YTIan 4pcs Voltage Meter 2 Wire 0.28" DC 2.5V-30V Digital Voltmeter Gauge Tester LED Display Panel Mount Car Motorcycle Battery Monitor 4 Colours

Low cost and useful to monitor adjustable supplies. There is a screw that can be adjusted for correction of readings.

These are not very accurate, one showing 12.1v and the output was exactly 12.0v, another showing 4.89v when the output was exactly 5.0v. They work well but the accuracy is off