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I do not think so: triacs and thyristors operate with AC. You can try dimming the bulb using a PWM circuit like used to control DC motors.
As you do not indicate bulb current or power, you can also use a power transistor operated as a variable resistor. Or you can connect several resistors in series and switch one after another with power transistors or MOSFETs.
All that requires quite complex electronics, but if you like to play...
When selecting a transistor switch for the lamp, you have to consider the rated current and the inrush current of a cold filament. BC547 most likely isn't able to supply it.
the idea of the project is to control the brightness of the bulb. this bulb operates on 6.8v 300ma. iam using pwm to control the brightness of the bulb. is it possible to control the brightness of the bulb. will uln2003 or which transistor will be suitable.
Yes, of course it is possible to control the intensity. The BC547 is a signal transistor, would not handle 300ma; since you are in India, I'd suggest an SL100 instead. Try with a frequency of 1Khz, and vary the duty cycle.
As you need to control a current of 0.3 A, a power transistor can replace a variable series resistor. My advice would be to use a LM317T variable-voltage regulator which can pass up to 1.0 A DC current. You can adjust the voltage or current with a variable resistor rated to a low dissipation ( e.g. a 5K potentiometer, 0.5 Watt).
If you wish to control the DC current from a uP, you will have to use an D/A converter with the above.
Refer to user's manual for LM317T, it works well as a voltage or current regulator.
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