Using espruino and nRF52840 nice!nano clones off aliexpress as custom tags (tenstar)

[enaon]

I am not sure anyone is really interested in this, but I believe Espruino is excellent for this task.

This is a simple example, one key only, but it is easy to add more. The tenstar nano draws ~18μΑ using this script.

//custom FindMy tag using espruino

D13.reset(); //disable VCC regulator, this saves ~50μΑ

const myBase64Key = "private_Key"; // add private Key here

function base64ToBytes(base64) {
  let binaryString = atob(base64);
  let len = binaryString.length;
  let bytes = new Uint8Array(len);
  for (let i = 0; i < len; i++) {
    bytes[i] = binaryString.charCodeAt(i);
  }
  return bytes;
}

function startAppleFindMy() {
  let public_key = base64ToBytes(myBase64Key);

  // 1. Construct MAC adreess from Key
  let ble_mac = [];
  ble_mac.push(public_key[0] | 0xC0); // Make sure  MAC starts with binary format '11'  (Private Random)
  for (let i = 1; i < 6; i++) {
    ble_mac.push(public_key[i]);
  }

  //  MAC to Hex String
  let hexParts = [];
  for (let i = 0; i < ble_mac.length; i++) {
    let hex = ble_mac[i].toString(16);
    if (hex.length < 2) hex = '0' + hex;
    hexParts.push(hex);
  }
  NRF.setAddress(hexParts.join(':') + " random");

  // 2. Calculate Hint byte from Key byte 1
  let hintByte = public_key[0] >> 6;

  // 3. Last 22 bytes (public_key[-22:])
  let advKeyBytes = public_key.slice(public_key.length - 22);

  // 4. construct RAW packet (31 bytes)
  let payload = [
    0x1E,       // Length
    0xFF,       // Manufacturer Data
    0x4C, 0x00, // Apple ID
    0x12,       // Find My Type
    0x19,       // Bytes Length
    0x00        // State 
  ];
  
  // add 22 last bytes from Key
  for (let i = 0; i < advKeyBytes.length; i++) {
    payload.push(advKeyBytes[i]);
  }
  
  // Last two Bytes(Hint  & Battery status)
  payload.push(hintByte); //  Hint Byte connected to MAC
  payload.push(0x00);     // Battery / Status status

  // diisable UART  & Scan Response
  NRF.setServices({}, { uart: false });
  NRF.setScanResponse([]);

  // Max TX Power
  NRF.setAdvertising({});
  NRF.setTxPower(8);

  // 5. Advertise RAW packet, interval 2 sec.
  NRF.setAdvertising(payload, { 
    interval: 2000
  });
}

// Start
startAppleFindMy();
 

[enaon]

and some photos, I use them for tracking stray cats.

[dennisdebel]

Wait, does this still work at all?

…

On Tue, 4 Aug 2026 at 13:28, enaon ***@***.***> wrote: and some photos, I use them for tracking stray cats. image.png (view on web) <https://github.com/user-attachments/assets/06b7eff8-c40b-497c-8733-39f06e15ccb3> image.png (view on web) <https://github.com/user-attachments/assets/514e68ae-a5c8-4c69-9c89-bf752bd662ea> image.png (view on web) <https://github.com/user-attachments/assets/0dc75a94-5259-407f-89d8-5546d4c4ba68> image.png (view on web) <https://github.com/user-attachments/assets/e66c3696-3795-4373-9448-3fb01c21afb6> — Reply to this email directly, view it on GitHub <#259?email_source=notifications&email_token=AA6AIUFDZ445CTEO7UANH635IFX7RA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TEMBZUZZGKYLTN5XKU43VMJZWG4TJMJSWJJLFOZSW45FMMZXW65DFOJPWG3DJMNVQ#discussioncomment-17889209>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AA6AIUH4TDC746ZJDIMNO735IFX7RAVCNFSNUABIKJSXA33TNF2G64TZHM3TGNZTHEYTCMBQHNCGS43DOVZXG2LPNY5TCMBVGQ4DSNRWUF3AE> . Triage notifications, keep track of coding agent tasks and review pull requests on the go with GitHub Mobile for iOS <https://github.com/notifications/mobile/ios/AA6AIUH7XPJQ53PPM7CD2OL5IFX7RA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TEMBZUZZGKYLTN5XKU43VMJZWG4TJMJSWJJLFOZSW45FKMZXW65DFOJPWS33T> and Android <https://github.com/notifications/mobile/android/AA6AIUGTREWBHBWFOPUAIC35IFX7RA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TEMBZUZZGKYLTN5XKU43VMJZWG4TJMJSWJJLFOZSW45FOMZXW65DFOJPWC3TEOJXWSZA>. Download it today! You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[enaon]

It works very well, should it not?

I am very new at this, just found out about findMy.py, I was using macless haystack till now, I am planning to switch. But I am experienced in Espruino, this script works fine, no need for multiple keys unless one is woried about privacy, I am not, the tags are on cats. This is the reason I uploaded, I am reading about things like esp32 and custom firmwares, and it is a shame. Espruino and nRF52840 is excellent for this task.

[dennisdebel]

I will give it a shot again, thanks. A few weeks ago i spend two days trying to get it to work but no dice (using esp32-s3)

…

On Tue, 4 Aug 2026 at 14:48, enaon ***@***.***> wrote: It works very well, should it not? I am very new at this, just found out about findMy.py, I was using macless haystack till now, I am planning to switch. But I am experienced in Espruino, this script works fine, no need for multiple keys unless one is wried about privacy, I am not the tags are on cats. This is the reason I uploaded, I am reading about things like esp32 and custom firmwares, and it a shame. Espruino and nRF52840 is excellent for this task. — Reply to this email directly, view it on GitHub <#259?email_source=notifications&email_token=AA6AIUA6QRQWBZT2ZIYPTED5IGBLNA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TSOJXUZZGKYLTN5XKOY3PNVWWK3TUUVSXMZLOOSWGM33PORSXEX3DNRUWG2Y#discussioncomment-17889997>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AA6AIUDXWTC6SNC47XNGOWL5IGBLNAVCNFSNUABIKJSXA33TNF2G64TZHM3TGNZTHEYTCMBQHNCGS43DOVZXG2LPNY5TCMBVGQ4DSNRWUF3AE> . Triage notifications, keep track of coding agent tasks and review pull requests on the go with GitHub Mobile for iOS <https://github.com/notifications/mobile/ios/AA6AIUEORC23W364ZBPL3RL5IGBLNA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TSOJXUZZGKYLTN5XKOY3PNVWWK3TUUVSXMZLOOSVGM33PORSXEX3JN5ZQ> and Android <https://github.com/notifications/mobile/android/AA6AIUBKAWTRMKZTPTPP24T5IGBLNA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNZYHA4TSOJXUZZGKYLTN5XKOY3PNVWWK3TUUVSXMZLOOSXGM33PORSXEX3BNZSHE33JMQ>. Download it today! You are receiving this because you commented.Message ID: <malmeloo/FindMy. ***@***.***>

[malmeloo]

This is really cool, thanks for sharing! 18μΑ is not bad for an off the shelf board, especially when using espruino on top of that.

Have you tested the battery life yet? With those batteries they should last you what, half a year or so?

[enaon]

This is really cool, thanks for sharing! 18μΑ is not bad for an off the shelf board, especially when using espruino on top of that.

Have you tested the battery life yet? With those batteries they should last you what, half a year or so?

Correct, ~5 months for the 90mAh batteries, way over a year for the 300(602030) ones. I have them working for over a year now. Measurements are made using the PPK2, and assume a good quality board. QC is not great, some Tenstar(red) boards are at 2μΑ when BT is in sleep mode, some go up to 40. Espruino has no impact on battery life, it can be a bit slow (that makes no difference for this use case), but consumption is top class.

I am using this to drop GPIO to 3.0V, so efficiency is good at low voltage(needs an updated adafruit bootloader) , I am using batteries that auto shutdown at 2.8 volts.

NRF.restart(function(){
  poke32(0x4001e504,1);while(!peek32(0x4001e400)); // enable flash writing
  poke32(0x10001304,0xfffffff8+4);while(!peek32(0x4001e400));
  poke32(0x4001e504,0);while(!peek32(0x4001e400)); // disable flash writing
});

I've made it so that it switches between modes, it emulates my custom tag protocol, apple itag protocol, and google tag protocol, in turns, every 5 seconds (advertising every 2 secs). This (the switching part) has minimal impact on battery life.(18 ->22)