Monitors the underpressure of your underwater housing and notifies you of any leaks.
The leak detector provides the following features:
- Optical and acoustical alarm. Red and green LED to indicate the current health. Piezo buzzer to get the user's attention.
- Powered by just one CR2032 coin cell.
- Very long battery life, works down to 1.8V cell voltage.
- Battery monitoring. Indicates low-battery during idle mode.
- Hackable design using a Microchip ATtiny microcontroller
- MEMS pressure sensor. Supports two different devices: Bosch BMP581 and STMicro LPS22HB.
- Demo mode. Test any changes by a simulated sensor.
- Optical UART debug mode. The red LED can be used as a serial port. The receiver consists of a photo transistor and a bipolar transistor in a darlington configuration and an UART-to-USB adapter.
- Different modes selectable by the reset button.
- Extremely small.
- Open Hardware and Open Firmware schematics and layout as well as firmware sources are free for everyone.
TBD
Most functionality revolves around the microcontroller polling the pressure sensor at a low frequency and then go (back) to sleep. The goal is to be in sleep mode most of the time to save precious battery power. The microcontroller runs with a system clock of 1MHz. The assumption is that a slower clock frequency will preserve more power. But it is not clear whether this is true; it might as well save more power if the microcontroller can go back to sleep faster. One would need to do power consumption measurements to evaluate which is better.
There are basically two different modes:
- Idle mode. The detector waits for a pressure drop to go into supervisor mode. In this mode, the pressure is polled every two seconds.
- Supervisor mode. The detector monitors the pressure at a faster rate and drives the dual-color LED/buzzer to show the state to the user.
Internally, the firmware consists of a state
machine. The state maps to the external indications
at any time. E.g. if the program is in state ALARM, the LED is flashing
red and the buzzer is enabled.
The firmware is able to count the number of reset button presses. Depending on this the firmware enters different modes:
- Normal mode
- Debug mode. Red LED is used to transmit debug information.
- Demo mode. Instead of reading the real pressure, predefined measurements are replayed.
- Demo mode with debug. Combines demo and debug mode.
- Power-off mode. Turn off periodic watchdog interrupt. Only an external reset can wake up the microcontroller.
The Timer/Counter0 is used to stimulate the piezo buzzer with a 2kHz frequency.
The Timer/Counter1 is either used in PWM mode to dim the LEDs or to drive the software UART. Both are mutually exclusive and the startup will choose between these two.
The Universal Serial Interface is used for the I2C communication with the pressure sensor. There is no receiving ACK handling (yet).
The Watchdog timer is used in interrupt mode as a wake up source. It will wake up the microcontroller either every 250ms or every 2s, depending on the mode. Additionally, it is used for timekeeping by counting ticks.
The Analog to Digital Converter is used in reverse mode (AREF is VCC and the measurement is the internal band gap voltage) to measure the battery voltage.
The *Analog Comparator` is unused and disabled on start-up.
The hardware is prepared to use interrupts via the *Pin Change Interrupt` but it is unused. The sensor supports to constantly measure the pressure and generate an interrupt if it is outside a predefined window. But it is very limited in this regard, thus it not used for the time being.
The firmware supports two different pressure sensors:
They share the same footprint and can be used with the same PCB. The BMP581 is more advanced regarding its features, but most of them are unused. One major drawback is the package and metal can of the sensor. There is a tiny hole in the can which sometimes clogs and the sensor will measure bogus values. Also, it isn't easy to convert the raw value of the sensor to millibars. It will need division routines which takes up a lot of code space.
The LPS22HB is a rather old sensor and consumes more power. It's package doesn't have any holes but an exposed silicon area to sense the pressure. Therefore, it is not prone to clogging like the BMP581. Also the register interface seems to be more thought through. There are also successors of the LPS22HB which need less power. Unfortunately, they aren't available at the moment.
TBD
The board features a Tag-Connect with the standard 6pin header AVR pinout. The microcontroller can be programmed using the simple SPI based programming protocol. There is no need for high-voltage programming.
The schematics are available here.
| Ref | Qty | Part | Footprint |
|---|---|---|---|
| R1 | 1 | 33Ω Resistor | 0603 |
| R2, R7 | 2 | 1kΩ Resistor | 0603 |
| R3 | 1 | 10Ω Resistor | 0603 |
| R4, R5 | 2 | 100Ω Resistor | 0603 |
| R6, R8, R9 | 3 | 10kΩ Resistor | 0603 |
| D1 | 1 | Dual-color LED | - |
| U1 | 1 | BMP581/LPS22HB pressure sensor | LGA-10 |
| U2 | 1 | ATtiny25V | SOIC-8 |
| SW2 | 1 | PTS810 Push Button | - |
| BT1 | 1 | CR2032 Coin Cell Holder | - |
| BZ1 | 1 | Visaton 3580 Piezo Buzzer | - |
If you need a PCB, a kit or an assembled and pre-programmed board, contact me at uwh-leak-detector@walle.cc.
The schematics and layout is licensed under the CERN-OHL-S v2. The firmware source is licensed under the GNU General Public License v3.0.