I create 100 times more complex circuit diagram in minutes. My offer is not only to provide the diagram needed, but to also support you in the process, help select the proper components, and make sure I get a happy customer, whatever it takes.
This is my first ever bid on the Freelancer, and my true motivation is positive review, not the money.
For example I would really suggest logic level MOSFET for this application, rather than bipolar NPN, because of their low ON resistance, and low voltage drop, and simpler control on the input side.
If you so desire I can provide diagram with NPN transistors, but there will have to be either major voltage drop (at least 0.5V) between collector and emiter in the ON state, or if allowed to go deep into saturation, the voltage drop may be as low as 0.3V, but the switch-off time will be longer, causing loss of resolution at higher outputs. To clarify, high pulse width values are likely to merge together into continuous ON state.
I would need a bit of clarification on your existing components:
1 - Are the light bulbs rated for 5VCD?
2 - Is your existing power supply capable of driving the light bulbs in their full ON state, with only minor drop of voltage?
3 - 2 lamps with 2.5ohm resistance have combined resistance of 1.25 ohms. Theoretically they will draw the current of 4 amps. Your power source has capacity of 3 amps, and is therefore underpowered. If both lamps have to be at max. intensity at once this could be a problem.
Thanks,
Dario