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decoupler question

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wejos

Member
hello guys i'm getting multiple readings on my pic, maybe this is because i'm using common ground throughout my breadboard. my question is if i increased the value of the decoupler (1uf) to say 47 uf, will this reduce the noise (i'm only assuming this was the correct term to use) and fix the multiple reading that i'm getting? i already placed 1-meg pull up resistor in my RB0.

thank you
 

Mikebits

Well-Known Member
A 1 meg pullup is useless. Post a schematic and a better description of your problem.
 

wejos

Member
eric gibbs taught me the resistor divider approach but was not able to make it to work so i went on using the 555. the 16f84 and the 555 have 1 uf decoupler. thank you very much in advance.
 

ericgibbs

Well-Known Member
Most Helpful Member
eric gibbs taught me the resistor divider approach but was not able to make it to work so i went on using the 555. the 16f84 and the 555 have 1 uf decoupler. thank you very much in advance.
hi,
Just to clarify a point, I suggested 3k3 and 2k2 as a divider to get from 12Vdc to +5Vdc for a PIC input for PORTB.0 ,,,
never any mention of 1meg resistors.:)
 
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wejos

Member
hi,
Just to clarify a point, I suggested 3k3 and 2k2 as a divider to get from 12Vdc to +5Vdc for a PIC input for PORTB.0 ,,,
never any mention of 1meg resistors.:)
yes eric and thank you for all your help. i realised now why it did not run because the signal was negative going, which at the time i wasn't aware of.

yes no 1meg resistor lol sorry
 

ericgibbs

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yes eric and thank you for all your help. i realised now why it did not run because the signal was negative going, which at the time i wasn't aware of.

yes no 1meg resistor lol sorry
hi,
If the supply to the coin mechanism is +12V and 0V is common/ground and all the coin switch does is switch the +12V on/off, how can the coin pulse be a negative voltage..:confused:
 
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wejos

Member
i have no idea but heres whats going on eric, when i attached the wire that sends out the pulse from the coin mech to the 555 trigger, the 555 will give me an output and the PIC would even detect. i have no idea and i can't explain why and how it worked. its just i'm getting multiple reading after the pic read the rising edge.
 

ericgibbs

Well-Known Member
Most Helpful Member
i have no idea but heres whats going on eric, when i attached the wire that sends out the pulse from the coin mech to the 555 trigger, the 555 will give me an output and the PIC would even detect. i have no idea and i can't explain why and how it worked. its just i'm getting multiple reading after the pic read the rising edge.
if its a mechanical coin switch you will get multiple signals due to contact bounce.

You need to debounce the switch signal, use a cap/resistor filter on the signal from the switch.
 

wejos

Member
okay eric i found a sample




let me guess "X" there goes to my RBO?


i found other diagrams but they are a bit complicated and uses 555 and schmitt triggers (i have no idea what schimitt is [it compares voltage according to my speed read]).

you think this diagram will do eric?
 

ericgibbs

Well-Known Member
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okay eric i found a sample



let me guess "X" there goes to my RBO?


i found other diagrams but they are a bit complicated and uses 555 and schmitt triggers (i have no idea what schimitt is [it compares voltage according to my speed read]).

you think this diagram will do eric?
no, it will make it worse.:)

I will do a sketch..

EDIT:
Something like this. Increase the C1 value as required.
 

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Mikebits

Well-Known Member
Your 1 meg is not actually pulling up anything since it is tied to ground, this would be a very, very, very weak pulldown.
 

Mikebits

Well-Known Member
A pull up is a resistor that tends to pull a line towards a positive level as in +v. A pull down attempts to pull a line towards ground level. The choice of a pull up or pull down is often selected based on many factors. Some are, open collector outputs, or if line may go tri-state, or the driver needs help sourcing or sinking current, these are but a few reasons.
 

wejos

Member
thanks guys eric mike. although i havent completely grasped the whole thing but i'll do some readings also and actual in-breadboard testing. my best kudos to you guys for helping a starter like me.
 

Mikebits

Well-Known Member
Eric is a really good engineer, I would listen to him before I listened to me any day. What are you still unclear on?
 

wejos

Member
the decoupler mike, the higher the value the less interference?

and how do you check a mylar capacitor if its good or not? does it charge like electrolytic?
 

smanches

New Member
The value of the decoupler will determine it's frequency response. Lower value caps can usually react faster and handle the higher frequencies better. Also, ceramics are usually better than electrolytics for higher frequencies as well due to the lower ESR. Sometimes if the environment is very noisy, you will use two or three decoupling caps, at different values, each handling a different frequency range of the noise.

Mylar caps charge just like other caps.
 
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