Hello,
is there any chance to switch S1-S4 in the following circuit with a N-Channel Mosfet? (I would like to use IRLB8743). I am afraid this is not possible, because I have to switch GND with a N-Channel Mosfet.
Thanks!
Please tell us more about this mysterious device. For signal level switching MOSFETs could be overkill, a
quad opto-coupler might work in that case. We have no way of knowing if the load is 1 microamp or 50
amps!
Switching +24V (a positive supply) to something is high-side switching, so nFETs are immediately out, as are NPN BJTs
The link to the device is here from S.13 on: https://www.meltem.com/fileadmin/user_upload/5301-13_Meltem_Installationsanleitung_M-WRG_SZ-S-24-EIB-KNX_KW22-2017.pdf (M-WRG-S/Z-24)
I can not find anything to load. But the +24V are only used for a signal, so I don't think that there can be much load.
Yes I guess signal level - maybe connect a multimeter in current mode across where the switch
goes and measure the current.
If its 10mA or less opto-couplers are a neat solution...
To make an N-MOSFET switch ON, you need to supply its Gate with a voltage higher than the voltage present at the Source. About 5-10V higher, depending on the MOSFET. This is VGS, a very important parameter for a MOSFET circuit.
If you have a power supply of 34V or so already in your design, then this is easy. Since you probably don't, it will be easier to go and buy some P-MOSFETS for this task.
Thank you very much! I will try to go on with Opto Coupler! (And post the next question to it 
MorganS:
This is VGS, a very important parameter for a MOSFET circuit.
Vgs is just the voltage between gate and source, its not a device parameter.
The key parameter is the value of Vgs quoted for entries listing the Rds(on). Logic level
devices normally have two entries, one with Vgs=10V and one with Vgs=4.5V, without
the later entry the device is not logic level and not suitable for gate drive below 10V.
Rds(on) is a the on-resistance.
If you see an entry mentioning Vthr, its the threshold voltage - don't be confused by
this it is not useful to you.
I said "circuit" not "device", but I gave MarkT a Karma point anyway because his explanation is probably more useful than mine.