Building a power screwdriver is a pretty simple thing if you have access to geared motors. If you live in Bangalore, you can get them at S.P road for about Rs. 180.  They come in a variety of speeds- 45 rpm to 500 rpm and are powered by a 12 V DC motor. 

I used a gearbox that has an output speed of 500 rpm ( according to the box) That ensured a good mix of speed and torque for the purpose it was being used for.

 The screwdriver end was salvaged from an old broken T-type screwdriver which had a nice hexagonal chuck kind of thing in which one can fix quite a large variety of bits ( flat/ star of varying sizes)

The "chuck" holding the actual screwdriver bit and connecting it to the drive shaft of the gearbox is a sort of connector which has  screws made of aluminum at the sides to secure it onto a shaft. 

The circuitry ( if you can call it that) used is just a simple H-bridge kind of setup ( for direction change) made with a special switch i found in an old  walkman and a push button switch in series with the motor.

 The special- ness  of the switch is because it is actually 4 switches packaged in one nice compact unit. It has 2 positions and 2 independent swiches are toggled at a time. Ideal to make a H-bridge!

The body is made of an old PVC drain pipe. Its a good thing that it softens quite easily on moderate application of heat. Nice for squeezing it into any shape you want.

I'm using an old UPS battery as the power source. Its a 12V sealed lead acid battery. I also have a power adapter of an old cable modem- this is what I use to power it most of the time actually.

The push-button switch ( for on/off)

The H-bridge switch for direction changing (at the back ) 

                                        Rear view (the proto-board holds the direction changing switch

I had to saw off the screw's head and make a notch like a grub screw (I made a messy job of it, I know)

                                                    Front view without the screwdriver bit

I heated the PVC pipe and squeezed it by hand so that it fit snugly over the geared motor assembly. No screws or fasteners required this way!

THough not exactly a substitute for good old muscle, It seems to do quite a decent job of unscrewing and screwing things. There were quite a few things that came undone pretty easily ( like my calculator's screws and the screw that holds the blade in place in a pencil sharpener)