søndag 3. mars 2019

Overload indicator

As the moog filter overloads nicely by feeding back the output, we need a way to know when the filter starts overloading.

We do know how high the maximum input signal may be, so overloading happens when the input passes this threshold.

The following circuit lights up the led whenever the input is too high. In the analog domain, the brightness of the led corresponds to how much of the signal is too high, as it will rapidly turn on and off.

If we choose to remove the led and connect the output to an MCU instead, a positive (5v) signal means overloading and 0 means not overloading - for a single signal cycle.

Doing this digitally lets us also filter the signal - we may trigger a "overloading" flag whenever the input is high, set a timer to clear the flag, and reset the timer whenever a new high input is received. That way we get a continous overload if the time between a low and a high overload input is short enough.

Some MCUs even have built in comparators, which lets us supply a reference voltage on one pin and the input on the other, meaning we do not need the pullup resistor or LM311.

In the XM8 I intend to have replaceable filters. For our filter, the reference voltage should be generated by the filter circuit using a trimpot or resistor divider, and the signal to measure should be tapped after the input mixer.

By doing it this way we don't need a separate comparator for each filter.

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