Lab 5: Post-lab
Trey Morris
801006456
ECEN 214-502
(Travis Eubanks)
10/16/2008
Procedure
Task 1: Build a latch.
1. Create the latch circuit using buttons for both ALM and RESET.
2. Test the circuit to makes sure it works because ALM will be replaced with a real input in a
following step.
3. Record the results in Table 5.1.
Task 2: Add buffer and LEDs to the latch circuit
1. Attach an op-amp used as a buffer to the Q output of the latch.
2. Attach a small LED circuit to the op-amp buffer. There are two LED colors, green and red.
These are to see when the circuit is tripped.
3. There is also a buzzer to be attached.
Task 3: Finish putting the security system together.
1. Use the provided PCB to prevent having to rebuild everything from Lab 4.
2. Power the PCB circuit and attach the photo detector and laser diode to it.
3. Connect the PCB output to the latch S input.
4. Measure the output voltage of the buffer op-amp and record results in table 5.3.
5. Watch the circuit make all sorts of noise when it is tripped.
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Data Tables
Table 5.1: Tasks 1
Take Voltage at:
ALM depressed
RESET depressed
S
L
H
Q
H
L
R
H
L
Table 5.2: Task 2
ALM on
RESET on
Q Voltage
11.93 V
1.62 V
Buffer Output
10.97 V
1.62 V
LED on
Table 4.3: Task 2
Output voltage (Measured)
1.62 V
10.96 V
With laser beam focused
Without laser beam focused
2
Green
Red
Sample Calculations
The only calculation required for this lab was to find the resistance value which would limit current
through the LEDs to 10mA.
0.01𝐴 =
12𝑉 − 0.7𝑉
𝑅
𝑅 = 1130Ω
Discussion
The whole security system is fairly simple. A laser is pointed at a photo detector. The photo detector
outputs a voltage which is fed to an op-amp which converts it to a voltage. This voltage is very small
however, so it is fed into another op-amp which acts as a voltage amplifier. Now we have a real voltage
to work with which is much different from the voltage generated from the photo detector current
source when the laser beam is obstructed. This makes it easy to tell when the beam is obstructed or
operating regularly. In order to monitor this voltage, it is fed into an op-amp which is acting as a
comparator. This comparator will output high positive voltage when the laser is not obstructed and a
low voltage when it is obstructed. This high or low voltage is fed into a latch. The latch will output low as
long as the comparator output stays high (beam unobstructed). As soon as the beam is obstructed and
the comparator output switches to low the latch will output high (tripped) until reset is pressed. The
comparator output is fed through a buffer to two LEDs. Green for unobstructed laser, red for system
tripped. The red LED is connected in parallel to a buzzer which also sounds when the system is tripped.
Laser
Diode
On
On
Photo
Detector
Receiving
Not
receiving
Current to
Voltage
converter
-0.446 V
-5.00mV
Signal
Amplifier
Signal
Comparator
Latch
Buffer
LED color
on
10.80 V
0.35 V
11.17 V
1.32 V
1.62 V
11.93 V
1.62 V
10.97 V
Green
Red
The results shown here are exactly as expected. The buzzer rings when the red LED is on since they are
attached in parallel. This happens when the latch output is high and indicates the beam has been
obstructed. The latch is necessary because otherwise the system would only be tripped for the few
moments when the beam is obstructed. Someone could then pass through the beam quickly enough for
no one to notice. Some security that would be…
Conclusion
In conclusion, I liked this lab. It was short and sweet. It taught important uses for op-amps. The circuits
were not overly complicated as could have been the case. The pre-lab was also very straight forward
and applied directly to the lab. I might just build myself one of these to play with at home.
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