CONSTRUCTION DETAILS
Since the MPXY80xx family sensors perform absolute pressure measurements, the sensor itself must be physically separated and enclosed in a hermetic chamber, which is pressurized during measurement. Current atmospheric pressure needs to be subtracted in software from the tire pressure indicated by the sensor to display correct differential tire pressure.
Mechanical construction of the first prototype is shown under figure.
The sensor, with its power supply decoupling capacitor, is placed on a separate PCB. Connection between the main board and the sensor PCB is achieved by a miniature connector pair. This enables easy sensor PCB extraction and replacement in the prototype system. The main PCB is secured in the enclosure by a single M3 screw, while the
sensor PCB is held in position only by the connector friction.
Prototype PCBs
Actual width of the main PCB is approximately 38mm
A photograph of the assembled gauge is shown under :
SCHEMATICS
Based on the component selection made earlier, we can now look at the detailed schematic diagram of the application
(see Figure).
The LCD and the sensor are connected to GPIO pins of the microcontroller. The LCD drive waveforms and SPI communication interface for the sensor are created in software.
The CPU is clocked by an external oscillator. To further reduce the overall system cost, the oscillator can be replaced by a low–cost resonator since accurate timing is not required or the CPU can be replaced by MC68HC908JL3 (as mentioned above).
The push button is connected to the IRQ pin of the CPU. Button depression can wake–up the CPU from low power stop mode even when the oscillator is stopped to minimize the power consumption.
During SW debugging, the code of the application is downloaded to the CPU through the SCI port (RS–232 protocol) by utilizing the serial bootloader. This path can also be used for future firmware upgrades.
The bill of materials is shown under Table.
| Item | Quantity | Reference | Part |
| 1 | 1 | BT1 | CR2032, 3 V Lithium |
| 2 | 2 | C2, C1 | 22n |
| 3 | 1 | C3 | 470n |
| 4 | 2 | C4, C5 | 100n |
| 5 | 1 | D1 | LCD, 2 x 11 Segments |
| 6 | 1 | J2 | Connectors and MPXY8010 |
| 7 | 1 | J3 | Header for Debugging |
| 8 | 1 | R1 | 330k |
| 9 | 1 | R2 | 10M |
| 10 | 4 | R3, R4, R5, R6 | 10k |
| 11 | 1 | SW1 | Push Button |
| 12 | 1 | U1 | MC68HC908GR8 |
| 13 | 1 | Y1 | Crystal, 32.768 kHz |
System Concept Tire Pressure Gauge: Digital Pressure Gauge
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Description of High Accuracy Digital Pressure Gauge Design
System Concept
From a systems point of view, a tire pressure gauge is relatively simple (see Figure 1). The heart of the application is the microcontroller. It reads data out of the pressure sensor and remembers the maximum value. This value is then shown on the attached display. The user can power–on the application or clear the display (reset the maximum value) by depressing the push button.
Figure: System Concept
Since the whole application is handheld and needs to be powered by a small battery, the power consumption is critical. Another important factor which governs selection of components is size — the application needs to be small in dimensions and lightweight.
For the pressure sensor, the CMOS absolute pressure sensor designed for tire pressure monitoring is a perfect match. The power consumption in stand–by mode is below 1.2 μA (typically around 0.6 μA in ordinary temperature range). The sensor features very small dimensions (10 x 7.5 x 4.2 mm) and is available in different pressure ranges. This makes the application easily adaptable for different tire pressure ranges by simply exchanging the sensor.
Selection of the microcontroller is also driven by low power consumption and a small package. In addition, it needs to have enough GPIO pins to interface to the sensor and the display. MC68HC908GR8 was chosen for the first prototype: it features very low power consumption in stop mode (below 3 μA, 1 μA typ.) and the 32–pin LQFP package is small enough while providing just enough I/O pins. MC68HC908JL3 with the RC–based oscillator can also be used to lower cost of the application.
Because of power consumption limitations of the system, an LCD was chosen as the display.
The choice of battery which powers the unit is a compromise between size and weight and available capacity. The CR2032 lithium coin cell with a capacity of 210 mAh was chosen. Since the system will typically draw only 1.6 μA in stand–by mode, the battery would be capable of delivering the stand–by current for approximately 15 years. Measurements on the prototype showed that when the application is running the system power consumption is around 2.2 mA. The expected lifetime of one battery under different conditions is shown in under table.
| Conditions and Usage | Lifetime |
| Typical power consumption values, frequent check of pressure in 4 tires (once per 10 days) | >15 years |
| Typical power consumption values, heavy usage (every day checking of pressure in 4 tires) | >15 years |
| Typical power consumption values, extra heavy usage (check of 40 tires per day) | 3.1 years |
| Worst case power consumption values, frequent check of pressure in 4 tires (once per 10 days) | 5.5 years |
| Worst case power consumption values, heavy usage (every day checking of pressure in 4 tires) | 4.3 years |
| Worst case power consumption values, extra heavy usage (check of 40 tires per day) | 1.3 years |
It can be seen that with the selected battery the product is suitable for home usage, however professional usage might require larger battery capacity. A battery holder was used in the prototype for easier testing and to enable the user to replace the battery after it is exhausted.