Ambient light sensor for heart rate sensing in wearables

November 22, 2016 // By Graham Prophet
Everlight (New Taipei City, Taiwan) has introduced an ambient light sensor that operates at 550nm with a very low signal calculation failure rate and a high current efficiency. An anticipated application for this device is heart rate signal detection in wearable electronics for the health and fitness sector.

Based on the principle of PPG (photoplethysmogram), the heart rate signal is calculated according to the current changes in transmission and reflection between a green light LED and the sensor to detect the systolic and diastolic blood vessel rythym. With a large detection area of 8.1mm ², improved signal strength is available from the ALS-PD50-42C. The larger sensing area can also improve the capabilities in instances when there are signal interferences caused by people’s skin colour, tattoos and hair on the skin.


The ALS-PD50-42C series features a very efficient current transmission. A current signal output of 0.8 μA at a light current of 100 Lux from a white LED (Vr = 5V) improves detection from capillaries. The device's coating design effectively reduces the interference from UV light below 400 nm and infrared light above 650 nm, thereby decreasing the effect of strong sunlight from outdoor exercise to ensure accurate measurements. The ALS can be operated within a supply voltage range from 0 to 5V in a temperature range from -25°C up to +85°C.