Simple Light-Driven LED Circuit

Below is a simple circuit that demonstrates the use of a transistor. The transistor drives an LED from a photocell at the input bias circuit. The intensity of the LED light depends upon the intensity of the light from the environment (ambient light) hitting the photocell. The greater the ambient light, the greater the LED intensity. If light is totally blocked on the photocell, very little or almost no glow coming from the LED.

Figure 1

This simple circuit can be expounded for many exciting applications in the real world. The photocell is just one of representations of sensors one can use to control the output of the transistor. The output can be other than an LED, say a motor whose speed can be variably controlled, or it can be an input to another circuit that does some other functions that depends on the input from the sensor.

Basic Operation:

The photocell has resistance that ranges from 5k (strong light) to 20k (dark). When light hits the photocell, more current flows to the bias resistor 10k, thus the larger base voltage of the transistor. That higher base voltage translates to higher collector and emitter current. The choice of resistors are such that enough current will flow at the output to make the LED glow. One can approximate the output current when the transistor is conducting to I_C = I_E = (V_BASE-0.7)/1K. LEDs typically can handle current up to 20mA. When the photocell is blocked, it has maximum resistance (20K), and the base voltage will be just a third of the supply. The resulting current at the output will produce very little glow from the LED. You may choose other values of resistors to suit your desired circuit response.

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