A Raspberry Pi hat using the Microchip MCP3424 analogue to digital converter. The hat also has connectors for I2C and 1-wire interfaces, and two switched outputs, for instance to control fans, motors or relays. A Honeywell HIH6130-021 or HIH6131-021 I2C temperature and humidity sensor can also be fitted.

The MCP3424 is a 4 channel 18 bit differential input analogue to digital converter. Sample rates are 3.75 SPS (samples per second) at 18 bit resolution, 15 SPS at 16 bit resolution, 60 SPS at 14 bit resolution and 240 SPS at 12 bit resolution. When single-ended inputs are used the resolution is reduced by 1 bit. A jumper block enables the I2C address of the MCP3424 to be selected from eight possible values. The ADC inputs are directly wired to a 9 pin D connector, 3 channels can be used for differential inputs, the fourth for single-ended measurements only. Optional decoupling capacitors can be added to all inputs; alternatively zero ohm resistors may be used to convert the differential inputs to single-ended.

The buffered I2C interface is made available via an RJ11 connector, allowing additional MCP3424 devices or other I2C devices to be connected, eg the AuroraWatchNet magnetometer sensor head (https://github.com/stevemarple/AuroraWatchNet/tree/master/hardware/FLC100_shield/remote_v2).

A connector for a 1-wire interface is provided. The 3 pin Molex connector also connects to the Raspberry Pi's 3.3V supply so that it is not necessary to use parasitic power operation.

All power is used is taken from the Raspberry Pi via the GPIO header. The 1 wire interface, I2C buffer and HAT EEPROM are connected to the Pi's 3.3V supply, all other power is derived from the 5V supply. Changes in the CPU load on the Pi can result in small supply voltage variations due to voltage sag from the Pi's power supply. When sensors and the ADC are powered directly from the Pi's 5V supply this can result in spurious measurements. To combat this effect a DC-DC converter and linear regulator can be fitted to derive a stable analogue voltage supply.

Two switched outputs are provided which can be connected to fans, motors, relays etc. Flyback diodes can be fitted to allow safe switching of inductive loads.