I'm trying to write an electrical engineering paper about different I/O on mobile phones. The inputs, such as the camera, microphone, accelerometer, GPS, can be considered sensors. What's the equivalent word for the outputs, such as display, speakers, vibration, etc? I thought it might be haptic, but haptics are only for outputs involving touch.
The commonly accepted term for this is "actuator," although technically that only applies to outputs which generate motion (for example, an LED is not an actuator).
A more general term for both inputs and outputs is "transducer," which is a device that converts one form of energy into another. For example: An LED converts electrical energy to light, a speaker converts electrical energy to acoustic waves, and photodiode converts light to electricity.
Transducer is also often used to mean "sensor," even when the sensor isn't really doing an energy conversion: A common pressure sensor is really a bridge resistor network, so electrical energy from the power supply is being converted to electrical energy to the amplifier/AD converter/whatever. The pressure of whatever gas is acting on the sensor isn't really being converted to another form of energy.
An output is the opposite of a sensor, as you say. And haptic does refer to touch. When I write about this kind of stuff I tend to say: visual output, or audio output. The word output should cover the general sense, if you want to get more specific I suggest simply naming the output method you are using e.g. 20x4 LCD Display
Signal. Speaking generally, 'signal' is the most appropriate word. It is not the opposite of sensor, but rather the semantic counterpart. A sensor senses a signal.
The most grammatically correct answer to the question a may be 'signaler,' or in certain cases 'signaling device,' but these terms are clunky and I think signal/sensor is best -- if only for superficial linguistic reasons such as the shared 's/n' and bi-syllable count.
Transducer may be the most technically appropriate antonym, but it fails to capture the essence of sensor usage in the world. When an accelerometer captures the movement of a person, it would be inappropriate to call the person a transducer, just as it would be inappropriate to call the world a transducer when sensing temperature, humidity or global position. Whereas it would be fair to call the light emitted from a diode a signal just as it would be fair to call the sound emitted from a speaker.
Output is certainly the antonym of input and I believe signal is the best antonym of sensor, even if the most appropriate term is 'signaler' or 'signaling entity.'