Actuating Pencil To Draw Waveforms

[Abdel]

I am looking to make a pencil whose tip moves to create a waveform as you move it across the paper.In essence, I want to be able to record a sound and then play the sound through the pencil whilst I draw with it, of course, I will use software to lower the frequency to make it more of a waveform and not just a vibration.

I have many ideas but I am thinking of using a solenoid to actuate a pencil which I can move across some paper. Sadly, I do not have access to one to experiment with so I was hoping someone could hook a small solenoid up to an amp and tell me the results before I buy one?

If not this, what is the best way (using an amp) that I could achieve the desired effect? I have considered using a subwoofer but this will be bulky.

I want to keep it simple which is why I am not considering the use of a servo.

Thank you

 

[berkeman]

I am looking to make a pencil whose tip moves to create a waveform as you move it across the paper.In essence, I want to be able to record a sound and then play the sound through the pencil whilst I draw with it, of course, I will use software to lower the frequency to make it more of a waveform and not just a vibration.

I have many ideas but I am thinking of using a solenoid to actuate a pencil which I can move across some paper. Sadly, I do not have access to one to experiment with so I was hoping someone could hook a small solenoid up to an amp and tell me the results before I buy one?

If not this, what is the best way (using an amp) that I could achieve the desired effect? I have considered using a subwoofer but this will be bulky.

I want to keep it simple which is why I am not considering the use of a servo.

Thank you

Welcome to the PF.

So you will be moving your hand from left-to-right along the "time" axis, and you want something to modulate the tip of the drawing instrument in the vertical "amplitude" axis, right? How much deflection do you want? +/- 1cm or less? Or some larger deflection somehow?

Can you post some Google Images pictures of the kinds of waveforms you want to be able to draw/print?

 

[CWatters]

I don't think a solenoid would be the best thing to use. Perhaps look at a small servo of type used in model planes. Mount it at the top of the pen with control rod down through the body to the tip.

 

[eq1]

You might want to first look into how things like phonographs and record players work. They're pretty simple in concept but in practice they can be hard to construct as one needs to do the alignment fairly precisely. You might be able to just hack a used record player to get what you need.

Or maybe look for used temperature and humidity chart recorders on eBay or something. I'll bet whatever moves the pencil just takes a voltage input like an oscilloscope channel.

 

[Abdel]

Berkeman,

Yes, you have the correct idea. +\- 1 cm deflection is great.

I would like something like this:
https://ak2.picdn.net/shutterstock/videos/22951306/thumb/1.jpg
But am happy with less detail, as long as it is recognizable as a waveform.

Thank you!

 

[davenn]

I am looking to make a pencil whose tip moves to create a waveform as you move it across the paper.In essence, I want to be able to record a sound and then play the sound through the pencil whilst I draw with it, of course, I will use software to lower the frequency to make it more of a waveform and not just a vibration.

Or maybe look for used temperature and humidity chart recorders on eBay or something.

yeah, so you want a penchart recorder

https://www.bing.com/search?q=pen+chart+recorders&src=IE-SearchBox&FORM=IENTTR&conversationid=Dave

 

[Abdel]

A portable penchart recorder yes, also one that doesn't cost in the hundreds. The same however.

Thanks

 

[Windadct]

Not a bad project, but it involves a few steps.

Waveform Capture ( tuning the sound into computer data)
There is software for the PC that let's you look at the input from the sound-card ( microphone) - is a good place to start. Example

Plotter
To make a waveform on paper, you need to move in 2 directions, axis. One can be the pencil on a servo or other moving part, for the other (time) you could scroll a roll of paper, easy in concept, but not everyone has rolls of drawing paper laying around - so most commonly the first axis is mount onto another axis. There are many ideas out there for a 2 axis plotter.

If you have an arduino - it also has a serial plotter function in the IDE, which is about as much of a plug and play way to get started that I can think of.

 

[CWatters]

Perhaps capture the sound and play it back more slowly so your hand doesn't have to move so fast.

Or just use a pen plotter?

 

[berkeman]

Ooo, ooo, I got it, I got it!

Use Google Glass to show you a heads-up version of the waveform, and you just trace it out with your pencil on paper! Why didn't I think of that before!

https://9to5google.files.wordpress.com/2015/11/google-glass-enterprise-edition.png?w=1495&h=674

 

[davenn]

A portable penchart recorder yes, also one that doesn't cost in the hundreds. The same however.

Thanks

pen chart recorders are old technology, everyone has gone digital
they are available on the surplus market, often at giveaway prices

It's been almost 20 years now since I used a penchart recorder for my seismograph ( earthquake recordings) ... digital being much more versatile

Dave

 

[Windadct]

If you can capture the data ( samples) in a spreadsheet - you can then just make a graph in excel. But we probably all would like you to define this a little better, for example, this can not be continuous, since you need to slow it down by a factor of 100 or even 1000 to work, that means a 1 sec plot is 100 sec of sound.

 

[arydberg]

Others would also like a cheap chart recorder. Perhaps you could hack a calculator with a attached paper tape. They sell for about $20 . You may need to live with many dots instead of a line.

 

[sophiecentaur]

pen chart recorders are old technology,

They are severely slew rate limited, too. Your frequency response will be very poor unless the pen is extremely light and the deflecting magnetic field is high at audio frequencies. Do you actually want to trace the audio waveform in real time or could you accept a sub sampled picture of the waveform. Sub sampling can give you excellent looking results if the input waveform consists of long strings of the same repeated shape. Alternatively, you could do a plot of the audio level - a much easier problem to solve. The x-axis scan will be pretty slow so you won't see more than a few tens of Hz, real time.

I have considered using a subwoofer but this will be bulky.

As you do not need an acoustic output, you could do without the sub-woofer enclosure; the drive speech coil of the speaker could move your pen directly without the cone being attached even. As long as you leave the 'spider' to keep the coil moving smoothly through the slot in the field magnet. That's essentially the mechanism of some fast pen chart movements. Old LF loudspeakers are pretty easy to get hold of. You will need a pretty hefty amplifier to deflect the coil by a cm or two. It could be a fun project, though.

 

[Abdel]

Thank you! This is exactly what I've done and it works! However, it cannot reproduce "soft" signals such as a sin wave signal bellow 5Hz but is able to reproduce a sawtooth signal as low as 1Hz. My understanding is that the built in amp cannot amplify a signal this close to a DC current? I need a very low-frequency amplification circuit, between 0-20Hz I think would be best.

I've dabbled in the idea of a simple transistor as an amp but I do not know enough about this to try it, can anyone help me?

Thank you!

 

[sophiecentaur]

Thank you! This is exactly what I've done and it works! However, it cannot reproduce "soft" signals such as a sin wave signal bellow 5Hz but is able to reproduce a sawtooth signal as low as 1Hz. My understanding is that the built in amp cannot amplify a signal this close to a DC current? I need a very low-frequency amplification circuit, between 0-20Hz I think would be best.

I've dabbled in the idea of a simple transistor as an amp but I do not know enough about this to try it, can anyone help me?

Thank you!

The amp you need is actually quite high spec (needing a large amplitude, linear performance). But there are many cheap high power amplifier boards available and I wouldn't bother trying to re-invent something like that. There is plenty of mechanical stuff that you will have to make for yourself. The problem could be that you need a DC coupled amplifier and many off the shelf amplifier designs are AC coupled with a lower frequency bound of 20 Hz or so.
Edit: PS (My memory just kicked in) you can buy electromagnetic actuators for School Lab use. Unfortunately, the three that I used in the Physics Lab where I taught were not very well made and they broke very easily when used by students. Not too hard to mend as they were basically loudspeaker drives. You will need to have a robust mechanism to transfer the linear motion of the coil in magnet to your output pen motion.

 

[Windadct]

I am reminded of a Sailing Analogy -- you can have Fast, Good or Cheap... Pick two... Response time = fast, accuracy = good,,, well cheap = cheap...

Keep experimenting!