Build an Analog Bar Graph Expanded Scale Voltmeter

An expanded scale voltmeter (ESV) can save your plane. That may be a strong statement, but it`s true. The crucial radio link that lets you control your plane relies on nickel-cadmium (NiCd) batteries in the transmitter and receiver. If either of these batteries goes dead, you`ll lose control and your plane will likely crash or fly away. Unlike car

bon-zinc or alkaline cells, NiCd cells have a very flat discharge curve. This means that their output voltage remains relatively constant (ranging from about 1. 28V down to 1. 17V) until they are almost dead, and then drops off suddenly. Testing the voltage with an ordinary analog voltmeter is therefore nearly useless, because it will always read close to 1. 2V no matter how much charge is left. A digital volt meter would be a slight improvement, because it has enough resolution to show the changes in this narrow range. However, another characteristic of NiCd cells is that they tend to show full voltage when they aren`t doing anything, even if they are almost completely dead. If you use a resistor to discharge a NiCd cell to 0V (this is not recommended), and then remove the resistor and wait a few minutes, measuring the voltage will still indicate about 1. 2V. First, it acts like a magnifying glass on part of the scale of an ordinary meter, namely the part of the scale we are interested in. For example, an ESV for testing a single NiCd cell would range from 1. 17V at one end of the scale to 1. 28V at the other end of the scale. With the full scale covering this narrow range, it`s easy to see voltage changes within the range. Secondly, and ESV applies a known load to the cells while it`s testing them. Only by testing them while they are doing a known amount of work can one accurately get an estimate of their state of charge...

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