Measuring the flashing time interval of Led lights in Toys

Abstract

Two methods have been developed at the Standards and Calibration Laboratory (SCL) to measure the light flashing time interval conveniently and accurately. One method uses high speed video recording and a synchronous counter. The method starts with taking a short video clip of the flashing LEDs under test, together with the reading of an in-house designed synchronous counter which is phase locked to the caesium frequency standard of SCL. The flashing time interval is obtained by viewing the recorded video to search frame-by-frame for the time interval between two successive LED state changes. The measurement uncertainty obtainable by this calibration method is better than 2 x 10-3. The second method uses direct measurement by time interval counter. A test circuit has been developed at SCL to measure the light flashing time interval accurately. A photo-transistor is used to convert the light signal to the electrical signal. A variable resistor is connected in series with the photo-transistor to tune the detection sensitivity such that it can work well in low luminance condition. The output signal is then amplified and fed to the laboratory time interval counter which is phase locked to the laboratory caesium frequency standard. The time interval measurement range is 1 Hz to 10Hz with measurement uncertainty obtainable by this calibration method is better than 1 x 10-6. A soft opaque silicon tube is used as a mask to select which LED is to be measured when there are multiple LEDs used in a toy.