Exercise A
In this first simple exercise, we will use a voltage source, a “Voltcraft” multimeter and a
voltage-divider circuit to measure the resistance of a resistor. Later we explore simple sensors
and data acquisition.
R
Figure 1: A resistor.
A simple passive element of an electronic circuit is a resistor, illustrated in Figure 1. The unit
of the resistance is [R]= Ohm (Ω). A resistor obeys Ohm’s law:
(1)
U = R× I ,
where U is the Voltage or electric potential difference or electric tension, unit Volt (V) and
I is the Current unit Ampere (A).
I
R
U
Figure 2: Illustration of Ohm's law.
Furthermore, you should know Kirchhoff’s law, that states that the sum of all currents flowing
into a node equals the sum of all currents flowing out of that node, i.e. no charge is lost.
R1
R1
R2
(a)
(b)
R2
Figure 3: Two ways of connecting resistors: in (a)
the resistors are hooked in series, in (b) in parallel.
Resistors can be connected in parallel or in series (Figure 3). We can derive from Ohm's and
Kirchhoff’s law, that
R = R1 + R2 ,
holds for resistors connected in series and
(2)
(3)
1 1
1
,
= +
R R1 R2
for resistors in parallel. In a voltage-divider circuit (Figure 4), two resistors are connected in
series to voltage source (battery).
R1
R1
Uin
R2
(a)
Uout
R2
Uout
(b)
Figure 4: A voltage divider (a). Voltage divider connected to battery (b).
•
Assume R1 >> R2 ; describe the resistance R of the whole circuit, when R1 and R2 are
connected in series and parallel.
•
The resistance and the precision of a THT resistors are given by a color code. How does
this code work? By looking at the color codes, determine values of provided resistors and
verify your results with a Voltcraft multimeter.
•
The resistance of a SMD resistors are given by a alphanumeric or numeric code. How do
these codes work? By looking at the codes (you can use magnifying glass), determine
values of provided resistors and verify your results with a Voltcraft multimeter.
•
Find 2 kΩ THT resistor ( R1 ) and measure exact resistance with a Voltcraft multimeter. Is
the measured resistance within the given precision? Do the same for a 1 kΩ resistor ( R2 ).
•
Setup a voltage-divider circuit with R1 and R2 connected in series. Use two AAA
batteries to provide a constant voltage U in = 3 V. Measure the voltage drop U out
across R2 .
•
Use Ohm’s law to calculate the resistance R2 as function of U in , U out and R1 ,
R2 = f (U in ,U out , R1 ) .
•
Apply a constant voltage U in = 1.5 V (single AAA battery) measure U out and calculate
R2 . Did you got same result as with U in = 3 V?
•
A thermistor is a type of resistor whose resistance is dependent on temperature, more so
than in standard resistors. The thermistor is a short form of thermal and resistor. Part
provided for exercise is negative temperature coefficient (NTC) thermistor.
RT
Uout
R1
Figure 5: A voltage divider with thermistor.
Setup a voltage-divider circuit with R1 = 1.5 kΩ and RT connected in series. Use two
AAA batteries to provide a constant voltage (3 V). Measure the voltage drop U out across
R1 . Place RT into boiling water and measure U out . Calculate values for RT in both
conditions.
•
Flexion sensors (from Latin 'flectere' = 'to bend'), also called bend sensors, measure the
amount of deflection caused by bending the sensor. Part provided for exercise conductive
ink-based (graphite) bend sensor adjusted with silver ink.
RFlex
R1
Uout
Figure 6: A voltage divider with bend sensor
Setup a voltage-divider circuit with R1 = 15 kΩ and bend sensors ( RFlex ). Use 3 V battery.
Measure U out for flat bend sensor. Bend the RFlex (ink outside) around provided Plexiglas
wheels (diameter 40 to 120 mm) and measure U out . Calculate RFlex values for flat sensor
and maximal bending (40 mm). Draw the φ = f (U out ) .
•
Change R1 = 4.7 kΩ. Measure U out for flat bend sensor and sensor bend around 40 mm
wheel. Change R1 again, now to 82 kΩ. Repeat the measurements. Calculate ∆U out for all
used R1 . Propose how to calculate value of R1 to maximize sensor response ( ∆U out ).
•
Connect multimeter to PC using serial port or USB to serial interface.
•
A photoresistor or light-dependent resistor (LDR) or photocell is a light-controlled
variable resistor. The resistance of a photoresistor decreases with increasing incident light
intensity.
RLDR
R1
Uout
Figure 7: A voltage divider with photoresistor.
Setup a voltage-divider circuit with R1 = 10 kΩ and photoresistor ( RLDR ) connected in
series. Use 3 V battery. Using PC acquisition software observe change of U out when
sensor is in darkness and exposed to light.
If needed replace R1 with a resistor of a different value.
Appendix
Figure A: Resistor Color Code (source: http://www.michaels-electronicslessons.com/).