TRIAC output deformed

[ashisalluneed]

hello everyone ,
i am working on an AC dimmer project and the triac gives me a deformed output signal whaterver the arduino code i upload .
if anyone knows any trick to help with the ouput ptoblem please help , any help is appreciated .
the exact problem is the amplitude of the negative alternation .
the schematic and result are shown below .

[bdunham7]

Your triac has a guaranteed senstivity of 50mA for quadrants I/II/III, but you are only giving it maybe 15mA peak.  That might be enough to trigger it in quadrant I, but not III.  So I'd suggest changing R3 to 220R or so.