Problem with MK160, unreliable function

When I tested the kit last spring it seemed to work as intended. However, when the winter arrived and I started using the kit regularly, the kit does not work properly. (I use the MK160 to fire up the diesel heater in my car by calling to a cellphone.)

The problem is that the MK160 activates the relay on a regular/irregular basis without me calling the phone. Sometimes the activation is completely without reason, other times this happens when I start the car. The latter case mekes me suspect that the problem is somewhat worsened by dips in supply voltage.

At first, I thought it was something in the kit that did not cope with cold temperature. And after reading on this forum, I found out that the PIC12C508A is not dimensioned for cold temperatures, which made me think this was the problem. Velleman kindly helped me programme a PIC with colder temp rating, but the problem was not solved, unfortunately.

After that, I read some more in the forum, which lead me thinking the other IC-circuit was the problem. I then purchased a TL071 industrial grade IC, but this actually worsened the problem!:

With the TL071 the MK160 output is always on, even in complete darkness and minimum sensitivity setting! The LED-diode glows with no light around. Strangely, the LED goes out when there is enough light around (i.e. point a flashlight at it or similarly).

Irregardless of using the TL071 or LM741, the problem seems to be that the kit turns output signal to ON too easily.

Does anyone know what the problem might be?


Did you try adding the resistor?

Are you sure that radiation from the cell phone is not at the cause of troubles?

I read the post, but I got the impression from the advice in the post that changing the IC to a TL071 would do the job just as good as adding the resistor. That is not the case?

I am not sure, will switch the phone off to see if that helps. But it is a Siemens ME45, if that information is useful. If it is that phone, is there a solution to the problem? External antennae?