resurface original bthrm app as bthrmlite - for https://forum.espruino.com/conversations/385841/#comment16951663

apps/bthrmlite/ChangeLog

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+0.01: New App!

apps/bthrmlite/README.md

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+# Bluetooth Heart Rate Monitor - Lite
+
+When this app is installed it overrides Bangle.js's built in heart rate monitor with an external Bluetooth one.
+
+The [`bthrm` app](https://banglejs.com/apps/?id=bthrm) is a much more sophisticated version of this app, however is uses a lot more
+RAM so may not be suitable for Bangle.js 1.
+
+HRM is requested it searches on Bluetooth for a heart rate monitor, connects, and sends data back using the `Bangle.on('HRM'` event as if it came from the on board monitor.
+
+This means it's compatible with many Bangle.js apps including:
+
+* [Heart Rate Widget](https://banglejs.com/apps/#widhrt)
+* [Heart Rate Recorder](https://banglejs.com/apps/#heart)
+
+It it NOT COMPATIBLE with [Heart Rate Monitor](https://banglejs.com/apps/#hrm)
+as that requires live sensor data (rather than just BPM readings).
+
+## Usage
+
+Just install the app, then install an app that uses the heart rate monitor.
+
+Once an app requests Heart Rate `bthrmlite` will automatically try and connect to the first bluetooth
+heart rate monitor it finds.
+
+**To disable this and return to normal HRM, uninstall the app**
+
+## Compatible Heart Rate Monitors
+
+This works with any heart rate monitor providing the standard Bluetooth
+Heart Rate Service (`180D`) and characteristic (`2A37`).
+
+So far it has been tested on:
+
+* CooSpo Bluetooth Heart Rate Monitor
+
+## Internals
+
+This replaces `Bangle.setHRMPower` with its own implementation.
+
+## Creator
+
+Gordon Williams

apps/bthrmlite/app-icon.js

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+require("heatshrink").decompress(atob("mEw4UA///g3yy06AoIZNitUAg8AgtVqtQAgoRCAwITBAggABAoIABAgsAgIGDoIEDoApDAAwwBFIV1BYo1E+oLTAgQLGJon9BZNXBatdBYRVFBYN/r9fHoxTBBYYlEL4QLFq/a1WUgE///fr4xBv/+1Wq1EAh/3/tX6/fv/6BYOqwCzBBYf9tWq9QLF79X+oLBDIOgKgILEEIIxBGAMVNAP/BYf/BYUFBYJSB6wLC9QLBeAQLBqwLCGAL9BBYmr9X+GAILBbIIlBBYP6/wwBBYMFBYZGB/4XDGAILD34vEcwYLB15HBBYYkBBYWrFwILDKoRTCVIQLCEgQXIEgVaF44YCoRHHAAMUgQuBNgILFgECO4W/BZCPFBYinGBY6/CAArXFBY7vDAAsq1QuB0ALIOwOABY0KEgJGGGAguHDAYDBA=="))

apps/bthrmlite/boot.js

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+(function() {
+  var log = function() {};//print
+  var gatt;
+
+  Bangle.setHRMPower = function(isOn, app) {
+    // Do app power handling
+    if (!app) app="?";
+    log("setHRMPower ->", isOn, app);
+    if (Bangle._PWR===undefined) Bangle._PWR={};
+    if (Bangle._PWR.HRM===undefined) Bangle._PWR.HRM=[];
+    if (isOn && !Bangle._PWR.HRM.includes(app)) Bangle._PWR.HRM.push(app);
+    if (!isOn && Bangle._PWR.HRM.includes(app)) Bangle._PWR.HRM = Bangle._PWR.HRM.filter(a=>a!=app);
+    isOn = Bangle._PWR.HRM.length;
+    // so now we know if we're really on
+    if (isOn) {
+      log("setHRMPower on", app);
+      if (!gatt) {
+        log("HRM not already on");
+        NRF.requestDevice({ filters: [{ services: ['180D'] }] }).then(function(device) {
+          log("Found device "+device.id);
+          device.on('gattserverdisconnected', function(reason) {
+            gatt = undefined;
+          });
+          return device.gatt.connect();
+        }).then(function(g) {
+          log("Connected");
+          gatt = g;
+          return gatt.getPrimaryService(0x180D);
+        }).then(function(service) {
+          return service.getCharacteristic(0x2A37);
+        }).then(function(characteristic) {
+          log("Got characteristic");
+          characteristic.on('characteristicvaluechanged', function(event) {
+            var dv = event.target.value;
+            var flags = dv.getUint8(0);
+            // 0 = 8 or 16 bit
+            // 1,2 = sensor contact
+            // 3 = energy expended shown
+            // 4 = RR interval
+            var bpm = (flags&1) ? (dv.getUint16(1)/100/* ? */) : dv.getUint8(1); // 8 or 16 bit
+          /*  var idx = 2 + (flags&1); // index of next field
+            if (flags&8) idx += 2; // energy expended
+            if (flags&16) {
+              var interval = dv.getUint16(idx,1); // in milliseconds
+            }*/
+            Bangle.emit('HRM',{
+              bpm:bpm,
+              confidence:100
+            });
+          });
+          return characteristic.startNotifications();
+        }).then(function() {
+          log("Ready");
+          console.log("Done!");
+        }).catch(function(err) {
+          console.log("Error",err);
+          gatt = undefined;
+        });
+      }
+    } else { // not on
+      log("setHRMPower off", app);
+      if (gatt) {
+        log("HRM connected - disconnecting");
+        try {gatt.disconnect();}catch(e) {
+          log("HRM disconnect error", e);
+        }
+        gatt = undefined;
+      }
+    }
+  };
+})();

apps/bthrmlite/metadata.json

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+{ "id": "bthrmlite",
+  "name": "Bluetooth Heart Rate Monitor (Lite)",
+  "shortName":"BT HRM",
+  "version":"0.01",
+  "description": "Overrides Bangle.js's build in heart rate monitor with an external Bluetooth one. Cut-down version that uses less RAM.",
+  "icon": "app.png",
+  "tags": "health,bluetooth",
+  "type": "bootloader",
+  "supports" : ["BANGLEJS","BANGLEJS2"],
+  "readme": "README.md",
+  "storage": [
+    {"name":"bthrmlite.boot.js","url":"boot.js"},
+    {"name":"bthrmlite.img","url":"app-icon.js","evaluate":true}
+  ]
+}