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The inductor in the middle is the coil of a single-coil latching relay, which requires a H-bridge to drive.

How do I add flyback to the 74HC04's internal MOSFETs? Or should I just rely on body diodes?

schematic

simulate this circuit – Schematic created using CircuitLab

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The absolute maximum ratings of the LVC family do not allow for 12V to be applied to an output (6.5V max), even of an OD device. You should either use a NMOSFET directly (since you seem to want to invert the input regardless) or use a 7406/7416 (since you want 3.3V interfacing) instead.

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  • \$\begingroup\$ So all okay, just replace the 74LVC1G06 with 74HC06? \$\endgroup\$ – Maxthon Chan Sep 22 '15 at 19:03
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    \$\begingroup\$ No. HCMOS devices cannot handle that high a voltage at the output either. You need to use the TTL 7406. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 22 '15 at 19:03
  • \$\begingroup\$ Which means that the gates on the inside won't work either. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 22 '15 at 19:06
  • \$\begingroup\$ Then can I use a few discrete MOSFETs then? a lone 2N7002 replacing the 74LVC1G06 and a complimentary pair AO4606 replacing the high-voltage 74HC04? \$\endgroup\$ – Maxthon Chan Sep 22 '15 at 19:10
  • \$\begingroup\$ Since the first MOSFET brings the voltage high enough to use the complementary pair, yes. But it won't be as efficient as an integrated H-bridge due to lack of dead time. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 22 '15 at 19:13
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Modern logic gates are not rated for 12Vdc ! The old HEFcmos series that would handle the volts has a very low gaurenteed Output current capability.Another hassle with old HEF is that inputs should be 12V so you would be up for some sort of level shifting .Why not use a mosfet driver like a mc4426 or similiar .They come in 8 pin package ,They are made by many manufacturers,They are generaly rated at 18V ,They accept 3v3 or 5v logic with thier mosfet inputs .You dont need pull up resistors or freewheeling diodes with these mosfet driver chips .It really is a full bridge .

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