You can use the circuit in this Design Idea to measure the attack and release times of photoresistor-type optocouplers (Figure 1). Such devices often find use in audio compressors or volume-control circuits. The design uses an oscillating Schmitt trigger with the optocoupler DUT (device under test) in the feedback loop. The photoresistor and resistor R₁ 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 R₁. With the component values in Figure 1, the durations of the output pulses are a 0.15-msec attack time and a 2.7-msec 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 R₁, the power-supply voltage, and the Schmitt-trigger 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 and V_T2 is the negative-going threshold voltage of the Schmitt trigger.

This approach lets you pick a value for R₁ so that the photoresistor range is suitable for your device. You can also vary the value of resistor R₂ to observe the LED-current-to-attack-time characteristic of the DUT but not affect the release time. Note that R₂ limits the current through the LED; if its value is too large, oscillation will not occur.