Oscilloscope and Multimeter Observations¶
This week, our group assignment is to use the test equipment in our lab to observe the operation of a microcontroller circuit board. We will use our lab’s Siglent SDS1202X-E oscilloscope as well as digital multimeters.
We started this assignment with the osciloscope. We first read the user manual to better understand how the machine works.
Our first step was to calibrate the machine. We did this by plugging the hook end of the probe into the 1kHz terminal, and the alligator clip end into the ground terminal below that. We then adjusted the level frequency until the wave did not move.
We then observed the function of a circuit using the oscilloscope. We created a simple circuit on a breadboard involving a potentiometer and a servo motor, and connected the oscilloscope to digital pin 9 and ground. We made this conneciton because the servo motor is controlled through PWM, and we can observe the difference in voltage by connecting it to here and ground. Our circuit looked like this:
When we changed the value of the potentiometer, the arduino will change the PWM output from pin 9 in order to set the servo to a different position. The oscilloscope measures the voltage difference between the data pin and ground and displayes it.
To adjust the time axis (x axis), we adjust the two horizontal knobs. To adjust the voltage axis (y axis), we adjust the two vertical knobs above the probe we are using. The upper knob is for scale, and the lower knob is for position.
What is PWM?¶
PWM stands for Pulse Width Modulation, and is a way to get analog values from a digital signal. PWM involves very rapid (hundereds of microseconds) changes in the state of a pin between low and high. On the Arduino Uno, pins capable of PWM have a small tilda. The value from PWM can be adjusted by the length of the high pulse vs low pulse. A high-low ratio of 1:0 will be 100% high, a 1:1 ratio will be high 50% of the time, and a ratio of 0:1 will be low 100% of the time. Different ratios can correspond to different actions - such as the very short high pulse to a longer low pulse that will move the servo motor’s position.
Our lab also has quite a few multimeters that we use to test components and our equipment. We primarily use multimeters to measure for connectivity in our circuits to test for bridges and to measure the values of different components. However, we also can use them to observe the operation of our boards, just not as precicely. This is often just as much use to us because multimeters are lighter and smaller. This is important to use when attempting to find continuity or any shorts. The continuity is important because any sort of diode, for example, such as an LED will only turn on when the positive and negative ends are in the correct position. There is usually some sort of marking such as a dot or a line which determines which side is which, but when a company manufactures millions of components, there are bound to be some incorrect markings. Therefore, it is vital to make sure that each component functions and if the polarity marking is correct (if there is any).
We observed the function of our blink boards from the electronics production week with the multimeter. We put the prongs across each end of the LED, and measured the drop in voltage across it. There is a large delay between the change in state of the LED and the measurement of the LED, so we will mostly use our oscilloscopes for timing-intensive measurements.