Selecting An LED

Like many robot builders and electronics enthusiasts, I’ve amassed a collection of various shapes and colors of LED (light emitting diodes) electronic components. When selecting an LED for a project, there are several things I need to know:

  1. What is the LED’s actual color when lit?
  2. How much current is needed for the desired brightness?
  3. What size resistor is needed to achieve that current/brightness?
  4. Which wire lead on the LED is the anode (positive) and which is the cathode (negative)?

I suppose some of that information is specified on the LED’s datasheet, assuming I have it. But, whether or not I approve of the color and brightness for a project is purely subjective. And, computing the appropriate resistor value is not as easy as it might seem, since the LED’s voltage drop varies based on current. As such, this information is usually obtained by direct trial rather than from the datasheet.

Normally, I set my variable power supply to 5 V and attach the LED via alligator clips in series with a potentiometer (variable resistor). The potentiometer is turned until the desired brightness is achieved. Then, the circuit is powered off to measure the resistance of the potentiometer using a multimeter. If I’m concerned that the LED is being driven beyond specification, the multimeter can be hooked up to measure current with the power turned on.

All of this can be a hassle, particularly if the multimeter and power supply are currently being used for other purposes. Furthermore, turning the potentiometer below a certain resistance can result in damage to the LED being tested. I decided it would be easier and more foolproof to create a standalone LED test tool.

LED Test Tool

LED tester

The LED tester uses a 9 V battery as a power source, which provides portability and doesn’t monopolize my bench variable power supply.

LED test PCB attached to a 9 V battery using double-sided sticky tape.

The PCB is attached to the 9 V battery using double-sided sticky tape. That’s not consistent with my normal methodology. Usually I prefer something removable, like attaching with fasteners.

However, the circuit doesn’t consume any measurable power when idle, and only uses between 2 mA and 26 mA when briefly lighting an LED. So, the battery should last for many years. I won’t mind scraping off the tape when it is time to replace the battery.