ME 140L Lab 6 Instruction

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ME 140 L: Mechatronics Lab Dr. Wei Li Lab 6: Introduction to Operational Amplifiers (Op-Amps) and Transistors Required Components  1  10 MΩ resistor  2  10 kΩ resistor  1  4 kΩ resistor  2  1 kΩ resistors  1  330 Ω resistor  thermistor (LM35)  741 op-amp  light-emitting diode (LED)  n-p-n transistor (PN 2222)  light-dependent resistor (LDR) Laboratory Equipment Featured in this Lab  benchtop digital multimeter  benchtop DC voltage supply  oscilloscope  function generator Pre-Laboratory Readings (sections 6.1 through 6.4) 6.1 Learning Objectives The learning objectives of this lab are to:  Understand the proper construction and application of a commonly used amplifier circuit – a non-inverting amplifier. This is used to increase the magnitude of insufficiently small voltage signals that typically are output from sensors (transducers).  Develop a basic understanding of how bipolar junction transistors (BJTs) work and how they are used in circuits: either as a switch or an amplifier. 6.2 The Use of Thermistors in Temperature Sensing Devices To aid in our study of amplifier circuits, we will use a temperature sensor that typically outputs a voltage that is insufficiently small for practical use and should thus be amplified. Specifically, we will use an LM35 temperature sensor, which is a 3-pin integrated circuit that contains a thermistor: a transducer whose resistance changes with temperature. The technical specifications for the LM35 are provided below.  Sensitivity is 10 mV/°C. As the temperature increases, the output voltage increases, e.g., a 25°C temperature produces a 250 mV signal  Applicable temperature range is from –55 °C to 150 °C Pinout description: 1. +V s : Supply voltage (5 V is sufficient) 2. OUT: analog output voltage 3. GND: ground 1

ME 140 L: Mechatronics Lab Dr. Wei Li 6.3 Operational Amplifiers: “Op-Amps” An op-amp is an integrated circuit (IC) that is used in many different types of amplifiers. In this lab, we will construct a commonly used amplifier, namely a non- inverting amplifier. The pinout for the 741 op-amp is shown below. 6.4 Transistors Transistors, like diodes, are semiconductor devices that are usually made of silicon. Silicon can be doped with other elements, which cause different types of changes to its conductivity properties, the main two of which are described below. n-Type Materials When silicon is doped with the chemical elements arsenic, phosphorus, or antimony, the silicon gains extra "free" electrons—ones that can carry an electric current—so electrons will tend to flow out of it more easily. Since electrons have a negative charge, silicon treated this way is called n-type (negative type). p-Type Materials When silicon is doped with other impurities such as boron, gallium, and aluminum, it will have a deficit of "free" electrons, so the electrons in nearby materials will tend to flow into it. Silicon treated this way is called p-type (positive type). Putting n-type and p-type Material Together: Diodes If we join a piece of n-type silicon to a piece of p-type silicon and put electrical contacts on either side, useful things start to happen at the junction between the two materials. If we turn on the current, we can make electrons flow through the junction from the n-type side to the p-type side and out through the circuit. This happens because the lack of electrons on the p-type side of the junction pulls electrons over from the n-type side . But if we reverse the current, the electrons won't flow at all – thus, this type of arrangement is a diode, which you used in the previous lab. Making a “Sandwich” of n-type and p-type Materials: Transistors If we use three layers of silicon, we can make an n-p-n sandwich. If we join electrical contacts to all three layers of the sandwich, we can make a component that will either amplify a current or switch it on or off, i.e., a 2

ME 140 L: Mechatronics Lab Dr. Wei Li transistor. The boundary between different types of materials is called a junction. Since we have two junctions, this is called a bipolar junction transistor, or BJT. The two contacts joined to the two pieces of n-type silicon are called the emitter and the collector , and the contact joined to the p-type silicon is called the base . When no current is flowing in the transistor, we know the p-type silicon is short of electrons and the two pieces of n-type silicon have extra electrons. The physical and schematic representations of an n-p-n BJT are shown below. The n-type material has a surplus of electrons, and the p- type material has holes where electrons should be. Normally, the holes in the base act like a barrier, preventing any significant current flow from the emitter to the collector while the transistor is in its "off" state. To turn a transistor to its “on” state, we input a small current to the base, I B , make the emitter negatively charged, and make the collector positively charged, using voltage sources, as shown in the figure. This gives rise to a relatively large output current, I E , from the emitter. By turning a small input current into a large output current, the transistor acts like an amplifier. But it also acts like a switch at the same time. When there is no current to the base, little or no current flows between the collector and the emitter. Turn on the base current and a big current flows. So the base current switches the whole transistor on and off. In this lab, we will use a PN2222 transistor, whose orientation, pinout, and schematic are shown below. 6.5 Laboratory Exercises and Experimental Summary Sheet Exercise 1: Building a basic amplifier circuit using a 741-op-amp Build the circuit shown below, which contains an op-amp. This is called a non-inverting amplifier. The gain of this amplifier circuit is determined by the values of the resistors R1 and R2. The theoretical gain of the amplifier is given by the equation 3

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ME 140 L: Mechatronics Lab Dr. Wei Li Test the amplifier circuit using a sinusoidal input voltage and by taking peak-to-peak measurements to determine the actual gain. Use different values of R1 (1kΩ, 4kΩ, and 10kΩ) to calculate the corresponding gains. Connect channel 1 of the oscilloscope to V in (the unamplified voltage from the function generator) and connect channel 2 of the oscilloscope to V out (the amplified voltage that comes out of the amplifier circuit). Create a data table below that contains the following: R1, Vpp channel 1, Vpp channel 2, actual gain, theoretical gain, relative error for the gain (% error). Why is the relative error between the actual and theoretical gain so large for the case where R1 = 10 kΩ ? Hint: what is the absolute value of the voltage used to power the amplifier? 4

ME 140 L: Mechatronics Lab Dr. Wei Li Exercise 2: Building a “digital thermometer” using a thermistor and a 741-op-amp In the previous exercise, you used a non-inverting amplifier to amplify an AC (sinusoidal) voltage. Amplifier circuits can also be used to amplify DC voltages. Build the circuit below, which contains a thermistor and an op-amp. The voltage output of the thermistor is too small to accurately detect changes in temperature at the thermistor, so its output must be amplified. Warning – if you incorrectly connect the terminals of the thermistor – it will start to dangerously heat up! Also, some thermistors and transistors look similar to each other – double check the markings on the thermistor before you power up the circuit. The thermistor has a sensitivity (calibration constant) of 10 mV/ ˚C . Using this information, along with the actual gain of the amplifier (from Exercise 1), use the circuit above to measure the temperature change under the following changes in the condition at the thermistor: (1) from ambient to placing fingers on the thermistor (should result in a temperature increase) and (2) from ambient to blowing air across the thermistor (should result in a temperature decrease). On the next page, create a data table that contains the following: description of the change in the condition at the thermistor, output voltage from the amplifier circuit, and the change in temperature. Do your calculated temperature changes seem realistic? 5

ME 140 L: Mechatronics Lab Dr. Wei Li 6

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ME 140 L: Mechatronics Lab Dr. Wei Li Exercise 3: Building a darkness detection system (variable intensity “night light”) using an LDR and transistor In Lab 5, you used a comparator to build a light sensor with a c, shown schematically below. In this lab, instead of using a comparator as the “logic controller”, you are going to use a transistor . And instead of a “brightness sensing system”, you are going to build a “darkness sensing system”, e.g., a night light. Examine the right-hand side of the circuit below. Recall that the higher the light level that the LDR is exposed to, the lower its resistance. Thus, in dark conditions, the LDR will have a very high resistance. This condition will allow a larger amount of current to flow through R3, which will then activate the transistor. 7 sensor indicator logic controller 4.0 V 4.0 V 10k Ω 0 Ω (wire only)

ME 140 L: Mechatronics Lab Dr. Wei Li Build the night-light circuit, as shown below. Make sure that you follow the pinouts for the transistor and that it is oriented correctly. Using the flashlight feature on your smartphone, test your system by subjecting the LDR to variable intensities of light. How does your LED respond to various light intensities? Is this the same behavior that you saw when you were using a comparator to control the LED? If not, how can you explain the difference? On the schematic above, draw boxes around each part of the night-light system , labeling the sensor, logic controller, and indicator , similar to that on the circuit at the top of the previous page. 8 4.0 V 4.0 V 10k Ω 0 Ω (wire only)

ME 140 L: Mechatronics Lab Dr. Wei Li Ask your TA to come to your workstation and demonstrate how your night light works. After you have successfully demonstrated the system, your TA will sign below, certifying that you completed Exercise 3. Be prepared to answer the following questions: 1. What would happen if the LDR had a very high (approaching infinity) resistance? 2. What would happen if the LDR had zero resistance? 3. What would happen if the led were flipped, or moved to the emitter side? 4. What would happen if the ground on the emitter side of the transistor were swapped for a 4V source? 5. What would happen if the 10K resistor (taking place of 100K) were changed to a wire? TA Signature ________________________________________________ 6.6 Post-Lab Activities There are no post-lab activities for this lab. Ensure that you include the pages for these lab activities in your deliverable. 9

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