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/).