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Simple IC to determine optocoupler's response time

Posted: 07 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:photoresistor? optocouplers? audio compressors?

You can employ the circuit in this design idea to measure the attack and release times of photoresistor-type optocouplers (figure). Such devices often find use in audio compressors or volume-control circuits.

The design uses an oscillating Schmitt trigger with the optocoupler device under test (DUT) 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.


Figure: An optocoupler's rise and fall times can be measured by incorporating a photoresistor in the feedback loop of an oscillator circuit.

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 the figure, 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 RP1 to RP2. The circuit sweeps these photoresistor values according to R1, the power-supply voltage, and the Schmitt-trigger thresholds, as the following equations show: RP1=R1VT2/(VCC?VT2), and RP2=R1VT1/(VCC?VT1), where VT1 is the positive-going threshold voltage and VT2 is the negative-going threshold voltage of the Schmitt trigger.

In the case of the 74HC14 logic family, you can determine the thresholds from the data sheet and your power-supply voltage, according to the following equations, which yield typical values: VT1=0.53VCC, and VT2=0.31VCC. Using 5V as a power-supply voltage and solving the following equations, you can determine the photoresistor range: RP1=0.45VR1, and RP2=1.13VR1.

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.

Using this circuit allows you to match custom optocouplers comprising green, superbright LEDs and an MPY7P photoresistor.

- Peter Demchenko

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