Sensor: The LDR
The Light Dependent Resistor (LDR), also called a
photoresists or photocell is simply a resistor whose
resistance value decreases based on the amount of
light which strikes its surface.
But in our case at least one of the values will vary;
which is exactly what we want.
This should leave us with a voltage varying based on
the amount of light reaching the LDR.
LDRs are in phones, TVs, cars, cameras, light fixtures
— anything that needs to react to the amount of
ambient light.
For our experiment we are going to use a 10K Ohm
fixed resistor, and, of course, 5V as the voltage source.
In order to use them with our Nano, we need to use that
varying resistance in order to vary the voltage on an
analog pin so that we can read a value. Once we have
that value we can associate it with an amount of light.
Although every LDR will react slightly differently, these
will generally vary from as little as 200 Ohm in full
sunlight to over 1,000,000 Ohms in near total darkness.
If we wire the LDR as R1, then at full brightness we get:
The easiest way to achieve that is to create a voltage
divider with the LDR and another fixed value resistor.
We briefly mentioned voltage dividers when we talked
about the poteniometer, and this is no different, except
we are replacing the knob with the LDR. Let’s look at
the voltage divider in more detail.
10000 / ( 10000 + 200 ) * 5 = 4.9v
A voltage divider consists simply of two resistors
connected in series between a potential and ground,
with a third connection at their junction point. The
voltage as measured from this junction point to ground
is the divided voltage, and will be reduced by a specific
amount based on the values of the resistors.
wikipedia.org
To determine how much the voltage will be reduced you
can use the simple algebraic equation:
Vout = R2 / ( R1 + R2 ) * Vin
When R1 == R2, then it can be simplified to:
Vout = Vin / 2
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and a full dark:
10000 / ( 10000 + 1000000 ) * 5 = .05v
Which is a pretty good range of 4.85v . Since
analogRead() returns 1024 values over 5v, 1 per .005v,
we should have around 970 distinct values.
Experiment: Feeling Lux-y?
Components
Wiring Diagram
✓Nano
✓10K Ohm Resistor
✓LDR
Connection Instructions
Connect the 10K Ohm resistor from Ground to an open column on the breadboard, to that same column
connect a jumper to pin A2. Also on that column place one leg of the LDR. Connect the other leg of the LDR
to 5V, either directly or or by using another open column on the breadboard.
Sketch(es)
LDR.ino
Analysis Questions
Programming Tasks
The provided program will dump the raw values read from the LDR to the Serial monitor. Experiment with
covering the LDR and exposing it to a bright light source ( flashlight or similar ), and observe the range of
values.
(optional — programmatically track the highest and lowest values, and continually output them)
Use that range and the map() function to also output number ranging from 0-255 based on the LDR’s raw
value. Try to use as much of the 0-255 as possible.
Now combine this circuit and software with either the Arrays and LEDs or Sound Detection program, so that
the overall brightness of the LED strip dims in low light and gets brighter as the ambient light does.
© 2016 Dataseam