Help with circuit to Ramp DC motor?

wabbit

Hello all,

I am working on a silly project to make a device to scratch my dogs back.

Basically it is a brush conected to a round disc so that it rotates slowly and scratches his back. I will use an ir or ping sensor to detect when he is standing under the brush to start the brush moving.

Here is my problem: I do not want the·motor to start abruptly. I think the sudden movement will scare him. I would like to ramp up the speed motor so that it starts slowly and works it way up to full speed.

(newbie alert!) To do this I am thinking of using a darlington transistor to take pulse signals from the basic stamp and apply pulsed 12v to the motor.
I bought a darlington transistor at Radio Shack, but once I got it home and started looking at it in more detail it says that the base current is 120mA. This is too much for the basic stamp, Right?

Here are some details of the project to help you answer my question:
The motor only needs to turn in one direction.
The motor needs to ramp up to full speed.
The motor I am looking at uses a gear head to create a 270:1 ratio for 20RPM at full speed
The motor runs on 12v
No load current is 65mA
Current at Max efficiency is 250mA

Is the darlington the way to go to send pulsed signals to the motor? I am under the assumption that a relay would be too slow to pulse?

Thanks for the help. Sorry for being such a newbie. I just dont want to fry my stamp.

Bill Chennault

wabbit--

You need a motor controller! Take a look at this Pololu motor controller sold by Parallax ($23.95). Be sure to look at the pdf documentation. It should allow you ramp up your gearmotor easily. I have one and enjoyed experimenting with it.

--Bill

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You are what you write.

wabbit

I have looked at the motor controllers, but they seem like a little too much for this project. I do not need to reverse the motor and the only thing the basic stamp needs to control is the sensor and the motor, so I am not too concerned about using up input pins on the stamp.

I was hoping for something that might be a less expensive so that I could make this permanent when I get it working.



Could I just use a switching transistor to apply current to the base of a higher power transitor to operate the motor? If yes, what do I need to do to protect the stamp from the kickback of the motor?

With a transistor setup like this can I send pulses to the motor?

Mike Green

For any switched motor or solenoid or relay, you need a power diode with a voltage rating about 2 times the supply voltage and any current rating over a hundred mA. The 1N4001 is cheap, ubiquitous, and rated at 50V and 1A and will work just fine. The diode is connected across the motor in reverse, so it normally doesn't conduct (anode to ground, cathode to +Supply). The Industrial Control tutorial manual downloadable from Parallax has examples around page 90 ... see: www.parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/List/1/ProductID/142/Default.aspx?txtSearch=Industrial+control&SortField=ProductName%2cProductName. You'll need to figure out the maximum and average current drawn by the motor to figure out what transistor you need.