Watchdog for asterisk: implementation (Part II)

Part II: Calibrating the Tone Decoder for Specified Frequency

The tone decoder schematic is given below (Vout is connected to the input of the timing circuit, to be described in Part III):


(Thanks to Linear Tech for providing a nice free simulator LTSpice, which I used to produce the above drawing. The IC is actually an LM567 equivalent, though the schematic has Linear Tech's logo on it.)

I have constructed this circuit on a breadboard for testing. The 10k potentiometer (R1) on the test board is from the kit that I purchased from NightFire. It has only one turn, so it is probably not suitable for production as it might be less precise. I have also bought a couple of multiturn trimpots from Fry's Electronics, which I will use in the final circuit. The following video show the calibration of the tone decoder. (The right half of the board is the timing circuit, which we will ignore for now.) As I turned the trimpot slowly while the test tone is being played from the host computer, the orange LED eventually lights up when the frequency is matched.



Related posts:
Part I
Part III