I've got a 2N3904 hooked up to a pin on a NodeMCU Board (3.3V logic). I am trying to short the button contacts on a fan remote to trigger the speed and such, however, when I pull the base high (3.3V) or low, nothing happens. The only way I can get the remote to send a signal is if I "manually" short the collector and the emitter. I have attached a schematic below for reference. I'm thinking it's the incorrect resistor value....
closed as unclear what you're asking by Andy aka, Enric Blanco, PeterJ, Voltage Spike, Dmitry Grigoryev Jun 13 '17 at 15:31
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An NPN transistor can only take the switch connection to within 0.2 volts of ground. Substitute a logic level FET with low R DS and it should work for you.
I eventually got it working by connecting both grounds together (NodeMCU and fan controller) and sending the signal to the transistor through a 4k7 resistor.