Sound tone detection with a PSoC
The “external” circuit is very simple, a bunch of passive components. On the microphones side there are three high-pass RC filters for DC decoupling and low frequency noise filtering. Three current limiting resistors drive the LEDs. A Schottky diode forms a peak detector together with a capacitor to measure the level of the sound, this kind of diode cause a lower voltage dropout than normal rectifier diodes. The R5 resistor purpose is to decrease the discharge time of C5 capacitor, in order to allow a faster multiplexing of the three inputs. Those external components were chosen instead of a peak detection block because all the internal available boxes are busy and this three component network is an acceptable compromise. A trimmer is used to set up the threshold of the analog signal that triggers digital out. A trimmer? Why a trimmer when you can use an up/down digital circuit with a led bar indicator of the position? Because it’s much more simple. Sometime I’m lazy.
Filtering capacitors and sockets complete the “bill of materials”.
The real board and the compoonents placing realized with CadSoft Eagle.
The real complexity is in the “internal” part of the circuit, as expected using this PSoC device. The next page describes every single block and connection.