This project is a hottub controller for an Aruba Spas controller. It was built with a Raspberry Pi Zero W.
The Raspberry Pi sits on the line between the existing hot tub keypad and the hot tub controller. It operates as a passive sniffer (to read data being transmitted from the controller to the keypad's LEDs) and has control to act as a button, to simulate keypresses on the keypad.
The hot tub controller is connected to the keypad with an 8-port connection, using 5v logic. The controller is a Motorola MC68HC912D60, running at 5V. The keypad uses a serial latch to read data via SPI and buffer it to the LEDs. This enables the controller to display different data on the keypad, depending on the state (eg: Displaying the current temperature, or the temperature that the hot tub is currently programmed to).
The pinout is the following:
| Description | Pin |
|---|---|
| Ground | 1 |
| Temp Up | 2 |
| Light | 3 |
| Pump | 4 |
| +5V | 5 |
| Data In/Temp Down | 6 |
| Clock | 7 |
| Enable | 8 |
Each of the buttons on the keypad correspond to a port. These are normally pulled up to high. When a button is pressed, these are drained low, with the following circuit:
To drive these inputs, the Raspbery Pi operate a BJT switch, capable of driving the line low. The BJT selected (2N3904) has a CE voltage of 0.2 when fully saturated. According to the hot tub controller spec, voltage must be at most 1.0V, (0.2 * VCC), so this falls within the range. The BE voltage is 0.7 when saturated, providing 2.6V across the base resistor (Raspberry Pi provides 3.3V). A 2kOhm resistor was selected, allowing for a 1.3mA base current. The BJT has a gain of 10-20, allowing for the button circuit to drain 10-20mA, within spec.
Note that driving a button low requires the Raspberry Pi to drive a voltage high, as the BJT inverts the signal.
The controller communicates to the keypad, sending it the status of the LEDs at ~60Hz. This protocol is sent over SPI mode 3 (CPHA=1, CPOL=1). These lines are spliced to a 74HC4050N high-to-low level shifter, allowing for the Raspberry Pi to safely sniff these signals as they change.
The Raspberry Pi runs a tornado webserver that emulates a keypad. It uses the pigpio library to read data from and write data to the GPIO pins.
The controller transmits 4-bytes at ~60Hz, resulting in a bitrate of ~2KhZ. This is too fast for the Raspberry Pi to sniff using software polling, so hardware interrupts are used.
The protocol 4-byte protocol contains the LED status (5-status LEDs, and 3 BCD digits for the 7 segment display).
| Byte 1 | Byte 2 | Byte 3 | Byte 4 |
|---|---|---|---|
| ----SSSS | S---AAAA | BBBBCCCC | -------- |
- S = Status
- A = Temperature (100's place)
- B = Temperature (10's place)
- C = Temperature (1's place)
Thanks to the following projects for providing the underlying infrastructure for the software: