Fluctuation in all appliances

Hello everyone...

i need help in my project CELLPHONE BASED DEVICE CONTROL...

in this project i use DTMF decoder for decoding tones and based on that switching particular appliances using electromagnetic relay, i use P89V51RD2 microcontroller to get DTMF codes and ULN2003 relay driver for relay ON and OFF. this circuit has also manual control option.

My problem is that when i connect normal load such as resistive load it working fine, but when i connect inductive type load such as fan, old chock based tube-light etc...it will make all devices turn off, even in my home anywhere this kind of load turn on or off it will generate fluctuation in my circuit that will change relay state...

please suggest me that what is actually problem...

and also i thing that if i use uln2003 than i can neglate flyback diode...? is that diode create that kind of problem...??

please help me...!!

Most likely you get unwanted µC resets triggered by relays contact arcing.

The actual problem is the insufficient interference immunity of your microcontroller circuit. There are many aspects like ground and power supply wiring, supply bypassing, components connected to the reset input. Adding suitable RC snubbers or varistors across the relay contacts usually helps to reduce contact arcing.

The best way to separate the sensitive electronics from power relays, etc., is to use optocouplers between logic outputs and relay drivers, and to enclose the microcontroller in a metal box, with good filtering of DC power. Use a "linear" power supply or a battery for it. If your relay has to actuate an inductive load, snubbers must be used and use higher-power contacts to make them live long. Separate always AC power lines from the sensitive electronics.
Locating your uP on the same board with AC power relays always invites problems you observed. Use separate boards.

Solution is RC snubber as FvM mentioned in post #2, also this will prolong life of relay contacts.


Best regards,
Peter