Need (cheap and easy) amplifier to drive solenoid from cell phone audio jack

Thread Starter

crochet

Joined May 23, 2018
3
Howdy--

I'm building an activity for a physics class that will create standing waves on a string. I intend to use a push-pull solenoid (amazon link) that can be driven with 9-24 V, consumes ~100mA at peak, and has a coil resistance of 43 ohms. I would like the driving sine wave signal to come from the audio jacks of the student's cell phones (link to an app for that). What would be a good solution for an amplifier? (I'm all ears if anyone wants to critique my choice of solenoid as well, or anything else about the idea)

Thanks in advance!
 

Externet

Joined Nov 29, 2005
1,667
An amplifier driven speaker is already sort of a solenoid. You can actually use any audio amplifier for the described load, as a domestic ´stereo´. Perhaps properly attaching the string to the cone would yield both the visual string vibration and the acoustics simultaneously to the student senses.
 

Sensacell

Joined Jun 19, 2012
2,908
The solenoid you linked to is a bad choice.

It's not really "push-pull" in the way you need- it's only "push" in the sense that the rod emerges from the rear.
This solenoid will respond poorly to AC, and will only move in one direction.

You need a moving coil actuator with a permanent magnet to create forces in both directions.
Just buy a big woofer speaker, carefully cut the frame and cone away so you are left with just the magnet and coil assembly.

Drive the coil with a powerful audio amplifier- done.
 

crutschow

Joined Mar 14, 2008
27,941
Just buy a big woofer speaker, carefully cut the frame and cone away so you are left with just the magnet and coil assembly.
The problem I see with that is, it's the cone the keeps the voice coil centered in the space between the magnet poles.
Without the cone there's nothing to control the coil position.

So instead, just attach (perhaps tape or glue) the string to a speaker cone.
The speaker type and size depends upon the standing wave frequencies you want to generate.
Do you know what they are?
 

Sensacell

Joined Jun 19, 2012
2,908
The problem I see with that is, it's the cone the keeps the voice coil centered in the space between the magnet poles.
Without the cone there's nothing to control the coil position.
Eliminating the paper cone will reduce the dynamic load on the voice coil, providing more useful output.

Woofers usually have 2 support structures- the paper cone itself and a smaller flexible structure below.
If you cut away the paper cone, you still have the coil centering structure below.
It's only supporting it at one point, so tilt alignment is still a concern.

I would glue a super light yet stiff push-rod to the center of the voice coil, then make a simple guide tube to prevent the rod from tilting the voice coil out of alignment, which would allow it to scrape against the magnet assembly.

You end up with a cheap and effective moving coil actuator.
 

Thread Starter

crochet

Joined May 23, 2018
3
Thanks a bunch, everyone for your help. I think I'm going to go with this amplifier (amazon link), which should be more than adequate for my needs. If I can get it all together in the next couple of weeks, I'll post the results here.

By the way, the idea of using an audio speaker for this purpose is already being marketed by a major lab equipment supplier here (link)...and for an incredible mark-up!
 

Thread Starter

crochet

Joined May 23, 2018
3
Sorry I forgot to return to this post for so long. That solenoid was fairly large and heavy, and I was not able to drive it with the setup laid out above. In the future, I may try simply repurposing and audio speaker for the job.

Thanks everyone for your help!
 
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