switch input to paralel port - Electronic Circuits Projects Diagrams Free

vlad777 · 22nd February 2008, 03:16 AM

I am having this problem.
When i click the switch (mechanical switch)

instead of getting this:
00000000001111111111

I get this:
00000101010101111111

when I successively read changes to bit 5 (mask 32)(pin 12) of the
status port(base+1) of the parallel port. ( LPT1: base=378h)

I dont think anything is ocsilating.
I am guessing that change of voltage on the switch is too slow
and that when it is around 1.7x V , 50Hz interference from the power grid
gets amplified at the base of an input transistor.
I didnt know that the capacitance of a switch is so high, or is it something
else?

I actualy want to measure RPM of a brushless DC motor so when I connect
a hall generator to the port I dont get this problem
(I am guessing because there is a schmitt trigger in it)
but this is only me testing with a magnet in my hand.
I am wondering how the voltage is chaneing when the motor is spinning.

So should I use the schmitt trigger at the input or the interference can be
filtered out diferently?
And I am also very puzzled that a switch is so slow.

Many thanks.

**Nigel Goodwin** · 22nd February 2008, 12:55 PM

Have you not heard of 'contact bounce'?.

vlad777 · 22nd February 2008, 05:31 PM

Nop. But something like that is what first came to my mind.
I didnt know how to call it so I didnt search it on the net.
Because of this I also tried to use a potentiometer as an input.
When I turn the thing fast enough I get less glitches maybe even
a clean transition.But when I turn the thing slovely until I hit
special voltage level then I get continualy changeing input.
This also happens when I touch the unconected base of the
transistor that is conected to the port.
(When I connect the base to the ground with a resistor then nothing
happens when i touch it)
I am guessing this is because of interference.

How do you fight this?
Anyway on wikipedia I saw a circuit that might satisfie me.
Thanks.

**Nigel Goodwin** · 22nd February 2008, 06:04 PM

Quote:

Originally Posted by vlad777

How do you fight this?
Anyway on wikipedia I saw a circuit that might satisfie me.
Thanks.

Once you knew what it was called, it's easy to find information on it!

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22nd February 2008, 03:16 AM	(permalink)
vlad777	switch input to paralel port I am having this problem. When i click the switch (mechanical switch) instead of getting this: 00000000001111111111 I get this: 00000101010101111111 when I successively read changes to bit 5 (mask 32)(pin 12) of the status port(base+1) of the parallel port. ( LPT1: base=378h) I dont think anything is ocsilating. I am guessing that change of voltage on the switch is too slow and that when it is around 1.7x V , 50Hz interference from the power grid gets amplified at the base of an input transistor. I didnt know that the capacitance of a switch is so high, or is it something else? I actualy want to measure RPM of a brushless DC motor so when I connect a hall generator to the port I dont get this problem (I am guessing because there is a schmitt trigger in it) but this is only me testing with a magnet in my hand. I am wondering how the voltage is chaneing when the motor is spinning. So should I use the schmitt trigger at the input or the interference can be filtered out diferently? And I am also very puzzled that a switch is so slow. Many thanks.

22nd February 2008, 12:55 PM	(permalink)
Nigel Goodwin	Have you not heard of 'contact bounce'?. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

22nd February 2008, 05:31 PM	(permalink)
vlad777	Nop. But something like that is what first came to my mind. I didnt know how to call it so I didnt search it on the net. Because of this I also tried to use a potentiometer as an input. When I turn the thing fast enough I get less glitches maybe even a clean transition.But when I turn the thing slovely until I hit special voltage level then I get continualy changeing input. This also happens when I touch the unconected base of the transistor that is conected to the port. (When I connect the base to the ground with a resistor then nothing happens when i touch it) I am guessing this is because of interference. How do you fight this? Anyway on wikipedia I saw a circuit that might satisfie me. Thanks.