# Blinking the Sentinel HL Key LED

This topic describes how you can implement functionality in your custom application to cause a Sentinel HL key (or HASP HL key) attached to a user’s computer to blink on and off.

## Overview

In a situation where multiple Sentinel HL keys are attached to a computer, it may be convenient to cause the LED in a specific key to blink (flash on and off) so that the user can easily identify the key.

The functionality of causing an HL key to blink exists in the Admin Control Center. This topic describes how you can implement this same functionality in your custom application.

The blink functionality is implemented using an HTTP request. The functionality is not part of the Sentinel Licensing API. This information is included here because you will most likely want to implement this functionality in the same application that issues calls to the Sentinel Licensing API.

Sentinel HL Basic keys do not have a HASP Key ID, so they cannot be made to blink. (They can be made to blink from Admin Control Center, but this is accomplished using an internal enumeration that is not available from other applications.)

## How to Blink the Sentinel HL (USB) Key

To cause a Sentinel HL key to start blinking:

Issue the following HTTP request:

`http://127.0.0.1:1947/action.html?blinkon=keyID`

where keyID is the HASP Key ID of the Sentinel HL Key that you want to start blinking.

To cause a Sentinel HL key to stop blinking:

Issue the following HTTP request:

`http://127.0.0.1:1947/action.html?blinkoff=keyID`

The HTTP request must be issued in program code or using an external utility. The request cannot be issued in a Web browser.

Ensure that the referer is specified in the HTTP header and that the value is: http://127.0.0.1:1947

## Example

The following command causes the Sentinel HL key with HASP Key ID 12345678 to start blinking:

`http://127.0.0.1:1947/action.html?blinkon=12345678`