Project of the Day – Thermostat Controller for Innovative AC Cooler

Today we have worked on a thermostatic controller which is used to open a solenoid valve when the temperature measured by a sensor exceeds a user set level.

programmable thermostat to control a 12V DC solenoid valve

This controller is for a telecoms company which have servers in a data centre which are cooled by air conditioning (AC). The AC units are located outside an office in an area from which heat cannot escape, and so it builds up eventually causing the AC units to fail.

Four years ago when we first heard from this company, they had set up a Hozelock irrigation system on a timer to spray water mist around the AC units from 8am to 7pm on workdays, but they wanted something thermostatically controlled so that water would only be used to cool the AC units when it was actually necessary.

12 volt dc solenoid valveWe provided them with a complete solution including enclosure, 12VDC solenoid valve, controller, and a waterproof temperature sensor (an LM335 we sealed with epoxy into stainless steel tubing – the method for its manufacture is explained here: Make a Waterproof Temperature Probe). This has worked very well ever since reducing the peak temperature of the data centre by 6 degrees Celcius. However, after 4 years in all weathers the temperature sensor has started to become unreliable, and the company also wanted some modifications to the programming of the controller so that the water mist can be triggered at lower temperatures.

The new controller which was pictured above now uses a commercial waterproof DS18B20 digital temperature sensor and we also substituted a MOSFET for the relay which was on the original unit so that the whole controller is now solid state. This controller should help to keep the AC units cool for a good few years to come.

The use simply uses the button on the controller to set the threshold temperature above which the misting is to be turned on (settable in 1 degree steps from 25 degrees Celcius). After 10 seconds of the temperature being measured over this threshold the misting will start, and it will run until the temperature falls by a couple of degrees.

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