Hi there,

I am new to this forum and am hoping some one can help me out. I am trying to build a circuit with my son that will activate a flexinol cable every 5/10 sec. If you have not heard of Flexinol have a look at their website (I don't want to post the address in case it is against forum rules). It is a shape memory alloy, basically you put a current through and it physically shrinks, once current is removed it goes back to it's original form.

My son found an old cheap dinosaur of his that he, off the top of his head, decided he wanted to bring to life with electronics. I really want to encourage this stuff, so we did some research and he found this memory wire, it would work perfectly, we can attach it on the inside of the neck of the dinosaur (it is sort of hollow) and apply a current and away it would go.

Before I get this thing though we want to create a circuit that will have this thing turn on (flex) for a few seconds and then off (relax) for a few seconds. I know a 555 timer would do this, but my son wants to run it off batteries, and it is my understanding that 555 timers are constantly 'on' draining the power. I remember using a capacitor and a couple of transistors as a timer once, but can not find any circuits online. I can build these things but am lost when it comes to designing anything. Is that the best way to go? the circuit ideally would be small form factor as there is not a great deal of room, and as I said running off batteries.

The flexinol technical specs say that their 0.002" dia cable (just randomly chose that size) has a resistance of 12.7Ohms per inch, we would use about an inch to make life easier.

Can anyone help design something simple? Or point me in the right direction please.

Many thanks in advance guys.

 

Welcome to AAC Carter_S!

Let us define your requirements a little more.

1) You want to apply power the Flexinol wire for x seconds then remove power for x seconds, then repeat for as long as the circuit is turned on, correct?

2) I assume x seconds is 5 to 10 seconds, not five-tenths or half a second, correct?

3) What voltage does the wire you plan to use operate at? Can you provide a link to a datasheet or include an attachment of the datasheet?

If 1) above is correct, the 555 should work well for your application. You'll probably need to use a transistor or MOSFET to work between the 555 and the wire depending on the voltage and current requirements.

You want the 555 configured in astable mode. Check out this link for more detailed information: http://www.kpsec.freeuk.com/555timer.htm.

When you say the 555 is constantly on, do you feel its current draw while on is too high to get appreciable running time from the batteries or do you think the 555 will consume power when the circuit is turned off? The latter is easily addressed with a SPST toggle switch or similar placed between the positive battery terminal and Vcc going to the 555.

 

Good for you for indulging your son's scientific interest. I'm sure that someone here will help, and maybe a little more information will spur some interest.

Flexinol is a power hog; what do you plan to use for power? If for example, you were to use an ordinary 9 volt battery (just a guess based on the description of your project), the one inch of Flexinol would draw about 750 ma from the battery while it is contracted, which would mean that a battery with a 500 mah rating would be depleted in a very short time.

 

Thanks for helping me with this.

elec_mech

1)Yes that is correct on/off every 5-10 seconds as long as unit is on.
2)Yes 5 to 10 seconds, any time around then really
This is the tech sheet for the flexinol. http://www.dynalloy.com/pdfs/TCF1140.pdf

tracecom
I did not understand that flexinol would be a power hog. That is a real shame as I liked the idea of the silent movement as well as the compact size. That was my plan,to use a battery, I had actually hoped to use small batteries so we could have everything hidden....guess you can't get everything you want....so all I need you guys to do is come up with a power unit the size of a button that could power a house....how's nuclear fusion comming?

I am open to suggestions, we did have our hearts set on using the flexinol, but if you guys think it impractical then I guess we can look at other ideas.

 

tracecom
I did not understand that flexinol would be a power hog. That is a real shame as I liked the idea of the silent movement as well as the compact size. That was my plan,to use a battery, I had actually hoped to use small batteries so we could have everything hidden....guess you can't get everything you want....

Carter_S,

First of all, I should have said that I have never used Flexinol and therefore don't have any first hand knowledge. My statement about it being a power hog was based on my use of of nichrome wire, which is similar in some ways. It was also based on your statement that the resistance of the .002" wire was 12.7Ω per inch.

When I look at the datasheet you provided, I see that the "Approximate** Current for 1 Second Contraction (mA)" of the .002" wire is 85 ma, which is much less than I had calculated using Ohm's law. (I don't see in the table what the length of the wire is.) So, obviously there is something I don't understand. Hopefully, someone will come along that knows what he/she is talking about.

I did note from the datasheet that the contraction of the wire is often only 4%, which means that without some mechanical advantage (lever, etc.) there won't be much movement. YouTube has some interesting videos showing Flexinol in operation.

Don't give up; it's a fun idea.

 

Hmm, I don't think it's not doable, but tracecom is right that batteries may not last very long.

If you stick with the 0.002" diameter, the datasheet says it takes 85mA for one second contraction. I won't pretend to know what that means, but some quick reading says you apply power to the wire to heat it up which causes it to contract. Removing power allows it to cool and expand. If you were to apply 85mA continuously, you would consume 85mA per hour or 85mAh.

Now, you're planning a 50% duty cycle meaning you currently plan to apply power in cycles with half the cycle powered and the other half off. In other words, 5 seconds on and 5 seconds off. Assuming you were to leave the circuit running for one hour, you'll consume 85mAh/2 = 42.5mAh. You're still providing 85mA to the wire, but not continuously for one hour - if you add up the on and off cycle times over an hour, you'll have it powered for half an hour at 85mA and off for the rest of the hour. This results in 85mA consumed for half an hour or 42.5mA over the course of a full hour. This doesn't include the power used by the 555, but we'll assume that is negligible for now.

So, let's say you use an alkaline 9V battery. If memory serves, they contain about 565mAh of current. Assuming you were to leave the circuit running until the battery died, you could run it for 565mAh/42.5mA = 13.2 hours. This is a WAG at best and not factoring in that alkalines don't like high discharges and die faster as a result (this is why alkaline batteries don't last long in digital cameras).

Now, let's say you go instead with four AA batteries. Alkalines are rated about 2500mAh, so you'd get 2500mAh/42.5mA = 59 hours.

Again, this is my poor understanding of power consumption and high discharges tend to reduce battery life/run time considerably. Hopefully someone will correct me if I'm way off base. However, these probably aren't too unrealistic if you're only going to power the circuit for a few minutes at a time anyways.

Lithium batteries would last longer if you're okay paying the higher prices as they withstand high discharges a lot better.

I found this site which has a 555 circuit used to power a Flexinol muscle wire:
http://www.alivingarchitecture.com/tag/muscle-wire-2/

If you opt to use 6V or 9V (double-check the 555 datasheet, but I think it can accept either), then you'll need to change R4. You'll probably have to change R4 anyway as you're not planning to use 6" of wire and I'm not sure what diameter they're using either.

I'd recommend giving the wire and circuit a shot using a wallwart as your power supply to test. Once you've got a working circuit, you can use a meter to determine the current consumption over a period of a minute or so. From there you can decide if batteries are a viable power source and which ones to use (9V vs AAA or AA, alkaline vs Ni-Cd/Ni-Mh vs Lithium).

 

Wow great stuff guys thanks.

The power stuff was where I was at a total loss, I think I understand what you guys are saying here, if we went for the 0.001" wire instead of 0.002" the current would drop down to 45mA which would mean a battery would last longer, up to 25 hours (using a 9V etc) correct?

Love the site you found thanks.

So the consensus is that a 555 timer is the way to go and not a capacitor and transistors. From what it sounds like you guys don't think that the current draw of the 555 is important compared to the flexinol right?

I think you are right and is worth an experiment, it is probably a good way to get my son (and me) to understand the process of designing something like this. He says thanks by the way.

If you guys think of any more info I am all ears, and will be checking in regularly as this project goes along.

 

For one inch, only 1.08 Vs are needed, say a Ni-mh rechargable battery, 1.25V, with a second low current 5 to 6V battery for the C555 driving a logic FET. A 2000 mAhr battery would run for about40 hr @50%. Power switch would be DPST.

.

 

I would like to note the short contraction(?) distance of each strand of wire.

How will you implement mechanically the wire and which body part will you move?

 

Also, the time for the wire to contract is less than the time to cool (expand). Also, if you read the application notes, to prevent the wire from permanently contracting, you need an opposing force (spring return).

May I suggest ordering a piece of wire, or one of the Flexinol kits to experiment? I have seen working examples of what you can accomplish - so it is not impossible!