Not the HC-SR04 you know

Two parts, one product name

Switch between them and watch the echo wire.

The HC-SR04 is a board of separate parts — a small microcontroller, a transistor pair driving the transmitter, a couple of op-amp stages amplifying what comes back. It works, it has worked since about 2011, and it needs 5 V to do it. Its echo pin therefore answers at 5 V, into whatever you have connected, and on a 3.3 V board that is a pin rated to 3.6 V receiving 5 V. Hence the divider in every tutorial.

The TK50 replaces all of that with one chip, an RCWL-9610A in a 16-pin package, and the chip is happy from 3.3 V. The board prints its own supply range on the back: 3.3V-5V.

Why the divider disappears

Because there is nothing on this board between the header and the chip.

Trace the four nets and it is a short list. VCC goes to two pins of the RCWL-9610A and to a decoupling capacitor, and that is all — there is no regulator. ECHO and TRIG are each a single chip pin brought straight out to the header, with nothing in the way.

So the supply pin sets the logic level. Feed it from a 3V3 rail and the echo comes back at 3.3 V; feed it from 5 V and the echo comes back at 5 V. The level question is not answered with resistors, it is answered by which rail you choose, and you choose the one your board runs on.

The other three things that change

Four interfaces, not one. An HC-SR04 does trigger-and-echo and nothing else. This chip also speaks I2C, UART and a single-wire mode, selected by two solder bridges. A board that arrives untouched behaves exactly like an HC-SR04, which is why old sketches run on it unchanged.

A longer trigger pulse. Every HC-SR04 example sends a 10 µs pulse on TRIG. The supported library for this part sends 500 µs. Ten microseconds does still start a measurement, which is why the old sketches appear to work — but 500 µs is what the maintained code does and what the sketches here send.

A different idea of range. Both parts claim 4 m. What is unusual is that Lonely Binary's own documentation for this one says plainly that 2–3 m is where it should be trusted, which is more honest than the category is used to and is worth taking at face value.

What to keep from what you already know

The physics. Everything in what the echo measures is true of both parts, and so is everything about surfaces and temperature later on. It is the wiring advice that has expired.