Noise on input signal

[aratti]

Did you set all the unused pins as outputs (using the tris register) and set them to low? If not please do it! Never leave floating inputs. If they cannot be set as outputs, then you must ground them.

Cheers

Al

[fanman]

Hi Al,

Went back and grounded the unused pins. Still get the pulse on the analog signal. Anything else I missed?

cheers

Mark.

[aratti]

Is C11 (decoupling) connected directly to pin 14 ?

Add a large capacitor to the output of your regulator ( say 220 uF)

X5-3 point goes to a pic pin. Is it an input? If yes and you have nothing connected to the terminal, then you have to ground or pullup also X5-3

Cheers

Al

[Gusse]

X5-3 point goes to a pic pin. Is it an input? If yes and you have nothing connected to the terminal, then you have to ground or pullup also X5-3

Another vote for pull-up or pull-down (some high but reasonable value would be OK). Floating input will see your LED transient (voltage/current change) and interpret that as an input signal (=working as a sensitive receiver).

-Gusse-

[MikeWinston]

If X5-2 is power to your sensor, then maybe try a capacitor there.
But first I would try what aratti said and use bigger caps by the regulator.

Mike -

[fanman]

Thanks guys. I will try your suggestions. I will double check the TRIS registers to ensure they have the correct setting. By the way C11 is connected directly to Pin 14 and the input signal is from a low impedance source.

[Jerson]

You need to check your grounding paths. If this circuit is built on a breadboard, it may not be easy to resolve this. Ground loops are a concern in most ADC problems. Try to get your ground routed like this
Input -> 7805 -> Output transistor -> Microcontroller.

Any changes in this sequence can contribute to the problem.

Another possible problem is the rate at which you read the ADC. Do you filter the readings you get ??