Fall Rise Detection

181

01:34:00

A circuit to detect if an analogue signal is falling or rising. 
The first capacitor on the left "stores" the value of the signal at that moment when the switch is opened. The left op-Amp compares the momentary signal with the stored value. 
If the signal is higher than the stored value, the op-amp will output +VDD, if it is lower -VDD. While the switch is closed, The op-amp outputs GND. 
To get a clear non-switching output, the following RC Lowpass makes the op amp output lag so that, the following comparator can output a clean high or low voltage. 
Adjust the RC values for your desired signal frequency range. 

NOTE: For a real-world implementation use an analogue multiplexer instead of a relay! (e.g. BC4053, 4052, 4051)

published 9 years ago

FitFat

9 years ago

This is OK, as the way of using sampling. But I wonder why you don't use differentiation circut. Plese see this if you like ( http://everycircuit.com/circuit/6071479092903936 ).

tompilot

9 years ago

Thanks for the hint! i think it depends on your needs. With differentiation, the amplitude of the differentiation stage is dependent of the input frequency, so the frequency range of the input signal may be limited. E.g. for audio signals the sampling method might be the right choice, otherwise i'd say the differentiation method is more elegant.

tompilot

9 years ago

edot: adding limiting diodes to the differentiation circuit may show some improvements regarding frequency dependence.

FitFat

9 years ago

As you say, the frequency rage of differentiation circuit will be limited, but when its sing, + or -, is only needed, the range will not be so narrow, I think. I think the sampling method may be useful when there is a clock pulse already, like an interface to a digital audio.

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