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I'm hacking the speed control on the TCM-20DV sony walkman. Service manual with schematic here: http://pdfstream.manualsonline.com/d/d92cbef9-a37f-4e2b-92e5-521fab68df58.pdf

Out of the box it has a small-range speed control with two speed tiers (normal and half-speed). I'd like to replace this with a single knob to cover the entire range of speeds. I've tried a few approaches, but I'm very new to electronics and was unable to produce satisfactory results.

The motor has red and black terminals. The red is always ~3.0 volts from the two AA batteries. The motor seems to be controlled by varying the voltage drop across the motor, using some sort of "governor" circuit in the walkman's IC. If the motor takes all 3.0 V, it goes as fast as possible. Conversely, the slowest setting is a drop of ~1.0 V. I found this out by measuring the voltage from the black terminal to ground. Ideally I could just hijack the terminals and make a circuit which lets me adjust the voltage drop across the motor.

My first idea was to use a voltage divider and a potentiometer in parallel.

First Attempt

Oddly when I wired this up, the whole voltage drop happened on R4 and the motor did not do anything. First question: why would the first resistor take the whole drop?

My second idea was just putting a potentiometer in series with the motor, to control the voltage drop across the motor because it seems to have some resistance itself. This sort of worked but the motor oddly wouldn't move without a nudge at higher (but still within range) resistance values. Second question: why would it need physical nudging to get started in this case?

And third question: what is the appropriate approach to this problem? I'm a bit lost, since there are many many techniques I am completely unaware of. And any reference material you think is relevant would be much appreciated!

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First question: why would the first resistor take the whole drop?

Because the motor has a very small resistance.

Second question: why would it need physical nudging to get started in this case?

Probably the series resistor didn't allow enough current to flow for the motor to produce sufficient torque to overcome static friction.

And third question: what is the appropriate approach to this problem?

Connect the motor to the battery with just an ammeter in series and see how much current flows. Gently apply some braking to the shaft with your finger and note how much current flows.

And any reference material you think is relevant would be much appreciated!

Recommending specific sources in answers is discouraged here. There is a lot of material on the internet. Whatever you can best understand is best for you. It is best to start by studying basic principals and then move on to the sort of thing you are trying to do.

The basic understanding that you seem to be lacking is that a voltage divider must always take into account the connected load. The motor is, in effect, a low resistance load. You can begin to understand that by measuring how much current the motor takes when loaded and unloaded. The motor is best understood as a very small resistor in series with a generator that produces a "back EMF" voltage that opposes the current from the supply and varies in proportion to speed. When the motor is not turning, back EMF is zero. At full speed, back EMF is near the supply voltage.

Controlling a very small motor with a variable resistance in series is fine for a beginner. Historically, that is how DC motors were first controlled. However that is very inefficient and does not provide very accurate control. You need to pay close attention to the power rating of the series resistor.

Someone else might recommend some reference material, but I believe you should learn to find on your own, material that suits your level of experience and style of learning.

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  • \$\begingroup\$ Recommending products is off topic. Recommending reading material or what to look for is not. And your third paragraph answers nothing. \$\endgroup\$ – Passerby Nov 25 '17 at 2:09
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The voltage divider is never a good solution to a power source issue.

If you want to use a pot in series, it has to be a very low resistance pot. A 50 or 100 ohm pot would work. Male sure it can handle the power (r * i^2). Too high and the motor stalls out, as you've seen. The reason it needed a nudge is that a motor or solenoid has two voltage requirements. A trigger and a hold voltage. The hold voltage is much lower than the trigger voltage. It has to overcome inertia, and more power is needed for that then for it to stay spinning.

Another approach is using pwm to control the motor. A simple method is a 555 timer to create the pwm based on a pot. But for 3V, you need a low power clone of the 555 timer. Or you could go digital. A microcontroller to read a pot and produce a pwm signal.

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