vne147
Member
They are available in all price ranges from a few dollars to several hundred. Generally the more expensive they are the more features, better quality and better accuracy. For this purpose you don't need anything high quality. You could probably find a cheap analog one at Radio Shack or Home Depot for $10 - $20. I have even seen multimeters for sale at a gas station convenience store in a big bin with a bunch on other tools for $2 each. You can find a cheap one if you look around. If you're really broke, just forget the whole measurement and calculation idea and do some old fashioned trail and error. Start with the 100Ω resistor like you were thinking. If the red LED is dimmer when the yellow light is on than when the green is on, decrease the resistance value. If it is still brighter, increase it. Just do that until you find a value that works. You won't break anything by doing that.