Speed switch using LM2907

[Udo Zaydowicz]

Hello,

I've got a problem with the above-mentioned f/U-converter (8-pin
version) using it as a speed switch.

Given is a car's speed signal (symetrical rectangular signal). Switching
should occur at a frequency of appr. 36 Hz. The speed switch is build
iaw the example in the datasheet/application note. The speed signal is
connected to the IC-input through ac coupling as described in the
application notes.
Schematic: http://www.zaydowicz.de/Forum/Schaltung.jpg

Well, it is switching as designed. However, if the car is decelerated to
a stop (frequency -> 0), the converter switches sometimes just after the
car stops.
Quite obvious, this happens each time when the positive flank of the
speed signal is 'caught' and the car stops before the negative flank.
The speed signal itself is clean, not spikes, no disturbences etc..
The picture at http://www.zaydowicz.de/Forum/Signal.jpg should
illustrate this (purple: speed signal, yellow: LM2907 f-input).

Can anybody help me with that?


Many thanks in advance, Udo

 

[Udo Zaydowicz]

Hello GPG,

GPG schrieb:

Udo Zaydowicz <zaydo@gmx.de> wrote in message news:<cnne5r$hhg$1@news1.ewetel.de>...

[...]
Schematic: http://www.zaydowicz.de/Forum/Schaltung.jpg

[...]
Quite obvious, this happens each time when the positive flank of the
speed signal is 'caught' and the car stops before the negative flank.
The speed signal itself is clean, not spikes, no disturbences etc..
The picture at http://www.zaydowicz.de/Forum/Signal.jpg should
illustrate this (purple: speed signal, yellow: LM2907 f-input).

[...]


Possibly interference from the supply getting to the input via R7, C4.

Rather not, I think.
1. The supply is stabilized and protected against the typical 'dirt' in
the car, and I could not measure any.
2. It only happens in the situation I described above. If the car is
stopped between the negative and positive flank of the speed signal,
everything is working fine.

Udo