power supply How do I select the accompanying components for an optocoupler

Posted on Feb 5, 2014

Using an optocoupler ( MOC3021 ) to sense the On/Off state of an electrial appliance using a microcontroller ATmega16L. How do i go about doing this My mains supply specs are 230V, 50Hz. How do I design the surrounding circuit and select component values, like the resistors Referring to the above schematic. The idea is to use this circuit to

power supply How do I select the accompanying components for an optocoupler

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determine whether the load is on or off. The output pin from the optocoupler connects to an external interrupt of the Microcontroller I`m using which is ATmega16L. The interrupt will Monitor the state of the load. After monitoring I can toggle the state of the load using a relay (relay acts as a Control mechanism) which connects to the same microcontroller. Now, I tried calculating the resistor values for R1, R2 and Rc. Note, microcontroller`s VIL(max) = 0. 2xVcc = 660mV and VIH(min) = 0. 6xVcc = 1. 98V and VIH(max) = Vcc+0. 5 = 3. 8V. To calculate Rc is quite easy. When the transistor is not conducting the output is high (at 3. 3V). When the transistor conducts the output is pulled low. so from microcontroller`s point of view, output high means load is switched OFF and output low means load is switched ON. Looking at the datasheet for SFH621A-3, using 34% minimum CTR at IF = 1mA. Therefore, at 1mA input, the output is going to be 340uA. So in order for the microcontroller to detect low voltage from the output of the optocoupler can I use resistor value of 1Kohm So that the output from the optocoupler will have a voltage of 340mV (which is below VIL(max) Aim: the aim is to keep the LEDs ON* for maximum period of time in a 10mS half period (20mS full period of 50Hz). Lets say LEDs have to be ON for 90% of the time, that means LEDs require at least 1mA of current for 90% of the time for that half period which means LEDs will...

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