Sarah Rosanna Busch
Engineering Technologist
ELEX 130 X03
Design a Logic Probe
with Altium Designer
Student: Sarah Busch
12/1/2014
Instructor: Todd Rayson
Elex 130 Assignment 3 – Design A Logic Probe
Elex 130 Assignment 3 – Design A Logic Probe
Simplified logic probe
A logic probe is a test instrument that will display the digital state of a particular point in a circuit. If the circuit point is at 5vdc then the circuit is "high" or a logic 1 and the red LED will light. If the circuit point is at 0vcd then the circuit is "low" or a logic 0 and the green LED will light.
3 gates of the LM324 quad operational amplifier are used. The first gate is used in a buffer or voltage follower configuration. This is to isolate the logic probe circuit from the circuit under test. The op-amp gives the logic probe a very high input impedance and capacitance so that it will not affect the circuit under test.
The first gate then passes the buffered signal to the next two gates. These gates are in a comparator configuration. The comparators detect if the input voltage is above or below threshold voltages set by the resistor string.
One or the other comparator will activate depending on the input voltage. Both comparators will not be on at the same time.
When a comparator activates it will supply 5vdc to the LED. The LED will light.
The 2N5485 provide current limiting to prevent the LEDs from becoming too bright and burning out.. The LEDs are rated to a maximum of 20ma of current each.
If the circuit under test is oscillating between a logic 0 and logic 1 state then the logic probe lights will flash alternately. If the circuit under test is oscillating fast enough then both logic probe lights will appear to light at the same time.
1
Elex 130 Assignment 3 – Design A Logic Probe
*Note that R2 and R3 are reversed in comparison to the BOM and Netlist shown in the assignment.
2
Elex 130 Assignment 3 – Design A Logic Probe
[
C1
RAD0.2-POL
1.0uF
]
[
C2
RAD0.2-POL
1.0uF
]
[
C3
RAD0.2
0.33uF
]
[
D1
LED-5608F
LN265RPH
]
[
D2
LED-5608F
LN365GPH
]
[
P1
PROBE
PROBE
]
[
P2
CP-202A-ND
CP-202A-ND
]
[
Q1
TO-92A
2N5485
]
[
Q2
TO-92A
2N5485
]
[
R1
AXIAL0.5
4.7M
]
[
R2
AXIAL0.5
1.5M
]
[
R3
AXIAL0.5
100k
]
[
R4
AXIAL0.5
68k
]
[
R5
AXIAL0.5
33k
]
[
U1
DIP14
LM324
]
[
U2
TO-220
LM7805CT(3)
]
(
+5
C1-1
R1-2
R3-2
U1-4
U2-3
)
(
+9V
C2-1
C3-1
P2-3
U2-1
)
(
GND
C1-2
C2-2
C3-2
P2-1
P2-2
Q1-G
Q1-S
Q2-G
Q2-S
R2-1
R5-1
U1-11
U1-12
U1-13
U2-2
)
(
HIGH
D1-A
U1-7
)
(
LOW
D2-A
U1-8
)
(
NetD1_K
D1-K
Q1-D
)
(
NetD2_K
D2-K
Q2-D
)
(
NetP1_1
P1-1
R1-1
R2-2
U1-3
)
(
NetR3_1
R3-1
R4-2
U1-6
)
(
NetR4_1
R4-1
R5-2
U1-10
)
(
NetU1_1
U1-1
U1-2
U1-5
U1-9
)
3
Elex 130 Assignment 3 – Design A Logic Probe
Discussions and Conclusions
In this assignment I learned that with Altium Designer it is possible to make a schematic document based on a given circuit design, even without knowing what half of the components are or what they do, as long as meticulous attention to detail is used. The schematic should be laid out in such a way that it communicates the functionality of the circuit.
I also learned that a printed circuit board can be configured and routed based on this schematic, as long as there are notes from the designer about any components that need to be positioned in a certain way. The PCB layout is different from the schematic layout because it needs to meet the practical requirements of fitting the components on a board of a certain size, in a way that is possible to route, and that will make sense for the functionality of the device.
The Altium Electric Rule Check and Design Rule Check are very useful for ensuring that the schematic and PCB documents are completed properly, but they cannot be relied on entirely. For example, in this assignment, there was a discrepancy between the pin numbers of the N-channel JFETs from the imported schematic library and PCB library which. Because of this they did not get connected to the circuit in the PCB until this was manually fixed in the project libraries. The error reports didn’t recognize that as a problem, though this would have resulted in a failed PCB design.
Based on this assignment the only real criticism I have for Altium is that there is no scroll bar on the files window, so the only way to see the bottom of the list is to minimize the stuff about it. Overall, however, I found Altium to be a well-designed program for communicating PCB design, and I had fun doing this assignment.
On the other hand, in writing this report I found yet another annoying quirk of MS Word’s: I didn’t set up my header/footer and page numbering until after writing this document, so the default page numbering started over from zero after every page break, and the only way to fix this was to go back to each page break and format the page numbers to continue from the last section.