Why a bumper
A distance sensor tells a robot how far away things probably are. A bumper tells it nothing at all until it touches something, and then it is certain. The TK17 is that bumper: a lever switch whose SIGNAL pin goes HIGH once the lever has been pushed in about a millimetre and a half.
Certain, and late
A distance sensor sends something out and times what comes back. It works until the obstacle does not send it back: the TK50's own handbook finds chairs and people poor ultrasonic targets, and an infrared sensor can be fooled by a black surface or bright sun. None of them says "I cannot see"; they report a distance, and sometimes it is wrong.
A bumper has one way to be wrong and it is easy to see. The TK17 is a lever switch at the front of the robot. Touch the lever and push it in, and the switch closes. Nothing else closes it.
Slide the robot forward. For the first 6 mm nothing happens, and nothing would happen at 6 m either: the block has no idea anything is there. When the wall meets the lever, it starts pushing it in. After about 1.4 mm the switch clicks, SIGNAL goes HIGH, and the red LED on the board lights.
What the click costs
That 1.4 mm is the difference between two numbers on the switch's drawing: where the lever sits at rest and where it clicks. Both carry a tolerance of 0.8 mm, so treat it as "about a millimetre and a half". It is how far the robot has already moved into the obstacle when your sketch finds out.
At a slow crawl that is nothing. On a fast robot it means the bumper needs room behind it: a lever that is pushed flat against the body before the robot stops is being used as a buffer, and the drawing gives no figure for how much of that it takes.
Where it belongs
The lever sticks out past the top edge of the board, so that edge faces forward. Use it as the last sense rather than the only one: a distance sensor to slow down for what it can see, and this for what it could not.
When it does not work
No. It is a switch: LOW until the lever is pushed in far enough to click, HIGH while it is held there. There is no reading in between and no warning before contact. For distance, pair it with a sensor that measures, such as the TK50 ultrasonic sensor, and keep the bumper for the things that sensor misses.
The switch's drawing gives an operating force of 70 gf, give or take 15: about the weight of 70 g resting on the lever. A small robot driving into a wall has far more than that. A curtain or a sheet of paper may not.
With its top edge forward. The switch hangs over that edge and the lever reaches past it, so it is the first thing an obstacle meets. Mount the board so nothing else on the robot sticks out further than the lever does, or the robot will hit with that instead.
Not to the switch itself: the lever is part of it, and a bent or loaded lever changes where it clicks, or stops it clicking at all. To cover a wider front, build a light bumper plate on the robot that presses on the lever, and leave room for it to move the lever by more than a millimetre and a half.
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