Dusk-to-Dawn Light

Overview This lab creates a “dusk-to-dawn” light; the circuit turns a light on when the ambient light level goes below a certain level. In the circuit we will build a photocell – a light-sensitive resistor – will be used to sense the ambient light level. A light emitting diode (LED) will be the light source, and a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) will be used as a switch to turn on the LED when the ambient light level becomes low. The circuit is powered by a nine volt battery. A schematic of the circuit we will implement is shown in Figure 1. The 47Ω resistor is used to limit the current to the MOSFET and BJT; this protects the devices from damage due to overcurrent conditions and limits the amount of current that will be provided by the 9V source.

R

47

ZVN2110A

+ -

9V

Photocell LED

Figure 1. Dusk-to-dawn lighting circuit. Symbol Key: Demo

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Data

Demonstrate circuit operation to teaching assistant. Analysis; include principle results of analysis in laboratory report. Numerical simulation (using PSPICE or MATLAB as indicated). Record data in your lab notebook. 1

©2015, Tim Hanshaw

Dusk-to-Dawn Light

I.

Pre-lab: a) Read the background materials relative to photcells, LEDs, and MOSFETs. b) Determine and record the expected range of threshold voltages for the ZVN2110A MOSFET from an on-line data sheet. A link to a typical data sheet can be found in the background material.

Data

c) Determine and record expected forward voltages and forward currents for a red LED. A link to a typical data sheet can be found in the background material.

Data

d) Analyze the circuit of Figure 1 to determine a relationship between the MOSFET gate voltage (VG in Figure 2), the 9V source voltage, the photocell resistor (RP in Figure 2), and the fixed resistor R. You can use KVL around the loop shown in Figure 2, or a voltage divider relationship between RP and R. Assume that the current into the gate of the MOSFET is zero.

Analysis

Figure 2. Circuit analysis to determine MOSFET gate voltage. II. MOSFET/LED Characterization In this portion of the lab assignment, we will measure the voltage-current characteristic for the MOSFET/LED portion of circuit shown in Figure 1. Specifically, we will measure the relationship between the MOSFET collector current and the applied voltage between the MOSFET gate and the LED cathode. The goal is to determine what voltage is required to make the LED to light up. This information will be used in Part III to determine an appropriate value for the resistor R in Figure 1.

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Dusk-to-Dawn Light

a) Connect the circuit shown in Figure 3. Two power supplied are used in the circuit. Use channel 1 of your Arbitrary Waveform Generator (AWG1) to apply the (variable) gate voltage, VG. Use the 9V battery to provide the MOSFET drain voltage; this power supply provides the drain current ID. The reference terminal for both the 9V battery and the measurement VG should be the negative 5V supply on your Analog Discovery. • Connect your DMM as shown to measure the drain current ID. • Use the Analog Discovery Voltmeter instrument to measure the voltage VG.

Notes: • •

The connections in Figure 3 allow us to use the Analog Discovery to vary the voltage VG from 0V to 9V. . AWG1 can be changed from -5V to +4V relative to the Analog Discovery’s ground. Since V- is -5V relative to ground, AWG1 (measured relative to V-) will span the voltage range 0-9V.

Figure 3. VCCS circuit schematic. b) Now characterize the MOSFET’s relationship between gate voltage and drain current. Starting at VG = 0V, increase the gate voltage at increments of about 0.5V up to a maximum of about 9V. Record the gate voltages and their corresponding drain currents. Record the lowest gate voltage which adequately lights the LED1. Plot the gate voltage vs. drain 1

This is, of course, a subjective decision.

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Data

Dusk-to-Dawn Light

current. Comment on your observations relative to the data, especially relative to how the circuit behaves like a dependent source. c) Compare your measured MOSFET gate voltage that results in the LED lighting up with your expectations based on the data sheets you examined in parts (b) and (c) of the pre-lab. Are the measurements consistent with the specifications? d) Demonstrate operation of your circuit to the Teaching Assistant. Have the TA initial your lab worksheet indicating that they have observed your circuit’s operation.

Analysis

Analysis

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III. Photocell Characterization and Design Task Next, use the data from Part II to choose an appropriate value for the resistor R of Figure 1. a) Characterize the photocell resistance by using an ohmmeter to measure the photocell resistance under normal lighting and “dark” conditions. Record the “light” and “dark” values of resistance on your lab worksheet. (You can easily create the “dark” condition by covering the photocell with your hand.)

Data

b) Using your mathematical relationship from part (c) of the pre-lab, the gate voltage required to light the LED from Part II of the lab, and the measured range of photocell resistances under light and dark conditions, choose a value for the resistor R in Figure 1 which causes the LED to light up in the dark and turn off when the surroundings are well lit2.

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c) Calculate the expected range in gate voltages that the circuit will experience in both light and dark conditions.

Analysis

IV. Circuit Integration Finally, we will implement and test the overall dusk-to-dawn circuit. a) Using the value for the resistor R determined in Part II, construct the dusk-todawn circuit shown in Figure 1. b) Measure and record the MOSFET gate voltage (VG in Figure 2) and the voltage difference across the diode. Comment on the agreement between the expected range of gate voltages (calculated in Part IIIc) and the measured range of voltages. Also comment on the diode voltage difference 2

You may prefer a “guess and check” approach to determining the desired resistance value, rather than attempting to create a rigorous solution. You’ll probably also want to choose your resistance such that it is relatively easily implemented using resistors in your parts kit.

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Data Analysis

Dusk-to-Dawn Light

measured in the overall circuit, and your expectations from the data sheets of Part c of the Pre-lab. Is the measured diode voltage in the circuit within the specified forward voltage ranges on the data sheet? c) Demonstrate operation of your circuit to the Teaching Assistant. Have the TA initial your lab worksheet indicating that they have observed your circuit’s operation

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Demo