MK138 - Changing the temperature range?

I want to use a few of these kits to control small still-air incubators for Quail eggs.

At the moment the incubators utilise a 240volt 7watt E12 night bulb running at 100 degrees F. Very basic, but problematic!

Unfortunately when the air around the incubator rises by N degrees so does the air inside the incubator - not good for the eggs!!!

I intend to use the MK138 running a 12volt 7watt bulb to control the temperature inside the incubator.

The thermistor will be fixed inside the incubator at egg level and the thermostat will control the light bulb heat source.

Therefore, can you please advise which thermistor would give me a temperature range that I can control between 99-103 degrees F (37-40 degrees C).

Would it also be possible to take power from the MK138 to run a mini 12 volt fan constantly inside the incubator? This would allow me to convert the small still-air incubator to forced-air incubator. If so, which would be the smallest 12v fan (or similar) that you have available or recommend?

I would appreciate any assistance that you can give me on this subject as it makes for an excellent school/home/hobby project. Many thanks.

Ron (UK)

Hi there

I did some experiment with resistors

R6 120K / R5 120K : temp range 11 to 33 Celsius

R6 120K / R5 98K : temp range 8 to 38 Celsius

R6 120K / R5 56K : temp range 16 to 60 Celsius (swap R5 R6)

R6 120K / R5 22K : temp range 20 to 70 Celsius

Try R6 120K / R5 77K it shout be temp range 12 to 50 Celsius

For the fan you can use a computer fan. You can buy them in a lot of sizes
With a resistor you can control the speed of the fan
Connect it to the 12 vdc connection (+ and - )

The 12v lamp on the NC contact and -12 vdc
COM you connect to +12 vdc

Hi, I found this old postand wonder if anyone can point me in the direction of getting the temp range even tighter around 37.5 degrees?

I want to really be able to finetune the temp, so could do with no more than (say) a 4-5 degree range.




Sorry, but I can’t help you with this as I don’t have that specific knowledge - perhaps a variable resistor/control knob set-up?

Unless some one else can jump in and provide a solution I would try and contact Groetjes Kees - also on this thread.

Also try the Velleman Project Support for basic guidance, although I don’t think they can give specific solutions for ‘safety’ reasons etc. Alternatively start a new thread in one of the other related Forums. Good luck with your project.


To increase the operating range of MK138 so it is suitable for use as an egg incubator thermostat (37.5º) with a hysteresis of approximately ±0.4º. Change the following components:-
Component Value Order Code
R4 4K7 1% 907-247
R5 3K9 5% 906-239
R6 2K7 5% 906-227
R7 10M 5% 906-610
RV1 1K 994-210