how does a knock sensor work?

[David Gravereaux]

Mark Agnello <m_agnello at yahoo.com> wrote:

>Actually, the sensor output is in units of electrical 
>charge called coulombs. In practical terms, the 
>electrical charge output looks like a very weak 
>voltage. It's very sensitive to the load it is 
>connected to.

Just say it has an extremely high output impedance.  That rings my bell, at
least :)

That can be solved with a good FET buffer as the input stage with a really high
input impedance around 40 MegOhms.  I reengineered the front-end to an AKG 451
condenser mic once.  The schematic looked something like this:

                         +Vb
                          |
                 o--------o----------o
                 |        |          |
                 |        |          |
                 |        |          |
                 |        |          |
                 |        |          |
       100pF     |    ||--|          |
IN  >---||---o---+--->||    T1       |
             |   |    ||--|          |
             |   |        |       o--|
             \   \        |     |/
       1G(?) /   / 10M    o-----|   T2
             \   \        |     |\
             |   |        |       o->|
             o---o        |          |
             |   |        |   510R   |   2.2 uF
             |   \        o---\/\/---o-----||----> OUT
             |   / 10M               |
             |   \                   /
   500pF(?) ---  |                   \  1.2K
            ---  |                   /
             |   |                   |
             o---o--------o----------o
                          |
                         -Vb
[Impedance buffer]
A =~ 0.97
T1 is some small signal JFET.  I don't remember what I used.
T2 is a boring 2N2222, 2N3906 or something.

--
David Gravereaux <davygrvy at pobox.com>

$ make war
make: *** No rule to make target `war'.  Stop.  Try `love' instead.
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