DC motor control

[Ralluph]

Hello everyone,

I would like to operate a small DC motor in a stalled condition and I need help to determine the component values in the attached circuit diagram.
My supply voltage is 5V at which the motor draws 40mA when stalled.
I want to be able to reduce the voltage to 1.5V at which the motor draws 10.8 mA when stalled.
The transistor I would like to use, is the 2N 3903.
Any help will be greatly appreciated.

Sincerely,

Ralluph

[KMoffett]

What do you want to happen after the "circuit" shuts the voltage down to 1.5V? You do know that starting the motor is essentially a stall condition with high current. Bypass the circuit during start up? What do you want to happen after a stall is detected and shut down? Auto restart after a time?...Manual restart.? It can help to tell us what the overall project is.

Ken

[Ralluph]

Hi Ken,

The overall project is a tonearm to play LP vinyl records.
All tonearms exhibit an undesirable tendency to want to "skate" toward the label area of the LP. There are various ways to counteract this skating force by using weights, magnets and springs. I want to use an electric motor with a pulley and a string instead of the above mentioned methods. The string would be attached to the pulley at some point and the motor would always operate in a stalled condition. in fact, the motor would be forced to rotate in a direction opposite to the direction in which it is wired. The motor would essentially operate like a rotary spring. This is essentially an anti skating device. The POT in the circuit would let me experiment with the amount of anti skate force to apply to the tonearm.
If you need more information, just ask.

Sincerely,

Ralluph

[KMoffett]

Interesting. Are you building a totally manual turntable?

Ken

[Ralluph]

Hi Ken,

I am only building the tonearm itself. My turntable is a Micro Seiki from the 1980s.
I have been working on a tangential tonearm since 1981. It took quite a while because it was always a hobby. When CDs appeared on the scene, I gave up working on my tonearm until, in 2010, I realized that LPs were making a come back. So I went back to designing and finally in 2014 designed what I consider to be the perfect tangentially tracking tonearm. Of course I am not the only one who claims that.
The model I designed and built in 2012, requires an electronic servo to operate. My 2014 model may work without the servo. I am now conducting tests to determine that.

Sincerely,

Ralluph