Seeking help with vibrating motor schematic

I am looking for some help designing a schematic for controlling (8) 3V vibrating motors off of (2) 12V batteries. My field is primarily mechanical design, with very little electrial experience. This project is for my own and I am just looking into feasibility of building my idea. I have a VERY basic schematic sketched up but would like some help on how to connect the motors as well as how to run them all using a variable speed control of some sort. If anyone would have advice on how I should lay out my schematic I would appreciate it. Thank You.

 

vib motor schem

Since you are driving 3V motors from a 12V source, put the motors in series in a set of 4 and eliminate the resistors. You can drive 2 chains of 4 motors in series (the two chains are in parallel) from a single mosfet if you don't need any kind of individual control. You are currently operating at a maximum efficiency of 25% and turning the rest of the power into heat.

If you are going to drive the gate with an analog voltage, the potentiometer will need to have the two ends connected to +12 and ground and the wiper connected to the gate(s). The way you have it now the mosfets will not turn on.

Driving the gates with an analog signal will probably not produce the results you are looking for. The portion of the motion of the potentiometer that goes from stopped to full will be a small fraction of the total rotation (can be fixed by using additional resistors). What you really want to do is to drive the gate(s) with a PWM signal, either from a 555 timer circuit or from an arduino or similar (google 555 PWM circuit). Using PWM will allow for the use of a smaller mosfet and will in general generate a lot less heat.

Take a look at this page: http://www.dprg.org/tutorials/2005-11a/

That circuit uses an NFET and contains one possible design flaw. If the 555 fails, the gate will be pulled high by the pullup resistor and the motor will run at 100% duty cycle. Depending on your application this might not be an issue, but to change the circuit to fail-off instead, put a PFET above the motor load with the source connected to +12V and the drain connected to the motors. If you are using 3V motors, start with 4 of them, put them in series and connect one side to the PFET drain and the other side to ground. Put an anti-parallel diode across all the motors (one diode between the drain and ground with the anode connected to ground (this should be a power diode, rated for a couple of amps). You should probably also put a 100uf or larger capacitor between +12V and ground, keeping the capacitor electrically close to the source of the mosfet and the ground connection from the motors. No other changes to the circuit are required, and the 555 can sink 200mA on the output so it can drive pretty much any MOSFET gate.

Once you get it working, connect the other 4 motors in series and then connect that set of motors the same way as the first one, + side to the drain and - side to ground.