I'm trying to replace the buttons on a radio control car transmitter. I wanted to isolate the transmitter voltage, 2.5 VDC, from the Arduino UNO board. I was going to use a relay, but the 5 VDC relays I found draw 80 ma. I wasn't sure I could use a solid state relay or a MOSFET where the switching voltage is less that my gate voltage.
I measured the buttons on the transmitter and it appears that they get pulled to ground to activate the control point.
My questions are:
Do I need to be concerned about isolating the 2 voltage sources?
If so any recommendations as to either a mechanical or electronic relay?
Instead of a relay of some sort could I connect the commons of the 2 voltage sources together and connect a digital out to the button and have the output pull it to ground?
What is the power supply for the transmitter - just a battery?
Unless there some sort of non-isolated power supply involved just use an NPN transistor or logic-level MOSFET,
You can switch an unrelated voltage so long as the device specifications aren't exceeded.
Using an Arduino pin you'd have to be careful never to output 5V to the transmitter which might easily be
destroyed if its a 2.5V device. That means using pinMode, not digitalWrite, to control the pin.
There's a whole bunch of 5V relays with low coil currents available:
Perhaps you could also use a N-channel MOSFET to connect the signal to Gnd.
Connect both system Gnds together, Arduino can drive Gate of MOSFET thru a 200 or 220 ohm resistor, connect Source to Gnd, and Drain to the button to be pulled low.