Servo actuators, sensors and external power.

Hi, please keep in mind I'm a total newbie in the electronic world before answering. I tried too read as much as I could before posting this and I'll try to make things as clear as possible. Right now, I'm not even thinking about code. I'm much more less comfortable with the hardware and I don't want to fry or damage my Arduino.

I'm planning on using an arduino to help me automate two things in my mini-greenhouse. FYI, the box is about 36" x 32" x 18" made of wood and a cover of plexiglas.

The temperatures are rising very fast when it's full sun so, I need to open the top cover. I was thinking of using an high-torque/geared servo and a rod to make an actuator or maybe a crank-window system. Would the servo be strong enough to lift it? In what range of torque should I be looking for ? Would it be strong enough to hold it closed when it's windy ? If not, what would be your suggestion ? I can't really afford those 100$ actuators like Firgelli even if it would probably the best solution. I understood that a servo that had pressure applied would draw more current. Possibly too much for the arduino so I would need an additional power source and activate everything with a relay, right?

Next step would be to had two sensors: Temp/humid and luminosity. Would it be possible to have all of these (3 total) powered through that external 5V source and have the arduino powered on his side from USB or 9v? I then would have to sum up those mA to know the 5VDC warth to buy.

I understood that a servo that had pressure applied would draw more current. Possibly too much for the arduino so I would need an additional power source and activate everything with a relay, right?

A couple of basics, you usually don't power servos from an arduino, and relays are usually not used with servos. To take the load off of the servo, counter balance the lid such that the servo does not have to hold any load, just move the lid.

If a servo says it has "1Kg/cm" of torque that means it can lift 1 kilogram at 1 cm distance from the center of rotation... or .5kg at 2 cm from the center of rotation, etc. Calculate your needs and purchase accordingly.

A servo has separate +5V and GND connectors in its three-wire connection; only the signal wire "pulls power" from the arduino and that amount is miniscule. You don't need a relay -- just a 5V power source. When you buy your servo you might want to look into buying a "BEC" for that 5V.

Temperature/light sensors use very little power and can pull that from the Arduino and USB (500ma) powering those would be fine.

If using multiple power sources make sure you connect the grounds together.

Thanks to you both.

If I understand, as long as the grounds are together, I shall no fear the servo trying to draw power from the arduino through the signal wire. I must only consider getting a wall-wart/batteries providing enough current.

@zoomkat

To take the load off of the servo, counter balance the lid such that the servo does not have to hold any load, just move the lid

Yeah, while writing it, I felt that it would definitively be best to not hold the load. I'm still figuring out how I could do this.

@Chagrin By BEC, you mean something like this ? http://www.hobby-lobby.com/castle_10a_bec_7145_prd1.htm I don't really understand what I would get out of this, more torque ? This one says 10A peak, I suppose I'd have to get the wart to match this?

cheesypic: @Chagrin By BEC, you mean something like this ? http://www.hobby-lobby.com/castle_10a_bec_7145_prd1.htm I don't really understand what I would get out of this, more torque ? This one says 10A peak, I suppose I'd have to get the wart to match this?

That particular one with 10A capability is more expensive than I expected, but basically a "BEC" just converts higher voltages (max 25V in this case) to 5V. Since you were looking for a power supply I thought it might uncomplicate your setup given that you were already shopping for a servo.

zoomkat:

I understood that a servo that had pressure applied would draw more current. Possibly too much for the arduino so I would need an additional power source and activate everything with a relay, right?

A couple of basics, you usually don't power servos from an arduino, and relays are usually not used with servos. To take the load off of the servo, counter balance the lid such that the servo does not have to hold any load, just move the lid.

That's the way to do it. Assuming the lid is hinged somehow, just attach a counterweight so that the lid+counterweight are roughly balanced around the hinge. When designing your servo/crank mechanism, aim for about 180 degrees of rotation on the servo and orient the servo arm so that it is inline with your pushrod/pullrod in the fully open and fully closed positions. That lets it hold the lid positively open or closed without requiring any effort from the servo, and also means it has maximum leverage to start and stop the movement of the lid. From the dimensions you're describing I'm not envisaging this lid being particularly heavy so I would have thought an ordinary RC model servo might well be up to the job. That's good, because they're cheap and easy to control using an Arduino.

That's the way to do it. Assuming the lid is hinged somehow, just attach a counterweight so that the lid+counterweight are roughly balanced around the hinge.

Here is a plan of what has been built. It can give a pretty good idea of the box. I don't need the lid to be as open as the picture shows. Adding a counterweight to the lid is a good idea to reduce the load. https://picasaweb.google.com/lh/photo/ktMrhWY_hSYqlTZyYuf41IP-wyk9SJexYxmJ0wxqBOw?feat=directlink

I would have thought an ordinary RC model servo might well be up to the job.

I was looking at the Parallax 900-00008 6VDC 47oz/in. Cheap enough for me, 360 degree, 140mA @ 6VDC no load. It seems slow at 50RPM, but I'm in no rush when I want close/open the lid. Seems ok to you ?

basically a "BEC" just converts higher voltages (max 25V in this case) to 5V

So that thing would allow me to use the 12VDC source I use for fans to power up the 6VDC servo at optimal voltage. Considering the 140mA draw of the servo, there's still room left. I wouldn't need the fans and the servo running at the same time anyway.

Thanks to those who took the time to answer this thread. For a newbie, it can really get things going.

I was looking at the Parallax 900-00008 6VDC 47oz/in. Cheap enough for me, 360 degree, 140mA @ 6VDC no load. It seems slow at 50RPM, but I'm in no rush when I want close/open the lid. Seems ok to you ?

You don't want a "360 degree" servo for your application. You need a large standard servo like below. Also the power converters are often refered to as UBEC.

http://www.hobbypartz.com/33p-solarservo-a105.html

http://www.ebay.com/sch/i.html?LH_BIN=1&_sacat=0&_nkw=ubec&_sop=15

That Solar servo seems like it could handle the job for sure. Can't find it's stall current though. Planning to get the AEO 6V 3A UBEC [http://www.hobbypartz.com/88e-6v-3a-ubec.html] from the same as the BEC, should be enough ?

So far, I draw a diagram for the servo (with BEC) and an RHT-22 sensor. But I'd like two add two things to that setup and I'm quite lost. First, I'll eventually have another sensor, probably 439-ADA TSL2561 Digital Luminosity sensor. This particular sensor is powered with 3v3 and the wiring seems particularly easy. Is it safe to have these two up when USB powered ? How would I wire up if both were 5v sensors ? Can I use a Digital pin as 5V output ?

And then I'd like to add a relay-controlled 12v fan to this. I'm quite lost to wire this. The fritzing relay example speaks for itself but adding these components to the ones confuses me out. I would now find myself in need for 3x 5v outputs or 2x 5v and 1x 3v3. I would think I should use 5v for the relay and digital outputs for the sensors.

This is probably horribly wrong but here's my diagram.

Thanks for your help.