diff --git a/usb/blink-test-led.js b/usb/blink-test-led.js
index c45b222d..59268204 100755
--- a/usb/blink-test-led.js
+++ b/usb/blink-test-led.js
@@ -1,25 +1,15 @@
 #!/usr/bin/env node
-'use strict';
-
 let uhk = require('./uhk');
-let [endpointIn, endpointOut] = uhk.getUsbEndpoints();
 
-var state = 1;
+let delayCycle = true;
+let areLedsEnabled = true;
 
-setInterval(function() {
-    state = state ? 0 : 1
-    console.log('Sending ', state);
-    endpointOut.transfer(new Buffer([uhk.usbCommands.setTestLed, state]), function(err) {
-        if (err) {
-            console.error("USB error: %s", err);
-            process.exit(1);
-        }
-        endpointIn.transfer(64, function(err2, receivedBuffer) {
-            if (err2) {
-                console.error("USB error: %s", err2);
-                process.exit(2);
-            }
-            console.log('Received', uhk.bufferToString(receivedBuffer));
-        })
-    });
-}, 500)
+uhk.sendUsbPacketsByCallback(() => {
+    delayCycle = !delayCycle;
+    if (delayCycle) {
+        return new uhk.DelayMs(500);
+    } else {
+        areLedsEnabled = !areLedsEnabled;
+        return new Buffer([uhk.usbCommands.setTestLed, areLedsEnabled ? 1 : 0]);
+    }
+});
diff --git a/usb/led-pwm.js b/usb/led-pwm.js
index 84c03cef..6bbd81ac 100755
--- a/usb/led-pwm.js
+++ b/usb/led-pwm.js
@@ -1,23 +1,16 @@
 #!/usr/bin/env node
 let uhk = require('./uhk');
 
-let [endpointIn, endpointOut] = uhk.getUsbEndpoints();
-let brightnessPercent = 0;
+let delayCycle = true;
+let areLedsEnabled = true;
 
-setInterval(() => {
-    brightnessPercent = brightnessPercent ? 0 : 100
-    console.log('Sending ', brightnessPercent);
-    endpointOut.transfer(new Buffer([uhk.usbCommands.setLedPwm, brightnessPercent]), err => {
-        if (err) {
-            console.error("USB error: %s", err);
-            process.exit(1);
-        }
-        endpointIn.transfer(64, (err2, receivedBuffer) => {
-            if (err2) {
-                console.error("USB error: %s", err2);
-                process.exit(2);
-            }
-            console.log('Received', uhk.bufferToString(receivedBuffer));
-        })
-    });
-}, 500);
+uhk.sendUsbPacketsByCallback(() => {
+    delayCycle = !delayCycle;
+    if (delayCycle) {
+        return new uhk.DelayMs(500);
+    } else {
+        areLedsEnabled = !areLedsEnabled;
+        let brightnessPercent = areLedsEnabled ? 100 : 0;
+        return new Buffer([uhk.usbCommands.setLedPwm, brightnessPercent]);
+    }
+});
diff --git a/usb/uhk.js b/usb/uhk.js
index 2c53c540..b25ab641 100755
--- a/usb/uhk.js
+++ b/usb/uhk.js
@@ -35,7 +35,26 @@ function getUsbEndpoints() {
     return [endpointIn, endpointOut];
 }
 
-function sendUsbPacket(packet) {
+class DelayMs {
+    constructor(ms) {
+        this.ms = ms;
+    }
+}
+
+function sendUsbPacketsByCallback(packetProvider) {
+    let packet = packetProvider()
+
+    if (!packet) {
+        process.exit(1);
+    }
+
+    if (packet instanceof DelayMs) {
+        setTimeout(() => {
+            sendUsbPacketsByCallback(packetProvider);
+        }, packet.ms);
+        return;
+    }
+
     console.log('Sending: ', bufferToString(packet));
 
     let [endpointIn, endpointOut] = usbEndpoints || getUsbEndpoints();
@@ -50,14 +69,27 @@ function sendUsbPacket(packet) {
                 process.exit(2);
             }
             console.log('Received:', bufferToString(receivedBuffer));
+            sendUsbPacketsByCallback(packetProvider);
         })
     })
 }
 
+function sendUsbPacket(packet) {
+    let isPacketSent = false;
+    sendUsbPacketsByCallback(() => {
+        if (!isPacketSent) {
+            isPacketSent = true;
+            return packet;
+        }
+    })
+}
+
 exports = module.exports = {
+    DelayMs,
     bufferToString,
     getUsbEndpoints,
     sendUsbPacket,
+    sendUsbPacketsByCallback,
     usbCommands: {
         getProperty: 0,
         jumpToBootloader: 1,