Design of NFC ultra-low power wireless reading head

Do you know the NFC ultra-low power wireless reading head?

1. After the NFC reader completes reading the tag, the card data must be sent to the receiver using wireless communication.

2. The installation method is shown below. Because the oil nozzle is hung in the holster when not in use, the oil nozzle mouth needs to be inserted when in use, so the size must not be too large.

3. Because it is used close to flammable and explosive environments, it complies with an explosion-proof design.

The reading head can only be powered by batteries. In such a small size, large-capacity batteries cannot be used.

For explosion-proof considerations, energy-type lithium-iron batteries with small discharge currents are used.

The batteries need to have characteristics such as a small short-circuit current and no risk of thermal runaway.

NFC Wireless Reader

Based on the above requirements, how to achieve ultra-low power consumption?

In view of the limitations mentioned above, the average power consumption and instantaneous power consumption of the read head are required to be controlled at a very low level.

1. In terms of instantaneous power consumption, there are two main modules that have greater demand for current, namely the radio frequency department of wireless communication and the tag reading module.

For wireless communication modules, after considering several aspects such as power consumption, size, price, simplicity, etc.,

low-power Bluetooth (Bluetooth) was chosen from various short-distance wireless communication methods such as LORA, Wi-Fi, Bluetooth,

and Zigbee. Low Energy, BLE). It has the advantages of ultra-low power consumption, low solution cost, and easy development and maintenance.

For the tag reading module, the high-frequency solution was selected among the low-frequency (LF 125-134.2 kHz), high-frequency (HF 13.56 MHz),

and ultra-high frequency (UHF 860-960 MHz) technical solutions. Its advantage is that it has a wide variety of tags,

is compatible with the NFC card simulation function of mobile phones, and its power consumption is within an acceptable range.

For the two modules with large current requirements, a staggered start method is adopted to further reduce the peak current.

After the tag reading is completed, the card reading module sleeps, and then the Bluetooth module starts to send data.

After the data sending is completed, it sleeps and waits for the next process.

2. Controlling average power consumption further tests low-power design capabilities.

On the one hand, we must start with the design of the hardware circuit. All IC devices must choose low-power consumption models;

all resistor devices must have a resistance value as large as possible while meeting the signal transmission requirements,

and avoid using electrolytic capacitors with large leakage current. , use tantalum capacitors with small leakage current instead.

On the other hand, the reading head needs to go into a deep sleep when the nozzle is mounted, wakes up when refueling is needed, and then starts tag recognition;

it sleeps normally after completing the identification and data-sending process once and re-enters deep sleep after multiple recognition failures or after mounting the nozzle.

Waiting to wake up. The normal sleep method is that the IC enters a low-power mode and can be woken up using a timer or IO;

in this mode, the core of the IC is still running and still requires a clock; in addition to the normal sleep method, a deep sleep design is further designed;

Add a low-power PMOS before the main circuit to control the power supply of the main circuit. By default, the main circuit is disconnected, with only a few resistive components and the power consumption of the PMOS itself;

add a mercury switch at the control end of the PMOS, when the oil gun changes from the vertical state hanging in the holster to the horizontal downward state inserted into the fuel tank, the mercury switch is turned on,

and the main circuit starts to supply power. After the system is powered on and running, the PMOS control terminal is locked to continue the power supply. The mercury switch fails;

when the oil nozzle fails to recognize the tag continuously, the system releases control of the PMOS and enters sleep. After the oil gun is hung back in the holster, the mercury switch is disconnected,

and the POMS will disconnect the main circuit and re-enter deep sleep.

PMOS

After controlling the instantaneous power consumption and average power consumption, the normal use time of the reading head can reach more than 3 months by replacing the battery once.

 

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