Schematic Design Review
4:47
Project
BOM
Datasheet
Review (ERC/DRC)
Reviewing schematics.
Analyzing reviews and components.
Examining power connections.
Reviewing schematic labeling and components.
Examining display connections and issues.
Identifying design issues.
Analyzing design inconsistencies.
Using Review (ERC/DRC) Tool.
Using BOM Tool.
Looking for "output type open-collector and requirement for external segment resistors" in 1809172041_Texas-Instruments-SN74LS47N_C129379.pdf
Looking for "intended display type (common anode or common cathode)" in 1809172041_Texas-Instruments-SN74LS47N_C129379.pdf
Looking for "CLR pin polarity and behavior" in sn74ls193.pdf
Looking for "UP and DOWN input levels and requirement to avoid floating inputs" in sn74ls193.pdf
Looking for "clear input polarity (CLR) active level" in sn74ls193.pdf
Looking for "UP and DOWN input truth and whether unused inputs must be tied high or low" in sn74ls193.pdf
Looking for "RBI and LT pins default/inactive levels, connection recommendations when unused" in 1809172041_Texas-Instruments-SN74LS47N_C129379.pdf
Looking for "Pin functions and required connections for LT, RBI, RBO when unused" in 1809172041_Texas-Instruments-SN74LS47N_C129379.pdf
Identifying schematic issues.
Here is a focused schematic-level review based on what is actually present in the project. Per Flux knowledge base guidance, I’m evaluating only observable connectivity, placements, and best practices, avoiding speculation, and providing concrete corrections.
Electrical and logical issues
-
555 clock output is floating
- Observation: U1 has no net on its OUT/Q pin in the netlist; only THR/TR/DIS/R/VCC/GND are connected (e.g., U1:THR/TR on Net 33; U1:DIS on Net 32; U1:R on Net 34).
- Impact: No clock reaches the counters; the design cannot count.
- Fix: Connect U1 OUT to the proper counting input path (UP or DOWN clock gating as intended).
-
Floating UP/DOWN input depending on switch position
- Observation: Up_Dowm COM is tied to VCC (Net 12). Its throws drive Ones_74192 UP (Net 30) and DOWN (Net 31) directly. The unselected input is left unconnected.
- Impact: The unselected clock input floats.
- Fix: Add defined bias on both Ones_74192 UP and DOWN (e.g., pull-downs or pull-ups) so the inactive input is not floating. Example: add 10k Resistor from each unused UP/DOWN net to the inactive state.
-
Duplicate designators for logic ICs (critical)
- Observation: There are three separate parts all named AND_7408 and two named NOT_7404.
- Impact: Designator uniqueness is violated; nets like “AND_7408:1A”/“1Y” become ambiguous and may short unintended pins across different physical ICs.
- Fix: Re-annotate so each physical IC has a unique designator (e.g., U2, U3, U4 or AND1/AND2/AND3, NOT1/NOT2). Then re-verify all connections.
-
Self-feedback/short connections on AND gates
- Observation examples from the netlist:
- Impact: Outputs hard-wired to inputs with no other defined source is unusual and likely erroneous.
- Fix: Review all nets where an AND gate output equals one of its inputs; break inadvertent loops unless you are intentionally creating feedback with proper biasing and timing.
-
7-seg decoder control pins appear unconnected
Component-level review
-
Segment current-limiting resistors are missing
- Observation: No resistors exist between segment outputs of Ones 7447/Tens_7447 and the segments on Ones/Tens. Only R1/R2/R3/R4 feed the common anode pins.
- Datasheet requirement: The SN74LS47 datasheet states the outputs are open-collector and “external segment resistors must be used to limit segment current to 25 mA or less.”
- Fix: Add one series resistor per segment line (seven per digit). Example: 220Ω Resistor on each segment net between Ones 7447/Tens_7447 outputs and Ones/Tens segment pins.
-
Parallel/duplicated pull-ups on ~LOAD
- Observation: R8 and R9 both pull Net 37 to VCC and that single net drives both Ones_74192:~LOAD and Tens_74192:~LOAD through the Reset button.
- Impact: Two pull-ups in parallel on the same net are redundant.
- Fix: Use a single pull-up resistor for Net 37 unless you intend separate control per digit (in which case split nets and give each its own pull-up).
-
Decoupling capacitors absent on every IC VCC
- Observation: No VCC-to-GND decoupling is present for U1, Ones_74192, Tens_74192, Ones 7447, Tens_7447, NOT_7404, AND_7408.
- Fix: Add a local 100nF Capacitor from each IC’s VCC to GND placed close to the VCC/GND pins.
-
Generic passives without values or MPNs
Power and signal integrity
-
VCC net unlabeled; unused +5V portal
- Observation: Power is distributed on “Net 12” (includes many VCC pins and J1:P1). There is also a 5V power net portal that is not tied to Net 12.
- Impact: Ambiguous power net naming; the portal does not function unless its net matches.
- Fix: Rename Net 12 to +5V or connect the 5V portal to the same net for clarity.
-
Potential SI/reliability risks from floating logic nodes
- Observation: As noted, the unselected UP/DOWN input floats and there are AND-gate self-loops with no defined sources.
- Fix: Ensure every logic input has a defined DC state (bias resistors or direct ties) and remove unintended self-feedback.
Schematic best practices and consistency
-
Designator uniqueness
-
Net naming and documentation
- Observation: Primary power net is “Net 12” and key control nets (e.g., decoder LT/RBI/RBO) are not labeled.
- Fix: Name critical nets (+5V, CLK, ~LOAD, ~CLR, etc.) and add brief notes where behavior matters.
-
Switch footprint missing
- Observation: DRC flags the Up_Dowm (SPDT) as missing a footprint.
- Fix: Assign a footprint to ensure manufacturability.
Datasheet-backed notes used for the above
- SN74LS47 decoder/driver: Datasheet states the outputs are open-collector and require external segment resistors to limit segment current to 25 mA or less. Your schematic currently lacks per‑segment resistors between Ones 7447/Tens_7447 and the displays Ones/Tens.
- SN74LS192 counters: UP/DOWN are separate clock inputs; only defined low-to-high transitions count. In your schematic, the unselected input is left floating when Up_Dowm selects the other side. Ensure unused clock input is biased to a defined logic level.
Targeted fixes checklist (actionable)
- Connect U1 OUT to the intended clock path for the counters.
- Add pull resistors on Ones_74192 UP and DOWN so neither floats when unselected by Up_Dowm.
- Re-annotate duplicate AND_7408 and NOT_7404 to unique designators; then review and correct the self-feedback nets (e.g., Nets 52, 61).
- Add one series resistor per segment for both digits between Ones 7447/Tens_7447 outputs and Ones/Tens segments. Example: 220Ω Resistor, adjust after current budget is set.
- Add local decoupling: one 100nF Capacitor per IC from VCC to GND, placed close to each device’s power pins.
- Tie Ones 7447/Tens_7447 control pins (LT/RBI/RBO) to defined levels as per datasheet if ripple blanking/lamp test features are unused.
- Replace duplicated pull-ups R8/R9 on Net 37 with a single pull-up unless you split ~LOAD per digit.
- Assign values for R1–R9 and C1 on the schematic, then select real MPNs/footprints.
- Unify power net naming: tie “Net 12” to the 5V power portal or rename Net 12 to “+5V” for clarity.
- Assign a footprint to Up_Dowm so it is manufacturable.
If you want, I can prepare specific per-segment resistor placements and add decoupling caps next.
Clarify 555 clock routing
Fix floating UP/DOWN inputs
Add segment resistors
Add decoupling caps
Resolve LT/RBI/RBO wiring
85 days
Ready.
Ones_74192
R3
Resistance
10k Ω
7432
7408
R2
Resistance
10k Ω
Ones
R8
Resistance
10K Ω
R1
Resistance
10k Ω
R4
Resistance
10k Ω
Ones 7447
Reviews
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24-Second TTL Shot Clock with Up/Down BCD Counters and 7-Segment Display
Properties
Pricing & Availability
Distributor | Qty 1 |
|---|---|
Arrow | $4.34–$7.39 |
Digi-Key | $14.45–$17.49 |
LCSC | $17.53–$19.37 |
Mouser | $19.20 |
TME | $16.17 |
Verical | $4.62–$7.84 |
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