• Automotive 24 V PWM-to-voltage and relay interface for an EPEC EC44.
• Status: Draft implementation.

• 24 V vehicle/heavy-equipment electrical environment.
• Converts two isolated 24 V PWM signals into nominal 0–5 V analog outputs and enables three relays only while an isolated heartbeat remains valid.

• Accept protected 24 V power.
• Generate regulated logic power and a precision 5 V analog reference.
• Isolate PWM1, PWM2, and heartbeat inputs.
• Low-pass PWM1/PWM2 near 10 Hz, assuming PWM frequency ≥500 Hz.
• Fail all relay coils OFF on heartbeat timeout or loss of 5 V control power.
• Provide isolated open-collector relay-coil feedback.

• Fused, reverse-polarity-protected, TVS-clamped 24 V input.
• LMR36520-family 65 V, 2 A buck regulator.
• Precision 5 V reference and OPA4171-Q1 filtering.
• TPS3823-50-Q1 watchdog and IRLZ44N low-side relay return switch.
• Two Omron G6K DPDT relays and one TE automotive SPDT relay interface.

Text
24V IN -> fuse -> reverse diode -> protected 24V -> relay coils
                                      |-> 5V buck -> watchdog
                                      |-> 12V analog LDO -> op amp + 5V reference
PWM1/2 -> PC817 -> 5V_REF pull-up -> 2nd-order LPF -> analog outputs
Heartbeat -> PC817 -> TPS3823 WDI -> IRLZ44N -> RELAY_GND
40A relay coil voltage -> PC817 -> isolated FEEDBACK_OUT/RETURN

• Input protection and power conversion.
• Precision reference and analog filtering.
• Three optically isolated command inputs.
• Watchdog/MOSFET fail-safe relay driver.
• Two low-signal DPDT relays and one high-current automotive SPDT relay connection.
• Isolated coil feedback.

• 24 V power input: +24V, GND.
• PWM1, PWM2, heartbeat: each signal plus field return.
• Analog outputs: AO1, AO2, GND.
• Relay contacts: COM/NO/NC terminals for each pole/contact set.
• Feedback: isolated open collector and isolated emitter return.

• Continuous vehicle supply; no battery runtime requirement.
• Relay coils remain on protected 24 V and are not powered by the 5 V reference.

• 0.5 A hold / 1 A trip, 60 V PPTC provides ample nominal margin, subject to automotive-temperature derating review.
• LMR36520 2 A capability greatly exceeds the small-signal 5 V demand.

• Professional mixed SMD/through-hole PCB assembly.
• High-current relay contact connector/copper sizing must be finalized during layout; do not assume ordinary 5.08 mm terminals are suitable for 40 A.

• None; watchdog is hardware-only. Heartbeat transitions must occur faster than the watchdog minimum timeout (0.9 s).

• Keep TVS/protection at power entry.
• Keep buck switch loop compact.
• Keep reference/filter circuitry separated from relay and switch nodes.
• Place flyback diodes at coil terminals and size high-current contact copper/connectors during PCB design.

• OPA4171-Q1 is powered from a dedicated nominal 12 V automotive LDO because a 5 V supply does not support accurate 0–5 V input/output operation.
• REF3050 cannot regulate 5.000 V from an exact 5.000 V source; the implementation uses an automotive REF5050-Q1 from the 12 V analog rail.
• Unity-gain Sallen-Key filters use R1=R2=113 kΩ, C1=200 nF, C2=100 nF for approximately 9.96 Hz and Q≈0.707.
• Exact library availability limitations are recorded in Design Notes.

• PWM and heartbeat field inputs are nominal 24 V and share a field return only on the LED side of the optocouplers.
• Input PWM frequency is at least 500 Hz.
• Heartbeat toggles at least once per 0.9 s under all healthy conditions.
• High-current relay contacts require a connector solution rated for the actual load, selected in PCB/mechanical design.

• Initial complete schematic architecture and engineering constraints captured.