Circuit measures optocoupler's response time

摘要

Here is a simple circuit you can use to measure the attack and release times of photoresistor-type optocouplers that often find use in audio compressors or volume-control circuits (figure). The design uses an oscillating Schmitt trigger with the optocoupler DUT (device under test) in the feedback loop. The photoresistor and resistor R, form a voltage divider that controls the input of the Schmitt trigger. The optocoupler's LED connects to the trigger output.You can measure the duration of the output pulses with an oscilloscope or a digital meter. The duration of the negative output pulses is equal to the switching on-time, or attack time. The duration of the positive pulses is equal to the switching off-time, or release time. The attack and release times depend on the value of R1;you can observe both by varying the value of R,. With the component values in the figure, the durations of the output pulses are a 0.15-ms attack time and a 2.7-ms release time. During oscillation, the resistance of the photoresistor sweeps in from R_(p1) to R_(p2).The circuit sweeps these photoresistor values according to R,, the power- supply voltage, and the Schmitt-trig-ger thresholds, as the following equations show: R_(p1)= R1 × V_(T2)/ (V_(CC) - V_(T2)) and R_(p2) = R1 ×V_(T1)/ (V_(CC) - V_(T1)), where V_(T1) is the positive-going threshold voltage andV_(T2) is the negative-going threshold voltage of the Schmitt trigger.