Download ECE112 - Lab 8

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