<aside>

IR sensors

• reflective sensor: there’s an IR emitted from this sensor, and a receiver to get the reflected light and strength. In a better reflective sensor module, it can even emitted IR in a specific frequency/pattern, and it will only detect the special IR it emitted, and eliminate the influence by environmental light. Omron is a good brand. Reflective sensor is measuring the “strength” of reflective.
• ToF measure the “distance/time” for the light to come back
• PIR is passively sensing the “change” of the environmental IR (there’s two receivers in the chip, and PIR detects the change, meaning the difference value detected by these two receivers), it’s passive and doesn’t emit light itself.
• Thermal Camera: really shows the thermal image, here’s a more delicate/high-resolution one. </aside>

<aside>

BLE (Bluetooth Low Energy)

• pros: low energy, pairing is optional

• cons: it’s slow

• the difference between BLE & Bluetooth Classic: totally different technology, using different protocol and frequency. The only shared quality is they are both defined by Bluetooth Special Interest Group.

• **2.4 GHz radio use:** within ISM radio band (Industrial, Scientific, and Medical spectrum: no license required, they are around 2.4Ghz/5Ghz, and that’s why WiFi in that bandwidth)

• In choosing protocol, there are two options and each comes with trade off:

  1. Transmit less data at once, the most extreme case will be transmitted a single bit, and the difference is only 0/1. Pros: can travel far without being interrupted; Cons: slow

  2. Transmit multiple bits at once, using the difference in amplitude to represent different signal. Pros: easily effected by noice; Cons: fast

     

     Case 1(Upper); Case 2(Down)

     (add the wifi chart)

• In like a concert situation, a communication without handshake is preferred, so bluetooth’s “broadcasting” characteristics is very useful in this situation

</aside>

<aside>

Extended resources

• An open source library to hack Apple
• LoRa: LoRa uses license-free sub-gigahertz radio frequency bands EU433 (LPD433) or EU868 (863–870/873 MHz) in Europe; AU915/AS923-1 (915–928 MHz) in South America; US915 (902–928 MHz) in North America; IN865 (865–867 MHz) in India; and AS923 (915–928 MHz) in Asia;[6] LoRa enables long-range transmissions with low power consumption. </aside>

<aside>

HOW DO APPLE DO THIS

iBeacon

• TX power: is a fixed number send out from iBeacon, indicating the “strength of signal” it should be when the iBeacon source is 1 meter away from the device. So when the device receives this signal, it can compare the signal strength it receives, and the TX power number, to roughly tell how far the iBeacon source (which is probably another Apple device) is.

AirTags

• When you are using your phone to find an AirTags, the AirTags sent out BLE when you are faraway, and with to UWB when you get close, so both precision and energy saving can be achieved.

Find My iPhone

• iPhone that agrees to be in find my iPhone network, will keep track of other iPhone’s position in the network, and the user has the special encrypted code that can locate specific iPhone position </aside>