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ECE112 - Lab 8 Purpose • Observe ”one-way” diode behavior • Use some LEDs in conventional and non-conventional ways • Use BJT transistors as amplifiers and switches Parts/tools needed: • Soldering iron and hand tools • Parts available in lab • Power supply • DMM Note: This lab is a little longer than others. Read through the steps and do what you can before coming to lab. Observe ”one-way” diode behavior In general, diodes only conduct current in one direction. In fact, almost all semiconductor devices exhibit some degree of polarity. In some cases semiconductors such as transistors may be damaged when excessive voltage is applied in the incorrect direction. Because of this, electronic devices that use batteries show explicitly how to put the batteries in to avoid damaging the device. An LED exhibits polarity as well. Its positive terminal is indicated by being slightly longer than the other lead. Figure 1: LED - Positive lead (anode) is slightly longer Let’s first observe the diodes one-way behavior by constructing the circuit below on a big pad protoboard. Use your power supply set to 5V output and a red LED for D2. 1 1N4148 D4 1N4148 D2 E C R4 1Meg R1 10k R3 1Meg B MPSA13 (bottom view) D1 1N4148 R3 100 D2 R2 1Meg LED (white) (sensitivity) R1 100 V1 5V d g (green) C tion" B D2 Q2 2N7000 D1 E LED (red) s Q1 MPSA13 E Figure 2: Diode Experiment One S G MPSA13 (bottom view) Once powered up, the LED should light. Measure the forward voltage Vd across the regular diode D 2N7000 D1 and the LED, D2 with your DMM. (bottom view) 1N4148 forward voltage: Red LED forward voltage: The 1N4148 is a normal silicon diode and exhibits a Vd of roughly 0.7V . However, LEDs exhibit much higher forward voltage. The Vf for red LED’s averages about 2.2 volts. Green and Yellow exhibit more. Measure the forward voltage of a yellow, green and white LED by replacing the red LED with each of the following: Yellow LED Vf : Green LED Vf : White LED Vf : (Note: Always have a resistor of at least 100Ω in series with your LEDs. They will be permanently damaged otherwise.) Now, with any color LED still in place and illumined, reverse the polarity of the power supply. Is the LED still illuminated? Is any current flowing in the circuit? Explain what you are observing. 2 The swizzle circuit It is possible to use some diodes oriented such that so that it makes no difference which way batteries are installed. In this experiment, we will use four diodes to allow a battery to power the LED circuit regardless of how the battery is connected by using the ”one-way” behavior in a clever way. The incomplete schematic below shows a circuit where a 5 volt source could be applied in either direction to a circuit that lights an LED. Actually, the LED can withstand considerable reverse voltage without damage, but let’s allow it to represent some polarity sensitive circuit. If voltage is applied in a backwards fashion, a swizzle circuit orients the positive and negative supply voltages so that its always applied to the circuit correctly. Complete the circuit schematic given to produce a swizzle circuit and then build it on your big pad protoboard. Be sure to clearly show your connection dots on the schematic. There are two missing wires. Your circuit drawing should be clear. The cathode end of the 1N4148 diodes are marked with a black band. D3 1N4148 D1 1N4148 V1 5V V1 5V R1 D5 5V V1 OR R2 100 R1 100k D LE LED(red) 100 note reversed polarity C touch pads B 1N4148 D4 1N4148 D2 E R3 1Meg B C Q1 MPSA E MPSA13 (bottom view) Figure 3: Swizzle Schematic to be Completed Once your circuit isV1 complete, try to light the LEDR4 with both polarities. If you have it designed D1 R3 5V 1N4148 1Megpolarities.100 and wired it correctly, the LED will light under both If not, figure out your mistake, R1 until it works properly. redraw the circuit and fix your circuit 10k D2 R2 1Meg LED (white) V1 Measure the voltage between the right side of R1 (+(sensitivity) lead of DMM) and the cathode 5V of the LED (lead of DMM). d g LED (green) Is it equal to the supply voltage? How much less is it? Explain the difference. C "air connection" B s Q1 MPSA13 E E S G B C Q2 2N7000 D1 D MPSA13 (bottom view) 3 2N7000 (bottom view) R1 100 D2 LED (red) BJT as a sensitive amplifier We have seen in class how a BJT can operate as a current amplifier. Some BJT transistors have exceptionally high current gain due to a special internal structure. We will use a super-beta transistor with a current gain (beta) of about 5000 to build a touch sensor. The touch sensor uses the tiny current path through the pad of your finger to provide the current path from the positive supply to the base terminal of a BJT. The BJT will amplify the current and turn it fully on. If an LED is placed in the collector to supply branch, a touch actuated LED switch is formed. A finger touch on the two pads will supply the current, and the LED will turn on. D3 1N4148 D1 1N4148 V1 5V R1 D5 5V V1 C touch pads B 1N4148 D4 1N4148 D2 E R4 1Meg R3 100 R3 1Meg B C Q1 MPSA13 E MPSA13 (bottom view) Figure 4: Touch Sensor Schematic D1 1N4148 Build the circuit on the blue protoboard. Use a white LED for D1. You can use two unconnected wires pads as your touch pads. Make D2 R2 or the protoboard R1 sure your circuit operates correctly. LED (white) 1Meg (sensitivity) V1 5V 100 Does the LED brightness change when you push harder on the touch pads or use moistened d D2 g the touch fingers on Q2 pads? een) D1 MPSA13 bottom view) D1 LED (white) LED(red) 100 R1 10k n" R2 100 R1 100k LED (red) 2N7000 C B s Q1 MPSA13 Explain why: E S Have your TA check off the operation of your touch switch: G D 2N7000 (bottom view) 4 Using an LED as a photo-detector (not a photo-emitter!) Most all semiconductor devices are sensitive to light, infrared through X-rays. This is why most are encapsulated in black epoxy or metal cases. LEDs are also sensitive to light. Since they are in transparent cases, they can both emit and detect light. LEDs are optimized to emit light but they can still allow a small leakage current to pass when they are reverse biased and light is applied to their PN junction. Look at the schematic in figure 5. It is similar to the touch sensor circuit except a green LED is put in the place of the touch pads for your finger. If light is applied to LED D1, a small leakage current flows into the base of Q1, turning it on. When D3collector of Q1 will be at about D1fully on, the 1N4148 Q2 which has a threshold 0.2 volts. Since the collector of Q1 is connected to1N4148 the gate of MOSFET V1 drain to source, voltage of about 2.5 volts, Q2 will be off. When Q2 is off, it passes no current from R1 5V thus LED D2 will remain off. For this circuit configuration, you can see that when Q1 is on,100k Q2 is off. When Q1 is off, Q2 will be on. R1 D5 V1 5V R2 100 D 5V V1LED D1, there LED(red) 100 current into Q1 and it will be off. However, if no light falls on the sensor is no base OR LE note reversed Under this situation, the collector of Q1 as well as the gate of Q2 is at 5 volts. This turns on Q2 polarity touch pads allowing it to pass current from drain to source, thus illuminating LED D2. C B Q1 MPSA E 1N4148 1N4148 R3 What we have created is a light source that comes on only when D4 the lights go Boff; an automatic D2 1Meg night light. C V1 5V R4 1Meg R1 10k MPSA13 (bottom view) D1 1N4148 R3 100 D2 R2 1Meg LED (white) (sensitivity) V1 5V d g LED (green) Q2 2N7000 D1 C "air connection" B S G B C s Q1 MPSA13 E E D MPSA13 (bottom view) 2N7000 (bottom view) Figure 5: Automatic Night Light Schematic Build the circuit on the blue big pad board. However, the connection between the D1’s anode and the base of the MPSA13 must be made in the air. Using the pad on the board allows the tiny cur5 E R1 100 D2 LED (red) rent to leak away from the base onto the board. Once built, see that it works correctly by covering D1 with your fingers. You can use the white LED you already used for D2. You can vary the sensitivity of the circuit by adjusting R2. You can also tailor the light wavelengths the circuit is sensitive to by using yellow or red LEDs. You can use multiple sensor LEDs in parallel to make it more sensitive to different wavelengths of light. Show your working circuit to your TA. Circuit works? 6