Page 109 - Programming the Photon Getting Started With the Internet of Things

map() returns a value on the scale of outFrom to outTo.

You can easily modify the map() function by converting a value into a percentage from

0 to 100—just change the output values.

Variable Resistors

Most analog sensors work like a potentiometer does, calculating the varying voltage as the

resistor changes its value. These are called variable resistors that resist the flow electricity
through a circuit. A good example of this is a simple light-dependant resistor or photocell
like in Figure 5.7. This changes the resistance based on the amount of light that hits it.
When the light increases, the resistance goes down; therefore, the voltage in the circuit

goes up. Take the light away, and the resistance increases and the voltage is reduced.

Figure 5.7 A typical photocell, which acts as a variable resistor.

In order to read sensors like these with the Photon, you will need to create a voltage
divider circuit and connect it to the analog input pins on the Photon board.

Voltage Divider Circuits

When you are working with different sensors that offer a variable resistor feature, you
need to create something called a voltage divider circuit. A voltage divider circuit converts
the variable resistance into a variable voltage so we can read this value from the input pins

on the Photon. In the schematic diagram in Figure 5.8 you can see a simple voltage divider
circuit.