Convert Thermostat to 12v Timer Switch
Convert Thermostat To 12v Timer Switch
Find out how to modify a battery powered thermostat to be used as a 12 Volt DC timer switch
electronics | electric circuit | generalIn 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).
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.
Article Last Modified: 09:40, 22nd Oct 2007
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