wiegand-rpi parts

wiegand-rpi.txt

relay $0.35 https://lcsc.com/product-detail/Power-Relays_Omron-Electronics-G5NB-1A-E-DC12_C225714.html
relay fet $0.03 https://lcsc.com/product-detail/MOSFETs_FUXINSEMI-2N7002KD_C880912.html
relay diode $0.01 https://lcsc.com/product-detail/Switching-Diode_TWGMC-BAV70_C727123.html
relay output polyfuse 750mA $0.06 https://lcsc.com/product-detail/Resettable-Fuses_BHFUSE-BSMD0805-075-16V_C976303.html
wiegand 12v output polyfuse 750mA $0.06 https://lcsc.com/product-detail/Resettable-Fuses_BHFUSE-BSMD0805-075-16V_C976303.html
wiegand input zener $0.03 https://lcsc.com/product-detail/Zener-Diodes_PANJIT-International-BZX84B2V7_R1_00001_C188540.html

total cost (just for those parts) for an 8 channel board: $4.16

no excuse anymore for using 8-bit $1.05 https://lcsc.com/product-detail/ST-Microelectronics_STMicroelectronics-STM32G030F6P6_C724040.html
or with plenty more pins $1.45 https://lcsc.com/product-detail/ST-Microelectronics_STMicroelectronics-STM32G030K6T6_C529331.html
(do what you want; it's ok to be wrong)

wiegand/relay spring terminals $0.25 https://lcsc.com/product-detail/Spring-Clamp-System-Terminal-Block_Cixi-Kefa-Elec-KF250-3-5-6P-1_C475158.html
power input spring terminals $0.11 https://lcsc.com/product-detail/Spring-Clamp-System-Terminal-Block_Cixi-Kefa-Elec-KF250-3-5-2P-1_C475154.html
fire alarm spring terminals $0.11 https://lcsc.com/product-detail/Spring-Clamp-System-Terminal-Block_Cixi-Kefa-Elec-KF250-3-5-2P-1_C475154.html

total cost for 8 channels of spring terminals: $2.22

Clearly the problem with any connector type is going to be fitting 8 channels of them within a reasonable pi hat footprint.
Those vertical spring terminals seem like a possible solution rather than any kind of board-edge connector.
Presumably this means that all of the SMD components will be on the bottom because the relays and terminals look like
they will consume basically the entirety of the top side. The alternative is that we could let the hat board span outside
the bounds of a typical hat aka we can go east past the bound of the usb/ethernet metal and go south past the bound of
the south edge of the rpi. In order to go further east we need more altitude to go over the top of the usb/ethernet metal
which means we need the 40p gpio connector to be taller than normal.

40p gpio header $0.30 https://lcsc.com/product-detail/Female-Headers_Liansheng-FH-00083_C2829905.html
red led 1608 $0.004 https://lcsc.com/product-detail/Light-Emitting-Diodes-LED_XINGLIGHT-XL-1608SURC-06_C965799.html


So this is looking like a ~$10 BOM, even with all the bells and whistles.
On Mouser I'm just guessing it looks more like a $35+ BOM, largely from connectors but also the trivial silicon and
obviously the relays which of course dominate the cost.