hey,
I want to replace the rubber dome membrane switches of my alarmclock with something like mosfets.
I need to short a wire to ground briefly. I just cant seem to figure it out. when I use jumper wires on a breadboard I can get it to work, but not with anything else
Mosfets have leakage current. Can it be done with optocouplers ? The Arduino with a resistor to the led of the optocoupler and the phototransistor output of the optocoupler instead of the switch.
If you don't care that the Arduino GND is connected to the clock ground, then a normal transistor can be used as well (with a resistor to the base).
Are you sure that the switch is connected to ground ? When the switches are in a matrix, then a reed relay can be used.
Some reed relais are 5V 10mA and can be driven directly from an Arduino Uno pin.
Whatever you try, we like to see a schematic. You can draw it on a piece of paper and make a photo of it.
Koepel:
Mosfets have leakage current.
All transistors have leakage currents -- including the transistor in an opto isolator. If leakage current is the issue, then the solution would be something like the reed relays Koepel suggested.
BUT, it seems hard to believe that transistor leakage current would be enough to register as a closed circuit. It's more likely that you're getting the polarities wrong, or it's "high side" switching [relative to whatever you connect your Arduino ground to], or the membrane switches are across resistors of different values in a voltage divider arrangement and the different switches are distinguished by various voltage ranges, or some such odd thing.
You need to get in there and do some voltage measurements. And don't assume that one side of the switch is at one of the power rails. See if you can find an actual ground. Then select NPN or PNP if using bipolar, or N-channel or P-channel, if you're using MOSFETs -- depending on if it appears to be high side or low side switching [ask if that makes no sense to you].
And, Opto Isolators are probably a good bet, because the absolute voltage levels aren't as important -- the output transistor just needs to be oriented with the correct polarity [i.e. for an NPN, the Emitter would go to the more negative side, and the Collector to the more positive side -- i.e. across the membrane switch, and measured when the switch is not being pressed -- and, in this case, *do* measure across the switch].
Also, the input current on the Opto needs to be high enough, to turn the transistor on enough, to cause a switching action. BUT, the currents in a device with membrane switches, is usually quite low, so, likely not a biggie.
Timklein:
hey,
I want to replace the rubber dome membrane switches of my alarmclock with something like mosfets.A rubber dome membrane switch is basically a push-button (probably momentary) switch, right?
A mosfet switch system will require a button as well, and that button is going to open or close a circuit that will drive the mosfet (to alter the resistance between the drain and source terminals). But also, a mosfet switch has particular voltage(s) requirements on either drain and source (or both), which makes a mosfet switch different from a basic relay switch.
It sounds like..... all you need is a momentary push-button switch ----- of any kind, that replaces the dome switch. No mosfet required.
Timklein:
I need to short a wire to ground briefly. I just cant seem to figure it out. when I use jumper wires on a breadboard I can get it to work, but not with anything else
No offense, but we must assume you are a noob, and as such nothing you say can be trusted [like "ground" and "jumper wires on a breadboard"] 
So, it would be grand to see a photo of your setup, and a wiring diagram of what you're calling "ground" and "jumper wires on a breadboard". So, we can get closer to determining what that really means.
Southpark:
It sounds like..... all you need is a momentary push-button switch ----- of any kind, that replaces the dome switch. No mosfet required.
Unless, of course, the goal is to programmatically "press" the switches, you know, like with an Arduino.
ReverseEMF:
Unless, of course, the goal is to programmatically "press" the switches, you know, like with an Arduino.
Yeah true! That would then be called .... replacing dome membrane switch with arduino device and relevant hardware as an alternative switching method for the alarm clock (or something).