When do you see false triggerings,

The false triggering events happen when we toggle any relay in the system though each of them has a snubber installed nearby (R=47 ohms C=100n) I even observed that the last key pressed was getting repeated ,

and where exactly?

Mostly with keypad 4x4 but other i/p pins also show false triggering,after
Addition of 100n cap on almost the pins of micon used as INPUT this is
Not happening


Have you tried to use a Faraday shield, connected to signal ground?

Faraday shied not yet tried , I guess enclosing the whole pcb with a metal sheet will solve the purpose , will definitely try and share the results
For signal ground I am sorry I didn’t made but yes I can definitely change the design with all inputs for good my understanding for signal ground is power ground separated with a low value resistance and filters is this okay ?

At which inputs do you have the 100nF caps installed? Take care, if such a cap is fed by an output of micro, this output sees a short circuit, resulting in huge ground bounce! The cable to the keypad and the keypad itself should be shielded. Filters can be used, but must match the keypad reading routine timing affordnesses, means must slew fast enough. You should add some protection circuitry at the keypad lines. Schottky diodes (BAT54S) to the supply rails, for instance.

The 100n caps are at i/ps pins only but in my keypad routine I cycle a high I a nibble
So should I remove from there ? the final keypad is a membrane one on aluminium base so shielding is not possible with that the Aluminium base is already grounded with the metal body ground I couldn’t understand the use of schottky diodes plese explain a bit more

We should have a look at the PCB. Do you have a solid ground plane? Enough distance to the relays? All inductive loads "free-wheeled" or "snubbered"?

The pcb and assembly photos are uploaded on the web on following link

http://www.itasveer.com/pool/mani...052-images

As such no ground plains because of routing tracks , but the relay PCB is a separate one , the inductive load are snubbered with 100n/600v and 47 ohms rc circuit

One question here as you see in my images the 16 o/p of ULN2803 are wired to realy board and a common +12v is separately wired does this make a problem (current loop ) if yes what can be possible solution for this to take parallel wires for 12v and gnd together or any other?