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Changing the speed of a walkman motor with motion

Discussion in 'Sensors and Actuators' started by foundry, Aug 4, 2013.

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  1. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    OK. You can get more control over how the light affects the motor speed on the 443 model if you remove the speed control trimpot and replace it with a circuit containing the LDR, one or more resistors, and possibly other components as well.

    The 423 model is not so easy to modify. Can you use two 443 models?

    I'm assuming you want the walkman to play at normal speed when the LDR is illuminated, then when people get close to the exhibit, they will block the light, and you want the walkman to slow down. Right?

    A simple connection that you can experiment with is as follows. Refer to the first diagram in post #16 on this thread. Remove the motor speed trimpot; connect the LDR between the trimpot pad that connects to R604, and the trimpot pad that connects to pin 11 of the IC; connect a resistor of around 33k between the trimpot pad that connects to R605 and the pad that connects to pin 11 of the IC.

    You may want to solder some thin insulated wires to the pads and make the connections off-board so you don't damage the board with too much heat.

    This will control the motor speed so that when the LDR is brightly illuminated, the motor will run fast - faster than normal, and when the LDR is relatively dark, the motor will run quite slowly. You can adjust the value of the resistor to change the scaling between illumination and motor speed; increase the value to get faster motor speed for a given amount of illumination. Or you could replace the resistor with a trimpot of around 100k.

    This might be kind-of what you want, but I assume you want the motor speed to be stable and correct over a moderate range of illumination on the LDR. Otherwise, variations in ambient light would cause changes in speed. This can be done, but not easily. Some external circuitry would be required, and it might need a separate power source.

    Actually that made me think of possibly a simpler solution. Just varying the DC voltage you supply to the walkman will vary the motor speed. This will probably only work for the 423 model, because the 443 uses a stepper motor which will probably just stop if the supply voltage gets too low.

    In the 423 model, the speed regulator tries to keep the speed correct by regulating the DC voltage to the motor. If the supply voltage drops too low, it can't provide enough voltage to the motor, and the motor slows down. This is exactly what happens when the batteries are nearly dead. So, varying the DC supply voltage might be a simple way to control the speed of the 423.

    Have a think about this, and see if you can describe the exact behaviour you want from the walkman motors, and how you think you might set up the LDRs.
  2. foundry


    Aug 4, 2013
    Thanks a lot for this KrisBlueNZ. Really appreciate your input.

    You are right, what you've described is exactly what i'm hoping to achieve. Whilst getting any kind of change would be nice, what i would really like is for it to play at normal speed when illuminated, and when the light is blocked, the motor slows down.

    I'm not too concerned about variations in ambient light changing the speed. The light in the space is very constant, so hopefully the only variations that will occur will be those that are caused by people walking nearby, which is fine.

    One slight concern i have about the motor running faster than normal is that the Walkman will be running for a few hours at a time, so i don't want to put to much strain on it. Would it be possible to have it just running at standard speed as default/in light, and then slow down in shadow?

    The suggestion you made about the voltage sounds like it could be a great idea. Initially i had this idea because i remembered as a kid my walkman would slow down when the batteries became low. How could we go about achieving that control over the voltage do you think?

    I'll have a look on eBay now and see if it's possible to pick up another 443 too.

    Regarding the LDR's i'm not sure exactly where i will position them yet, but someone made a good suggestion to me that i could put them in tubes so that i could direct a beam of light towards them more accurately.

    Another thing i was thinking was that it might be good to have 2 on each Walkman, for 2 points of interaction. Do you think this might be possible?

    These were my ideas, but very much open to any suggestions you might have, again, i really appreciate your help and advice.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    The 423 and the 443 use fundamentally different motors and can't be controlled in the same way, so for simplicity, I think it would be easiest to have two of the same type of Walkman. As long as this isn't a problem from an aesthetic point of view.

    The 423 can be controlled by varying the supply voltage, which requires no modifications and is probably the simplest. So you might want to look for a second 423 instead of a second 443.

    Regarding running the Walkman at higher than normal speed. I shouldn't have mentioned that. The control method (whichever it is) can be adjusted so that at normal light levels, the motor runs at normal speed.

    Yes, using a tube on the LDR is probably a good idea, and having two LDRs won't be a problem. I was thinking that you might want to put them near the floor, so people's legs will interrupt the light. That would be more confusing for viewers of the exhibit, because I think people are less aware of what their legs are doing than what their body is doing, so they would be less likely to connect their movements with the change in speed. So it might seem more mysterious or random. I'm not sure if that's what you want...

    I think if possible, you should have a specific light (or lights) directed at the LDRs, rather than relying on the ambient light. Mainly because they work best when they're well-lit. Can you arrange a test setup for this? If so, can you measure the LDR resistance when illuminated and when a person is blocking the light?

    Controlling the supply voltage to a Walkman from an LDR is not difficult but will require some circuit assembly. Here's a simple design. Do you think you could build it, or get it built?


    This circuit is powered from a 5V DC supply rated at 1A (or more). Chargers for cellphones and similar items often have those specifications; maybe you can use a charger from a defunct cellphone. One supply can power two circuits, for two Walkmans (or should that be Walkmen?).

    Here's a brief circuit description.

    U1 is a shunt regulator (supplied by R1) that regulates Q2's base voltage to about 3.7V (set by R2 and R3). This voltage is buffered by Q2 and supplies the power to the Walkman. This voltage is nominally about 3V.

    Light falling on LDR1 and LDR2 causes current to flow through them, producing a positive voltage on Q1 base. VR1 pulls this voltage down, so when there is less light and less current through the LDRs, this voltage drops.

    When Q1's base voltage falls below about 3V, Q1 conducts and drags Q2's base voltage down. Q2's emitter voltage (the supply voltage to the Walkman) decreases by an equal amount.

    C1 provides smoothing to slow down the circuit's response slightly and to help reject noise and hum picked up by the LDRs and the wiring to them.

    All the components for the circuit are available from Digikey; links below. The circuit can be built up on stripboard - Google stripboard layout for examples of how this construction method works.

    Q2 will probably get warm during operation. (This depends on how much current the Walkman draws.) If it gets too hot to touch, you may need to use a heatsink (see below).

    VR1 (100k trimpot)
    C1 (100n capacitor)
    Q1 (BC557C)
    Q2 (BC337-40)
    R1 (330 ohms)
    R2 (22k)
    R3 (47k)
    U1 (TL431CLP)
    Heatsink for TO-92

    Attached Files:

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I've made a correction to the design in my previous post.

    Two trimpots are really needed to fully control the circuit's response to illumination. I've added VR2, and changed Q1 from a BC557C to an MPSA64 or BC516. (Digikey don't stock the MPSA64 any longer.)


    VR2 (1M trimpot)
    Q2 (BC516)

    It's a bit difficult to describe what VR1 and VR2 actually do. With VR2 fully clockwise, darkening of the LDR will have no effect; as you turn VR2 anticlockwise, darkening of the LDR will have an increasing effect on the motor speed. VR1 kind of controls the illumination threshold. Between the two of them, you should be able to get the behaviour you want.

    There will be a deadband over which the motor speed will not change, and you can adjust the two trimpots to set the maximum speed reduction and the illumination threshold, but they are not really independent of each other. In other words, twiddle them until you get a feel for what they do, and adjust them for the behaviour you want.

    Attached Files:

  5. foundry


    Aug 4, 2013
    Hi KrisBLueNZ,

    A huge thank you. What you have done is here is fantastic. I'm massively grateful.

    Sorry i couldn't post sooner, i was called away to work on a project over the weekend.

    I need to read over all of this properly, but i think i should be able to build it. I haven't done anything like this since school (about 20 something years ago!) but i think i can still remember how to put this sort of things together, plus your instructions seem really clear.

    I can only say thank you again, i'm really excited to give this a try.

    I'm actually heading down to an exhibition space now to go and have a look, so i will be able to see what the light is like down there.

    I should have time properly look into this and order the components this week. Unfortunately i'm based in UK so they may take a little while to arrive.

    I did actually have a couple more questions; regarding the power supply, i should be able to source something like you mentioned to power the circuits, but could you also advise me on what power supply would suit the Walkman itself? There is a socket for this, but i can't find the official Sony one anywhere. I'm guessing i can find some kind of replacement, but i'm not entirely sure where to look.

    Also, one more thing i've been wondering about (as i have to complete a risk assessment for the project) If i'm running the Walkmen from a power supply, is it safe to have the insides exposed? Aesthetically it looks really great to see the cogs spinning etc, but if someone was to touch it, would they get a shock? I've been working on them with batteries inside, and had no problems, but with a power supply? Would that be a problem?

    Thank you so much again for all your help.
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I don't understand why you want a power supply for the Walkman. The circuit I drew up provides the power to the Walkman. You don't need a separate power supply.

    If you want to power a Walkman without my circuit, you just need a 3V DC adapter. I'm not sure how much current a Walkman draws, but I doubt it would be more than a few hundred milliamps, so an adapter rated at 0.5A or more would be fine. You should be able to use a modern lightweight regulated switching supply, or the older type that uses a mains transformer. Some adapters come with a collection of plugs, and you should be able to find one that suits the Walkman you're working with. Make sure the polarity is correct: centre positive.

    Power supplies (switching type, and mains transformer type) are explicitly isolated from the mains, so there is no safety issue. But I would avoid cheap no-name Chinese adapters; these are often poorly designed and constructed.

    You're welcome; good luck!
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