Small and powered from a separate source, this green LED voltmeter is a handy monitoring module.
We've had situations where we want to watch a voltage, but don't want to dedicate or mount a full meter to the projects. This little device fills the need. Small, compact, and inexpensive, it lets us watch for system voltage sags without using up our bench meters. The red (pwr) and black (gnd) wires are used to power the meter and the yellow is for the voltage measurement. Make sure that your ground is common between the meter and the voltage you are measuring!
If you have to install it a distance from your measurement point and you are concerned about voltage drop, there is a trimmer on the backside so you can dial in the compensation for the line loss powering the display. That said, please calibrate your unit before using it!