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I am new to IoT. I want to connect multiple smartphones (simultaneously) to single IoT device (chipset), wirelessly, but without internet.

Diagram of trying to connect smartphones to a chip controlling a light.

I want to turn on/off some electric device (for example, a fan). which hardware should I use?

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This is a pretty broad question, and depends a lot on your use case.

  • As @jsotola wrote, you could have your device generate a Wi-Fi network to which the phones would connect. No app needed, but may be slightly cumbersome to configure, especially if you have security. A QR code or NFC tag for automated Wi-Fi config may help. Or you could have an app that will help configuration. Of course, it's problem if the phones need to be connected to another Wi-Fi network.

  • Or you could use BLE. This requires an app to be installed on the phones, but then the app can discover your device and connect to it without the user having to do anything in the phone's config. Be aware that even though most chips that support BLE have the hardware capability to support multiple clients, some BLE stacks do not manage that well.

Note that you mention "lights" initially, then go on writing about switching electric devices on/off. This can raise a lot of security issues (like setting your place on fire), so unless you really know what you are doing, you're better of buying some sort of smart switch off the shelf. There are quite a few supporting either Wi-Fi or BLE.

If you have multiple devices to control, I would probably go for Zigbee, though that will require a gateway, and possibly a separate AP.

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  • actually i want to make socket which can be operated by smart phone for AC supply. something like this link but also want to connect more than one smartphone. and as you mentioned BLE harware which hardware do you prefer? Jun 12 '20 at 9:31
  • Make sure you really know what you are doing before embarking on this. Mains power can be deadly and destructive. I strongly recommend you buy one off the shelf. If you really know what you are doing and want to go ahead, any chip which supports BLE should be fine (ESP32, nRF52, and probably many more). However be aware that many BLE software stacks don't support multiple connected clients at the same time (IIRC the standard Arduino BLE library doesn't), so you may have to use the low-level development environment for that chip (ESP-IDF for the ESP32, nRF SDK for the nRF chips, etc.).
    – jcaron
    Jun 12 '20 at 9:38

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