Hi all! I’m working on making a simple sort of pulley thing with flexinol muscle wire, but for right now I’m just testing out what I need to make the wire contract safely. I’ve got a bit of an odd situation; I got it *working*, but I’m unsure *why* the set-up I’ve got works! I’d appreciate some help if anyone’s got it to offer, since I’ll likely be working with muscle wires and the like for some time, and I’d like to know where I’m making mistakes in my designs!

This is the circuit diagram I have to work with, with one notable change – I moved the collector resistor (R_{c}) to between the switch and the flexinol wire:

The Arduino is an Arduino Uno. The Flexinol is 10 cm of 100 micrometer diameter wire (which I calculated to have 15 ohms of resistance and need 180-200 mA of current based on the datasheets I have), and the transistor I’m using is a PN2222A NPN transistor.

I had some trouble working out what the h_{fe} of my transistor would be at the currents I needed it (especially given I couldn’t calculate the voltage across its parts without knowing what current would come out of it), so I decided to just build the thing and test it out, magic smoke or not.

Based on this guide, I worked out I’d need around 27.7 ohms of resistance on the collector to get 180 mA on the wire; I stuck with 25 ohms because I knew I could safely let the current go a bit higher. I stuck 100 ohms on the base resistor because we all get a little stupid and impatient sometimes – that was swiftly taken off.

At 25 ohms on R_{c} I was only getting around 140 mA on the wire. I only got to a range of 180-220 mA when I dropped down to my 4.7 ohm resistor.

Final relevant values for the above diagram:

I_{c}= 180-220 mA

R_{c} = 4.7 ohms

R_{b} = 0 ohms

So, my questions:

Why does this work, and perhaps more importantly, will it continue to?

One particular point of concern – how far above the recommended current for flexinol wire can one safely go? The datasheets I have did not specify a maximum or minimum value, and in fact gave two different values for “recommended current” – 180 mA or 200 mA.

Another concern – could my Arduino be putting out less than 5V? This thing’s been through… *a bit*.

Is there a more reliable way to calculate resistor values for flexinol transistor circuits?

Is it safe to run this amount of current through the wire for an extended period of time?

In addition, any other information on designing with flexinol (or those snazzy fishing-line-based artificial muscles) is heartily welcomed, as are scathing criticisms of this circuit in general.

Thanks so much for your time and help!