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The PIC schmidt trigger CCP input is a good FET type input, and you can connect it to most things just using a resistor, and/or through a cap. However the signal needs to be >3.5v or so peak to peak.
Otherwise for small signals it is common to use a single FET to amplify the input wave before going to the PIC pin. Many of the freq meter projects do this, and as Nigel suggested a google search will show you heaps of schematics or good working designs.
Very nice Mr RB I understood what you said.
I have a small question.Why most people adding a capacitor (ex:-470pF) in front of a transistor base in many frequency counters?
Ok guys here is the interface circuit I'm going to use.
I just told you that NOTHING will operate better than the input of the micro.
No-where did he mention that he wanted to detect signals less than TTL level.I need an interface circuit that will couple frequency to the PIC pin safely.
No-where did he mention that he wanted to detect signals less than TTL level.
He just asked for "safely."