Measuring Voltage with a Multimeter
When something isn’t working on your BeagleBone, measuring the voltages around your circuit can be a good start on finding the problem. Why isn’t the LED lighting up, for example? Maybe not enough voltage is being supplied to it. If you know that the LED needs about 1.8V to light up, and your multimeter reading is a lower value, the issue may be a depleted battery. You can also use the multimeter to check it.
To measure voltage, you have to connect your multimeter in parallel with the component you want to measure, such as a battery, an LED, or a resistor. After setting the mode to V (which is indicated by the dash with three dots underneath for DC), connect the red probe (which should be connected to a socket with a V next to it) to the positive side of your component, which is where the current is coming from. Connect the black probe (which should be connected to the COM socket) to the negative side.
Placing the multimeter in parallel means placing each probe along the legs of the component you want to measure.
Keep in mind that the COM probe is simply a point of reference for your measurement, which means that if you’d connected the probes in the reverse order of what’s described earlier, the multimeter would display the same voltage with a negative value. Also, you can measure voltage along more than one component in series. If the first component has a voltage drop of 3V and the next one has a voltage drop of 5V, the value you see on the display is the sum of both. In this example, the value would be 8V.