Timely adjustment of tire pressure

Driving a car tire safety performance indicators, while the tires subject to pressure, temperature, load, speed, traffic, continuous driving time, and many other factors, one of the most critical is the air pressure. To ensure that vehicle safety when driving at high speed performance, through effective methods to control speed and continuous driving time to adjust tire pressure and temperature, so that tire pressure is safe driving range.   Functional Analysis: Table-type tire pressure gauge is a very useful little equipment, can accurately read the tire pressure, and has good durability to ensure that the correct tire pressure can be used to enhance road safety.

As for the tire pressure gauge itself, it is a phenomenal little auto gadget.  It’s well made, feels sturdy, and the plastic that it’s made out of actually feels good in your hand.  Although a little large, it obviously would fit perfect in a glove compartment and since it’s solar powered you never have to worry about keeping batteries changed.

Some features that I immediately noticed was the red LED light positioned right at the tip of the gauge.  This enabled easy finding of the valve stem in the dark or in poorly lit areas.  Likewise the digital screen is backlit with a low red glow which makes for easy reading in any light.

As for accuracy of the digital tire pressure gauge, it appeared spot on.  I checked pressures against a traditional tire pressure gauge and found no descrepancies, other than the digital being more specific.  Also, the readings were returned immediately, no waiting necessary so less of a chance to lose air when checking tire pressure.

All in all I would recommend this $7.34 auto gadget to anyone, even if you already have a traditional tire pressure gauge.  It definately makes a no brainer for my wife and serves as a pleasant reminder to keep our tires in proper pressure condition.  Visitors from The-Grayline.com are also able to get a coupon for their purchase by using the code CM5OFF4, which will give a 5% discount at checkout.

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 :

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.

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.

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.