HELP! im a newbie in electronics... I have here a schematic which i have seen from other sources in the internet from Swagatam.. anybody could help me if where shall i attach the ground of VARIABLE 0-12 INPUT FOR SPEED CONTROL..is this correct?..
are R4 and R3 correct? any comment on the schematic?..

your comments and suggestions would be highly appreciated..

Thank you very much..

 

where shall i attach the ground of VARIABLE 0-12 INPUT FOR SPEED CONTROL..is this correct?..

The ground of the input signal would be connected to the "0" reference of the circuit. There seem to exist some strange things in this circuit which I don't have the time to go through right now.

Example:
R1 and R2 are two 1k resistors in series with the motor? Why is this?
Emitter and collector of T1 are inverted.

As for R3 and R4, they are part of the 100Hz astable multivibrator, whose output triggers the second timer circuit.

 

The circuit does have problems.
R1 and R2 in series with the motor would cause torque to be anemic at best.
T1 is connected inverted, and there is no base resistor; the person who drew the schematic probably thought that R1 & R2 would serve instead.

A Darlington is not the best choice, as the Vce will be high (voltage measured on the collector referenced to the emitter), as compared to using a MOSFET, which would have a low Vds (voltage on the drain, referenced to the source terminal).