You can probably experiment with a magnet a little to see exactly where the sleep sensor sits.
Often, that will be near the right of left edge, just off the keyboard
Run a magnet around the edge of the base, until the screen suddenly shuts off (it's sleeping!) - same action as closing the lid. You may also sense the magnet in the lid (your magnet will stick, or be repelled), which will help you find where the sensor lives in the base.
Just a suggestion, if you want to avoid the screen suddenly shutting off - you'll know why it happens - and where.