How to reduce noise!!!!!!

I am working on a circuit in which 8051 micro controller is used.

I have used 2 interrupts and one i/o pin to detect the signal and according to presence of the signal the corresponding alarms will be generated.

This is working fine also.

But the problem is even if in the absence of the signal the alarms are generating. And this happens only when there is a switching.

Please suggest how to get rid of it.

thanks,
paddy

Heya paddy_p,

It sounds like switching transients are inducing sufficient voltages on your inputs/interrupt lines to just cross over the input logic thresholds, erroneously fooling your micro into believing a desired signal transition has occurred. *Usually* these sort of problems can be made to go away with varying degrees of the following:

a) Make sure your micro has sufficient power supply decoupling, in the form of (at least) 100 nF right across the supply pins.
b) Ensure all unused inputs are pulled high/low as appropriate.
c) Make sure the RESET pin is driven appropriately, either by an RC network (with C > 10 nF) or a dedicated supervisory IC, and
d) If you have long input lines carrying low-bandwidth signals (such as to sensors/switches etc), these can potentially act as antennas for switching transients. Either:

* Electrically isolate them from the micro (by using optocouplers etc), or
* Add RC filters, with a series R in-line with the signal, and shunt C across the micro input. Naturally, adding an RC network will smear nice square edges into an exponential type slew, but provided the time constants aren't excessive (<10ms, say), your micro probably won't complain. Choose the largest time constant your circuit can tolerate (and still transmit the required data rate) for the best interference immunity.

E.g. For a burglar alarm with reed switches connected directly to the micro, a 1-10ms time constant would be entirely reasonable. Perhaps 10K and 100nF? An added bonus of RC networks on the inputs is that some degree of ESD protection can also be afforded by the addition of a transient suppressor or zener diode on the sensor side of the resistor to ground.

Hopefully that works for you!

Heya paddy_p,

Another problem can arise from the way you have lay out your circuit , For example if you have done it on a bread board and the wires near to the Interrupt wires are switching , In worst case you can assume that there are two wires near to the Interrupt wire that are simultaneously switching , This will cause induced voltage on the interrupt line giving false triggers , Your case is worsened by the fact that the interrupt wire is a high impedance node , (open at the sensor end and perhaps connected to the gate of a MOS at the other ) , Such nodes can have very unpredictable voltages due to mutual inductances alone , I can suggest you three solutions

1) Add RC to the interrupt input in configuration as in the previous post

2) Reduce the mutual inductance by proper routing of wires

3) Shield the Interrupt wire by adding a ground trace(PCB) nearby or twist a ground wire with the interrupt wire (bread Board )

Let me know if any of these work for you