LED analyser

in Blog

21 February 2016

A simple device to compare several LEDs simultaneously at different forward currents,
with a forward voltage read out.

A need recently came about to match LEDs for colour and brightness. This device contains ten constant current sources and a voltage read out for one of the LEDs. The current can be set from 0.25mA to 25mA using a single jumper connection for all ten LEDs. It is powered by a 9V PP3 battery.

The device uses a Texas Instruments LM3914 LED bar graph display driver (which handily has 10x constant current outputs) and a generic voltmeter module from ebay. The voltmeter is referenced to ground but the LEDs are connected directly to the positive supply voltage, so a Texas Instruments TLV2372 op amp is used to invert the LED forward voltage so it is referenced from ground instead. The forward voltage read out is used to calculate resistor values for a given LED for new circuit designs.

The circuit was built using standard veroboard. Once built up and working it was glued with a glue gun to some hardboard taken from a broken picture frame. The schematic was laminated and glued to the underside for future reference.

The device has already proven useful in checking batches of 5mm diffused red LEDs from ebay – one batch was sold as having a maximum forward current of 20mA but this was clearly too much for the LEDs which turned very bright yellow and became hot to touch. It turns out that these LEDs must be of the high-efficiency variety because 1mA of forward current is plenty for very bright illumination.

Texas Instruments LM3914 display driver
Texas Instruments TLV2372 op-amp

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