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Filtering noisy 12V to a logic level

M

Miles

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).

The circuit is powered by a LM2937 automotive regulator. The timming
is done with a 555 in one-shot mode. I think the problem is lack of
filtering on the trigger wire. The trigger wire will be switched from
an open, to 12V.

Any ideas on a really solid noise immune way of converting that noisy
12V line to a 5V logic level for the 555? My revised idea is to use
the following signal path:

1. 12V unregulated
2. 20V TVS
3. LC low pass filter
3. Voltage divider (Divide signal by 4)
4. Pull Down resistor
5. 5.1V Zener
6. Schmitt Trigger output to 555 timer trigger pin

Do you think this will work? What values for my low pass filter
should I be looking at?

Thanks for your help.
 
S

Stepan Novotill

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).

The circuit is powered by a LM2937 automotive regulator. The timming
is done with a 555 in one-shot mode. I think the problem is lack of
filtering on the trigger wire. The trigger wire will be switched from
an open, to 12V.

Any ideas on a really solid noise immune way of converting that noisy
12V line to a 5V logic level for the 555? My revised idea is to use
the following signal path:

1. 12V unregulated
2. 20V TVS
3. LC low pass filter
3. Voltage divider (Divide signal by 4)
4. Pull Down resistor
5. 5.1V Zener
6. Schmitt Trigger output to 555 timer trigger pin

Do you think this will work? What values for my low pass filter
should I be looking at?

Thanks for your help.

A dedicated 12 volt line leaves your circuit and goes to the switch.
The signal from your switch goes back into your box. First to a
pulldown resistor and possibly diodes to VCC and GND to clip any
spikes, then it passes through a 1K resistor, then there is a 1uF cap
to ground, and then possibly through another series resistor to
prevent latchup if using a cmos 555.

Or just use a coaxial cable to feed the switch and forget all the
other stuff.
 
K

Kevin McMurtrie

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).

The circuit is powered by a LM2937 automotive regulator. The timming
is done with a 555 in one-shot mode. I think the problem is lack of
filtering on the trigger wire. The trigger wire will be switched from
an open, to 12V.

Any ideas on a really solid noise immune way of converting that noisy
12V line to a 5V logic level for the 555? My revised idea is to use
the following signal path:

1. 12V unregulated
2. 20V TVS
3. LC low pass filter
3. Voltage divider (Divide signal by 4)
4. Pull Down resistor
5. 5.1V Zener
6. Schmitt Trigger output to 555 timer trigger pin

Do you think this will work? What values for my low pass filter
should I be looking at?

Thanks for your help.

The inputs have a very high impedance. You could probably trigger it by
swatting a cloth against the wire (electrostatic generation).

An RC filter and pull-down resistor will do the trick. Of course you
don't want to pull the trigger higher than pin 8, so your pull-down
resistor and RC filter might need to combine into a voltage divider.


switch ---- R --+--+--- Trigger
| |
R C
| |
+--+--- GND


I'm not sure exactly what your regulation is. A simple snubber won't be
enough. Negative spikes in the input power will trigger the timer too.
Another RC filter is the simplest solution and it works quite well too.
 
B

Boris Mohar

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).

Your problem could be the vibration sensitive switch.

--

Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
Aurora, Ontario
 
T

Terry

Jan 1, 1970
0
Boris Mohar threw some tea leaves on the floor
and said:
Your problem could be the vibration sensitive switch.

It could also be spikes from the ignition, so check that its not
sensitive to that kind of thing ?
 
J

Jim Thompson

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).

The circuit is powered by a LM2937 automotive regulator. The timming
is done with a 555 in one-shot mode. I think the problem is lack of
filtering on the trigger wire. The trigger wire will be switched from
an open, to 12V.

Any ideas on a really solid noise immune way of converting that noisy
12V line to a 5V logic level for the 555? My revised idea is to use
the following signal path:

1. 12V unregulated
2. 20V TVS
3. LC low pass filter
3. Voltage divider (Divide signal by 4)
4. Pull Down resistor
5. 5.1V Zener
6. Schmitt Trigger output to 555 timer trigger pin

Do you think this will work? What values for my low pass filter
should I be looking at?

Thanks for your help.

See "NoiseBlank.pdf" on the S.E.D/Schematics page of my website for a
variety of ways to handle ratty edges.

...Jim Thompson
 
J

John Fields

Jan 1, 1970
0
---
If that's true, then the first problem is that the 555 doesn't trigger
on a positive-going edge, but on a negative-going edge, so you're more
than likely triggering on negative spikes when the switch bounces, and
every once in a while when the trigger input is driven low by noise.
The trigger input of the 555 is the floating base of a PNP, so if you're
not using a pullup on the trigger input, it will normally be off, but
noise could certainly trigger it.
 
J

James (Jim) Meyer

Jan 1, 1970
0
Hello,

I built a circuit for a motorcycle which cuts off power to the
ignition coils for several milliseconds when a switch is pressed to
fire an air shifter. It works well, but sometimes it triggers for no
reason (ie. switch was not pressed).
Before you go off and flog the input for noise, try an experiment.

Make all the connections, power and grounds included, but disconnect the
signal input wire right at the input to your circuit. Just leave it floating at
the circuit end but connected at the switch end.

Then run around for a while and monitor the output of your circuit. If
you still get random actuation, then you have a "ground loop" problem and even
the most well designed filter circuit in the world won't do Jack.

Jim
 
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