Space-saving sensor for fitness trackers

The component is ideal for use in fitness armbands to monitor fitness levels, as well as for use as an ambient light sensor.

The IR Topled D5140 is a further development of the SFH 2440 photodiode, which Osram Opto Semiconductors recently introduced for fitness sensors. A pre-molded package soldered to the bottom of the package forms the basis of the component, whereas earlier packages had to be soldered to side legs. Because these legs are no longer there in the design, the Topled D5140 is approximately 1.4mm shorter, now measuring 5.1mm.

The electrical and optical properties of the photodiode are identical to those of its counterpart, the SFH 2440. Along with the short switching time of 90 nanoseconds (typ.), the spectral sensitivity optimised for visible light is particularly noteworthy, with a maximum sensitivity of 620 nanometers. However, in the infrared spectral range the sensitivity of the two photodiodes is greatly suppressed.

Precise heart rate measurements
Just like the SFH 2440, the IR Topled D5140 is also ideal for optical sensors for pulse rate measurements on the wrist, for example. It works by shining visible light on the surface of the skin, some of which light is absorbed or reflected to the detector. Because arterial blood absorbs more light than the surrounding tissue, the strength of the detector signal changes with the volume of blood through which the light passes. The periodicity of the signal then indicates the heart rate. However, infrared light that shines onto the measuring point from the surroundings and disperses in the body also reaches the photodiode.

The Topled D5140 offers a signal-to-noise-ratio because it registers the reflected light particularly well while at the same time suppressing the infrared light. Thanks to its short switching time, the light signal modulated with the heart rate can be time-resolved perfectly. “The IR Topled D5140 allows our customers to design in more compact sensors, preserving the high signal quality offered by the SFH 2440 so far,” said Dr. Chris Goeltner, Project Manager Infrared at Osram Opto Semiconductors.

Ambient light sensor
Applications that have been using large-area photodiodes such as the SFH 2440 or SFH 2430 as ambient light sensors can also be made smaller with the component. Osram plans to offer a photodiode with broadband spectral sensitivity in the new package for its automotive customers as well.