Transistor as Light Activated Switch
Because an LDR is rugged in nature, it can easily be used in rough external environments.
LDRs are made of a high resistance semiconductor material like Cadmium Sulphide that exhibits photoconductivity.
In day light the resistance of the LDR decreases. The current received by the transistor is now too low to turn it on. This prevents the flow of current through the light bulb, keeping it dark.
The transistor is "off" and functioning like an open switch.
Test it out
Tap the diagram for more information and click on the buttons to change a component and see what happens!
 Make it Day Make it Night Remove LDR
Example calculations
In order to work with Light Activated Switches, electrical engineers need to understand and work with a variety of calculations. Here are just a few:
Current through the load at saturation:
$I_C = \frac{V_P}{R_C}$
$R_C = \frac{V_{cc} - V_{(BULB)} - V_{CE(sat)}}{I_{C(MAX)}}$
The Collector Saturation Level:
$I_{C(sat)} = \frac{V_{CC}}{R_C}$
 Collector $C$ Base $B$ Emitter $E$ Collector Current $I_C$ Base Current $I_B$ Emitter Current $I_E$ Light Dependent Resistor $LDR$ Positive Voltage $V_P$ Load / Collector Resistor $R_C$
You probably know that street lamps, outdoor garden lights and car headlights that automatically turn on when it gets darker are activated by a change in daylight, but have you ever thought about how that actually works? As an electrical engineer, using a bipolar junction transistor (BJT) together with a Light Dependent Resistor (LDR) can be a useful way of making a switch in your circuit.
Our first step in understanding the use of a transistor as a switch, is to understand that when zero signal, or current, is applied to the base, or gate, of the transistor, no current flows is allowed to flow through the transistor itself. The transistor is "cutoff" is not conducting any collector current. So, it turns "off", acting like an an open switch.
When a positive signal is applied to the base, the transistor is then "saturated" and is conducting maximum collector current. Here, the transistor is "on", and acts like a closed switch, allowing the current to flow through to the output device we want to work.
While we can also create transistor switches that are controlled and activated by heat or sound passing signal to the base, for our demo we'll focus on light. Ultimately however, they all function in a similar way.
In this type of circuit, we are simply adding an LDR to help control when and how the current flows through the base, activating the transistor, and turning the output device on. Luckily, the basic idea behind an LDR is that it becomes more or less resistant depending on the intensity of the light that falls on it. When the light is more intense, it's resistance becomes low, while in darkness it's resistance goes up.
In this way, we can use it's light sensitivity to turn not only street lights and car headlights on and off, but also appliances such as fans, and air conditioners.
Practice Questions
Test your new knowledge on light activated switches by answering these questions.
1. An LDR is:
2. In the above scenario when is a Transistor "off"?
3. If the collector voltage is 5V, V(Bulb) = 1.9V, VCE(sat) = .1V, and the current through the collector is 15 mA then the resistance of the light bulb is:
Formula:
$R_C = \frac{V_{cc} - V_{(BULB)} - V_{CE(sat)}}{I_{C(MAX)}}$