In previous articles we have discussed the problems in finding an affordable 12 Volt Programmable Timer Switch to control low-voltage renewable energy powered lighting etc. One of the best solutions is to use a battery-powered programmable thermostat as discussed in our article Use Thermostat as 12V Timer.
The one disadvantage of this approach is that the thermostat unit has to be kept within a fixed temperature range – typically 5 to 35 degrees Celcius – to operate as manually programmed. This necessitates some kind of physical insulation (bubble wrap or similar) for outdoor applications such as Poultry Lighting, or irrigation systems etc. Naturally this adds to the cost and complexity of the finished system, and makes human monitoring of what should be an automatic system essential.
We therefore decided to open up one of the timer thermostats (for sale in our REUK Shop) to see if we could make it easier to use as a dedicated battery powered timer switch for renewable applications.
Converting a Thermostat Timer
Our aim was to permanently disable the internal thermometer in some way to make the thermostat think the ambient air temperature is fixed at room temperature. We could then program the thermostat with target temperatures of 5 degrees for the times when the switch is to be OFF, and 35 degrees when it is to be ON (or visa versa).
As far as the thermostat is concerned, the temperature will always be 25 degrees (even on a freezing February night), and therefore it will turn the switch ON and OFF exactly when programmed to do so.
The front cover of the thermostat can be removed very simply following the instructions in the user manual. We expected to find a temperature sensor of some kind (such as the LM335Z used in our solar water heating pump controller), however instead we just found a simple thermistor*.
* A thermistor is a type of resistor which has a resistance which varies strongly with changing temperature.
Therefore, all we need to do is replace the thermistor with a fixed-value resistor with a resistance corresponding to that of the thermistor at room temperature.
Using a digital multimeter we measured the resistance of the thermistor at a range of temperatures (generated by breathing hot air in its vicinity) and obtained the following results.
| Temp. (Celcius) | Resistance (KOhms) |
| 23.5 | 10.5 |
| 30.5 | 8.2 |
| 36.0 | 6.8 |
It appeared from these results, that the common and easy to obtain 10K resistor (Brown-Black-Orange) would give us a thermostat temperature reading of around 25 degrees Celcius. Therefore we cut the thermistor in half and connected the two contacts across different resistors for testing.
Above is a photograph of an 8.2 KOhm resistor used in place of the thermistor and corresponding to a thermostat temperature of 30.5 degree Celcius. With the 10K resistor 25.5 degrees was displayed on the screen of the thermostat as expected.
All that then remained was to solder the 10K resistor in place of the thermistor (as pictured above) giving us a battery powered timer switch which is completely unaffected by ambient temperature (at least within its specified non-condensing operating temperature range of -10 to 60 degrees Celcius).
Powering Timer Directly from 12VDC Source
This thermostat is powered by 2 AAA batteries. Therefore, it will be necessary to change the batteries from time to time, and quite frequently if the thermostat is to be used in a cold environment (in which batteries will drain more quickly). Click here to read our article Renewable Energy Powered Timer with instructions on putting together a 12V to 3V voltage regulator which can be used to power the thermostat directly from your 12 VDC supply.
Disclaimer
This guide is specific to the Towerstat RSP thermostat – for other thermostats the resistor required will most certainly be different, or this technique may not work at all.
Replacing the thermostat thermistor with a 10K resistor is very easy – it takes literally a couple of dabs of solder and the job’s complete – however, to do so will immediately void any warranty on the thermostat.
We are not liable if you break your thermostat following in our footsteps! If you are interested in purchasing a ready-converted thermostat, contact neil@reuk.co.uk and we’ll do it for you.
2009 Update – ECO ET2 Themostat
We are now using the ECO ET2 programmable room thermostat for our modified timer switches as we no longer have a supplier for the Towerstat RSP, and the ECO ET2 is a better unit. ECO ET2 can be programmed with two on/off cycles per day whereas the Towerstat RSP strangely only had two per day on weekdays but just one on weekends.
Pictured above is the ECO ET2 with its thermistor replaced with a 10K resistor. This makes the thermostat think that the ambient temperature is a permanent 25.5-26.0 degrees Celcius exactly as per the Towerstat RSP described earlier. It is also easier to convert one of these thermostats to be powered from 12V than was the case with the Towerstat RSP – see here: Renewable Energy Powered Timer Relay.
The instruction manual for the ECO ET2 can be downloaded here as a PDF: ECO ET2 Thermostat User Instructions.
Contact neil@reuk.co.uk if you are interested in purchasing one of these thermostats converted or otherwise.
Converting a Mains Powered Timer
Click here to read our new article Convert Digital Mains Timer to Low Voltage to find out how to make an alternative low voltage digital programmable timer with more on/off cycles per day and the ability to set on/off times accurate to 1 minute resolution.