the top with current and resistance at the bottom. This arrangement
represents the actual position of each quantity in the Ohms law
formulas.
Ohms Law Triangle
Transposing the above Ohms Law equation gives us the following combinations of the same equation:
Then by
using Ohms Law we can see that a voltage of 1V applied to a resistor of
1Ω will cause a current of 1A to flow and the greater the resistance,
the less current will flow for any applied voltage. Any Electrical
device or component that obeys “Ohms Law” that is, the current flowing
through it is proportional to the voltage across it ( I α V ), such as
resistors or cables, are said to be “Ohmic” in nature, and devices that
do not, such as transistors or diodes, are said to be “Non-ohmic”
devices.
Ohm's Law Example
For the circuit shown below find the Voltage (V), the Current (I), the Resistance (R)
Voltage [ V = I x R ] = 2 x 12Ω = 24V
Current [ I = V ÷ R ] = 24 ÷ 12Ω = 2A
Resistance [ R = V ÷ I ] = 24 ÷ 2 = 12 Ω
Georg Simon Ohm, Alessandro Volta, André-Marie Ampère
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