AVR Freaks

Well, when dealing with 220 VAC it is often wise to start with a transformer unless you have a good reason not to.

This drops the voltage down to levels useful for present day circuitry designs, and provides electrical isolation from the Mains supply.

If you use a small power supply transformer, say 230:12 V, CT (Center tapped), then ignore the 12 V output and use the center tap and one end you have a 6V AC output.

This is still too high for 5V logic, so drop it through a resistive divider to get something smaller.

Now either AC couple it to an op amp, or DC couple it to an op-amp set up as an adder to add in a DC offset to raise the AC to the mid-point of the supply.

The gain doesn't really matter too much. You want it set up so that 20 VAC, attenuated, then gained, is in the linear range of the op-amp.

The 0-10 VAC is now also within the linear range.

Anything over perhaps 30 VAC will saturate the op-amp to the rail, which for this purpose doesn't matter at all.

The output now feeds a "window" comparator, made from the remaining op-amps on the chip selected. Perhaps just two comparators instead of a window detector.

V <= 10 output. V<= 20 output. Neither of the above is then V > 20.

This circuit would give you pulses with each cycle of the power supply. Not an issue if a micro is sampling the circuit and storing the input status for the Main program.

One could, however, do more in the analog front end, either a sample and hold or a "detector", (RC filter); but these days that is usually done in software, especially for such a low frequency signal (50/60 Hz), which I presume this is.

This is a lot of parts, but with 1% resistors it can be "calibrationless".

If you are only building 10 units it might be easier to put a pot in the circuit, using the optos, and tweak each PCB manually. Then put a dab of glue on the pot.
This is certainly less expensive that transformers, and still gives you isolation.

The opto will also give you pulses outputs.

An ADC can easily track the output in an ISR and again set a value for the Main program to use, Valid, Undefined, High.

Many of the micros also have an analog comparator which could also be used, once the signal is isolated and attenuated and the "negative" voltage is blocked or offset, (recall that AVRs DON'T like negative voltage on their input pins!).

JC

Well, another option is to skip all the op-amp stuff and go digital.

Use a simple diode to R-Zener voltage regulator, with a cap across the zener, (or similar).

Use this to power a Tiny. The Tiny measures the incoming voltage via a resistive divider and its ADC.

The Tiny outputs a digital signal via the opto-coupler. This could be a USART driving the opto, sending data packets, or it could be frequency modulated, or pulse width modulated, etc.

For example, < 10 V: 500 Hz square wave output.
10-20 V: 750 Hz output.
> 20 V: 1000 Hz output.

Two items to be aware of:

The Tiny sits on the incoming power supply, it is NOT isolated. This may or may not be a problem.

The 0-10 V input range is problematic for this method, as the input is what is powering the Tiny and the LED inside the opto-coupler.

If No output signal is acceptable to indicate 0-10 V, and the Tiny only outputs a signal for 10- 20 V, and > 20 V then this method works.

This is essentially calibrationless, which is an advantage.

If you have not worked on "live" circuitry before and you elect to go this route then say so, and perhaps you will get safety some pointers.

For example:

Keep one hand in your pocket when probing the circuit.

Use a battery powered O'scope, or one designed for testing Mains supplies, or you can inadvertently fry your scope's front end.

Be careful how you keep fingers from touching live parts of the circuitry when it is done, and put in a case. A "Caution! High Voltage Inside!" sticker is helpful, as is potting the entire high voltage section. What you do depends upon who is likely, (or even unlikely), to be tinkering with the device in the future.

Keeping the two "halves" of the circuit physically separated on the PCB.

etc.

JC