Campaigns139–144: reader files

Reviewed educational designs; no physical or target execution claimed. Keep package folders intact. Native CAD and manufacturing qualification remain separate.

• What a Piezo Sounder Draws at Different Frequencies
• Ethernet Split Pairs: Two Cables, the Same Continuity Map
• Ceramic Capacitor DC Bias: Measure Two Small Steps
• SN74HC125: Let One Output Release Before the Other Drives
• SN74HC164: Hide an LED’s Intermediate States While the Register Shifts
• SN65HVD230: Two Transmit Requests, One CAN Bus State