rotary actuators


I want to regulate the temperature in a cure rooom to 21*c to 25*c. The room is essentially a walk-in freezer with no controls. The room need to be at 95% Relative humidity by the use of misting heads. The water temperature going thru these heads is what keeps the room at the right temperature.

I found a product to do this but is pricey at $2000.
It is essentially a mixing valve tied to an actuator that turns more or less hot/cold water on based on the needed temperature and a feedback temperature.
Here is an example:

I assume this would work by measuring the room temperature and if it falls below 21* then the picaxe would roate the valve/actuator via a motor so more hot water gets in.

I have not yet worked with servo motors but would this be the type of motor for an rotary actuator? At first this seems like a simple task for the picaxe. Any thoughs on how this program would work?


Senior Member
A servo or a stepper motor. It sounds like a somewhat complex problem if you need to maintain a temperature AND a certain humidity, and all you can add is more water. What do you do if the temperature falls but you're already at 95% humidity??

You can certainly easily get humidity and temperature sensors to interface with a picaxe. You might need a somewhat bulky motor to control an actual valve though. Good luck, it sounds like it might be doable, but maybe others here can help you more.


Senior Member
It would be easier to control the temp by using a common mains powered room heater. Picaxe would turn it on/off as needed.

Humidity could be controlled by having small fan blowing. You drop droplets of water into the path of the airflow when you want to increase humidity. You could use a simple solenoid controlled water valve for this which you can get from any irrigation /garden supplier. Maybe have a irrigation mister head on the nd of the line in the fans air stream. Or you can have some sort of cloth mat in the airstream and drop water into that which will then evaporate into the air. Either way you can control the water flow with the solenoid valve.


Senior Member
If the room has been at 25 for a while and the hot water line has gone cold,
and the temp is starting to drop.
Do you need to divert the cold water till the hot starts to flow?
Do you need to conserve this cold water ie. pump it back to the boiler?
Or is there some residual flow all the time.
95% is fairly damp, will the sensor have much resolution at this level and what it the reaction time of the hygrometer? I would imagine that there is going to be a fair amount of hysterisis in the setup.
Might need some of that fuzzy logic?

Grant Fleming

Senior Member
I designed and built a sucessful PICAXE system keeping 5 water tanks at different constant temperatures. I used hot/cold water mixers you mention but used 9 volt motors driving threaded rods that had the mixer handles attached with a central nut. To achieve correct temperature (monitored by Dallas digital temperature sensors) the motors were driven so many seconds this way or that, worked really well.
I tried a servo first as a prototype but of course conditions change in the loop and the position called for with a servo no longer would correct the temperature.
Let me know if you require any more info.

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Assuming I've read correctly (unlikely), and that you are limited to hot sprayed water for heating/ humidy then a ready-made watervalve solution may be to use central-heating type zone valves or water tank valves. (The sort that plumbers use).
Upside: no faffing, properly made, designed for long use in sealed c/h sytems*.
Downside: Mains only (I think), so a relay/triac would be needed (big deal).

But if you want to join servos and cog wheels etc. to a tap that's up to you.

Sensing - piece of cake with PICAXE and forget the fuzz.

PS. 2 grand for that thing... pheweee!

* Note: these are for sealed c/h systems which recirculate the water with corrosion/limescale inhibitor. I would guess that prolonged use with 'fresh' hot water may suffer from liming up unless softened - just my little caveat to get me off any hooks.


New Member
similar project

I had environmental storage rooms that had to be 37 C - 40 and 60 - 70 - 80% RH. We added a small mister that emptied into a drain - sprayed a wall with copper sheet (inhibits bacterial growth) into and all copper drain. For heat we used base-board electric heaters (three - one above another on one wall) with a small staging control I built from op-amps for proportioning control - 555 astable using the timing cap to modulate the reference into the temperature sensing op amps.

To remove XS humidity we used a large prepackaged water chiller (overgrown water fountain chiller) and circulated water through spiral copper tubes a fan blew through (all fabricated by an AC contractor to our specification). Condensate removed via a copper drain system.

Stainless "Metro" wire shelving held the product, and two large ceiling mounted fans (not "ceiling fans" but circulating fans supported on thick reinforced rubber straps to decouple vibration from the ceiling). Fans were coupled to a light switch so would run only when the lights were off so we didn't loose the humidity when someone was in the room.

Humidity sensors were (initially) chilled mirrors that measured light scattering at the dew point and outputted a linear 0-5 control signal that went into another proportioning controller. We later switched to on-off control with horse hair humidistats because the high tech sensors would require careful periodic cleaning.

Still getting to the valves - the chilled water valves were far too stiff for any RC servo. We had more or less standard 3/4" bronze ball valves with SS balls. There are actuators made specifically for that type of valve - only turn through 90 degrees (the valves had no stops - or the stops could be removed). They open and close with 24 VAC control with a small shaded pole gear motor and close via a spring - I don't know how they keep the motor from burning up when it reaches the end of its travel but it stayed open (or closed) as long as power was applied - to give a fail safe in the event of power failure.

Anyhow check to see what is already out there - no point in reinventing the wheel if you don't need too. Our valves were costing us something like 100 USD each valve and actuator. Bulletproof industrial design.

Our other choice was to just incorporate separate circulating pumps for each chilled water line, but it was felt the maintenance would be higher for that system and we'd still need a lot of valving so as not to bring down all the rooms when one pump needed service.