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2020-07-22 4:16 pm
This is going to be a fun project. I have an illusion in my magic act where playing cards rise slowly and mysteriously from a deck placed in a wine glass. at the end, all the cards shoot out of the glass in a fountain-like manner. Typically the solution is to weave a thread through the deck in a certain way that when the thread is pulled, the cards rise. The speed depends on how fast or hard the thread is pulled. Usually a hidden assistant pulls the thread. I'm going to make use of an Arduino and a DC motor attached to the underside of the table as my hidden assistant. The motor will have a spool that will wind the thread onto it when activated.
I attached the motor to an H Bridge with hopes that controlling the speed of the pull could be done with enA. The idea was to have two buttons. Pushing the first would start a delay, then pull the thread slowly and for enough time to cause one card to rise. Pressing the second button would cause a fast pull, thus shooting all the cards from the glass in a fountain. The first problem occurred when I found that with analogWrite to enA at a very slow speed, the motor lost all of it's torque and couldn't really spin anymore, it just sits there and buzzes. The top speed works beautifully for the finale. I can't really use a stepper motor which works with good torque at a very slow speed, but they are not fast enough for the last part of the trick. Maybe my DC motor isn't high enough quality.
So basically the goal is to run a motor (with the same or similar amount of torque) at two different speeds at the push of buttons, one to reel the thread very slowly and one to reel it very rapidly. I would love to hear ideas and thoughts from you.
Also, shhhh. It's a secret.
2020-07-29 8:50 pm
An easy way might be to use a speed sensor (this can be optical or magnetic) on the motor as feedback. Some motors can be purchased with them already built in or you can add a speed sensor to an existing motor.
Rather than trying to guess what current the motor needs for a specific speed, the sensor gives the MCU feedback that allows it to ramp up the current until the desired speed is achieved and maintains the desired speed using the speed sensor as an indicator if more current is needed.
Sounds like a fun application 😊
2020-08-01 1:24 pm
You could use a gearbox to get better torque at lower speeds at the cost of losing some of your top speed. Maybe playing around with different spool sizes would let you find that sweet spot where they still pull out fast enough but have the torque for the slower part.
Another option would be using two different motors. Have a worm/geared motor for the slow one and your current motor for the finale. (May require two spools/threads?)
You can grab a motor and gearbox out of an old Power Wheels car. They're 12V 550 motors (RC Car style) usually and have plenty of torque as low speed. They can also handle bursts of 24V, especially with that little of a load on them. You can find Power Wheels for free on if you don't need the battery/charger. Go grab one, pull the gearboxes out, salvage anything useful you may find (Lots of good quality 10 or 12 AWG wire in them) and throw out the rest. I find most of mine on Marketplace or if I notice one in someone's yard all sun-faded and looking like it's been there for years, I knock on their door and ask if they want to get rid of it. Almost always free. =)
Side note, those gearboxes aren't the quietest things in the world, so have a plan to insulate them in something that'll muffle the noise.