init, Add README with notes and ideas

README.md

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+A Raspberry Pi-compatible PCB that extends the GPIO header with protected I/O, relays, and various features for versatile use in prototyping and project development.
+---
+
+**Notes:**  
+
+# Name
+- Pi-multipurpose-Adapter
+- Pi-multi-function-adapter
+- Pi-extension-board
+
+- universal-Pi-interface-board
+- pi-peripheral-interface
+- raspberry-extension-board
+- Pi-protected-connection-hub
+
+
+
+# RPI Specs (Zero, 3+)
+- Available (old) Pi: Raspberry Pi 2 Modell B v1.1 
+- GPIO
+  - Low: 0.9V
+  - High: 1.6V
+  - Input current: 5uA
+  - Input capacitance: 5pF
+  - Output Current +-17mA
+  - Pullup / Pulldown 50-65k
+
+- EEPROM GPIO0 GPIO1, leave unused
+
+
+
+# Feature Ideas
+## Supply
+- Input protection
+  - TVS
+  - reverse polarity
+  - Fuse
+  - buffer
+- Pass through power supply
+- 3.3V, 5V, 12V, 24V Regulator
+
+- power by barrel connector (24V 3A)
+- power by usb
+  - Switch between power by USB or power supply?
+- power by battery
+  - liion cell connector
+  - BMS / charge controller
+
+
+## IO
+- Input
+  - GPIO
+    - voltage protection
+        - 330R, Z
+    - wide voltage range 1.6-24V
+    - Low pass filter
+    - Optocoupler input
+  - ADC
+    - external IC
+    - multiplexer?
+    - protection
+    - scale input voltage
+      - fixed inputs: 24V 12V 5V 3.3V ?
+      - trim potentiometer?
+- Output
+  - Relay
+  - Mosfet
+  - Optocoupler output
+  - GPIO with over current protection
+  - GPIO level shift (24V output)
+  - PWM pins
+  - shift register
+  - DAC
+
+- LEDS indicate I/O state
+- Testpoints
+- ESD protection
+
+
+## Bus
+- UART/RS485
+- RS232 Connector
+- I2C Pullup
+- SPI
+- CAN
+- Serial to usb (if Arduino)
+- OWB default GPIO4
+
+
+## UI
+- Input
+  - Buttons
+  - DIP switches?
+  - Temperature sensor
+  - Encoder
+  - Potentiometer
+- Output
+  - Display
+  - Buzzer
+  - LEDS
+
+
+## Mechanical
+- Barrel plug (24V 3A supply)
+- Screw terminals / spring loaded?
+- pin header angled?
+- pin header for extension
+- pass through all pins?
+
+## Housing
+- RPI ports
+-  banana sockets for power out 
+-  screw terminal exposed
+- buttons, leds: need to open up?
+-  BNC? []
+
+
+## Other
+- Extension board with multiplexer
+- Breadboard compatible?
+- SD Card
+- Onboard MCU for standalone use?
+- separate bought MCU via i2c  for additional GPIO, DAC, ADC in one