I don't think it's a regulation problem, but rather the motor is inducing noise on the power supply that is causing the 555 to reset and otherwise operate erratically.

Separate power supply is not an option because there is no space to fit external battery, I hope this problem can be solved.

 

You guys are of the impression that the "pulse" feature is electronic. From the Metabo catalog it shows that it is mechanical. Like a mini impact wrench. vesko.net/pdf/metabo/66-83_metabo_en_01.pdf

 

Well, the one in the video didn't seem mechanical. But also, that's why I said the torque control drills have has a similar functionality.

To avoid the motor noise you could use a diode between the battery and the 555 circuit, including a bypass capacitor, and preferably a smallish value resistor and a 15v zener just in case to suppress the back emf from the motor.

By the way if the tool has a variable speed switch then most likely it already has an internal flywheel diode.

What do you guys think of this circuit?

 

@ vrainom - may be this will convince you. Don't know why my first link doesn't work.

http://www.metabo.com.au/4932.0.html?&tx_ttnews[tt_news]=17894&cHash=9985c86cde

Here is the first link again(it does work). Page 3 shows the switch for the gearing to the impact feature. http://www.manualslib.com/manual/102370/Metabo-Cordless-Drills-Screwdrivers.html?page=3#manual

 

@shortbus, yes in that sliced view of the tool it's quite evident that it's a mechanical implementation, but in the video smilem linked in post #1 it's clear that the "impuls" switch is nowhere near the gear train and a led blinks with each pulse, so probably not mechanical and definitely a different tool to the one you're linking: http://www.toolstop.co.uk/metabo-sb...puls-combi-driver-2-x-3ah-li-ion-batte-p10342

 

In my video http://www.youtube.com/watch?v=86l4Ft_xAr0
the Impuls functions is not mechanical because you just push simple push-to-make sw

 

What do you guys think of this circuit?

Can somebody breadboard this? I would like to know if it works. In your circuit the 555 timer will eat the battery eventually, because timer is not switched off when pulse mode is off. That should be corrected.

I know using the switch like in my original circuit would be OK because you switch off the 555 timer power pin when you turn the impuls off. the only problem is that the MOSFET is used when the impuls is off that means possible overheating since you can't fit it to a heatsing in tight space inside screwdriver.

So I made this second circuit it does not have any improvements for back emf but it does not use MOSFET when impuls is off.

 

Well yeah but a cordless tool motor drains a lot of juice in dc, so you'd need to make the impulse switch very robust to withstand the motor's demand. I've drawn this for you:

But if you really wanna get practical about the circuit, is there enough space inside the tool to actually implement this circuit?