To make a servo motor turn 90 degrees, you will need to provide a specific signal to the servo motor using a microcontroller or a servo controller. The signal that is provided to the servo motor will determine the position of the servo motor's shaft, and by providing the right signal, you can make the servo motor turn to a specific angle.

In general, the process of making a servo motor turn 90 degrees involves the following steps:

1. Determine the pulse width range for your servo motor. Most servo motors have a pulse width range of 1 to 2 milliseconds, but this can vary depending on the specific servo motor that you are using.

2. Calculate the pulse width that corresponds to the angle that you want the servo motor to turn to. For example, if you want the servo motor to turn 90 degrees, and the pulse width range for your servo motor is 1 to 2 milliseconds, then you will need to calculate the pulse width that corresponds to 90 degrees using the following formula: pulse width = (2 - 1) * (angle / 180) + 1. In this case, the pulse width that corresponds to 90 degrees would be 1.5 milliseconds.

3. Provide the calculated pulse width to the servo motor using a microcontroller or a servo controller. Most servo motors have three wires that are used to control the servo motor: a power wire, a ground wire, and a signal wire. To provide the calculated pulse width to the servo motor, you will need to connect the power and ground wires to a power source, and then connect the signal wire to the microcontroller or servo controller.

4. Use the microcontroller or servo controller to generate the pulse width signal that corresponds to the angle that you want the servo motor to turn to. Most microcontrollers and servo controllers have built-in functions that allow you to easily generate pulse width signals, and you can use these functions to send the calculated pulse width signal to the servo motor.

5. Monitor the position of the servo motor's shaft to ensure that it is turning to the desired angle. Most servo motors have built-in feedback mechanisms that allow you to monitor the position of the servo motor's shaft, and you can use this feedback to ensure that the servo motor is turning to the desired angle.

In summary, to make a servo motor turn 90 degrees, you will need to provide a specific signal to the servo motor using a microcontroller or a servo controller. The signal that is provided to the servo motor will determine the position of the servo motor's shaft, and by providing the right signal, you can make the servo motor turn to a specific angle.