Light-Activated Switch with MOSFET

MOSFET is a type of unique field-effect transistor. As compared to BJTs, these transistors are voltage-controlled devices because BJTs are current-controlled devices. Generally, MOSFET includes three terminals; gate, source & drain whereas BJT includes base, collector, and emitter. Whenever the voltage is applied to the gate terminal, then an electrical field can be created that simply controls the flow of current throughout the channel in between the two remaining terminals like the source & drain and current doesn’t flow from the gate terminal into the transistor. MOSFETs play a key role in making different circuits like voltage regulators, motor speed controllers, solar trackers, light-activated switches, and many more. This article discusses how to design a light-activated switch with MOSFET.

Light-activated switch with MOSFET

The main concept of this circuit is to design a simple circuit to turn ON/OFF a load like an LED depending on the light intensity. Here the load in the circuit is controlled through a MOSFET.

Required Components

The required components to make this light-activated switch with MOSFET mainly include; IRFZ44N MOSFET, LDR, 4.5Mohm Resistor, 12V LED strip Load, and 9V battery Power supply.

Light-Activated Switch Connections

The connections of the light-activated switch with MOSFET follow as;

Light-Activated Switch Circuit with MOSFET
Light-Activated Switch Circuit with MOSFET
  • The drain terminal of IRFZ44N MOSFET is connected to the negative terminal of the LED.
  • The 4.5Mohm Resistor negative terminal is connected to the source terminal of MOSFET and the positive terminal is connected to the positive terminal of LED.
  • The LDR positive terminal is connected to the gate terminal of MOSFET and the negative terminal is to the source terminal of MOSFET.
  • The resistor one terminal is connected to the battery’s positive terminal and the gate terminal is given to the negative terminal of the battery or GND.


A nightlight is a very small electrical light fixture, used to give convenience in dark areas or where some areas become dark in some specific times like in an emergency or at night. The MOSFET is used to make a homemade light-activated switch. The lighting fixtures with high efficiency can be adjusted based on many factors such as; availability of daylight or occupancy.

An LDR or photoresistor is a variable resistor that is controlled by light. The function of this resistor is photoconductive which means the resistance will change based on the light intensity. When the light intensity increases, the resistance of LDR will decrease. Based on the photoconductivity, a material conductivity can be enhanced once the incident light intensity is enhanced. Photoconductive materials exhibit such kind of property, so this is used in switching circuits like light & dark-activated and also light-sensitive detector circuits.


A light-dependent resistor is simply made with a high-resistance semiconductor. When light drops on the device have a high sufficient frequency, then photons that are absorbed through the semiconductor provide sufficient energy to bound electrons to move into the conduction band. So the resulting electrons will conduct electricity to decrease resistance. The LDR in this circuit must be connected outside because its sensing surface must be exposed to the ambient region from where the level of light is necessary to be detected.

The photosensitivity of the photoresistor can fluctuate very much based on the surrounding temperature. In addition, these resistors will show some delay normally 10 msec or below or in between light exposure to the subsequent drop within resistance.

In the above circuit, an LDR (light-dependent resistor) is a variable resistor whose resistance value is reduced by increasing the intensity of incident light. Based on the intensity of light, the circuit will turn ON or OFF an LED which means when LDR detects high light intensity then the LED will turn off automatically & at a low light intensity, the LED will turn on. Here the load-like LED can be controlled with the help of a MOSFET. This simple light-activated switch automatically works by turning ON the light at night & turning OFF in the daytime. The light-activated switch is less costly and very effective when used daily to automate your home, garden, etc.

Advantages & Disadvantages

The advantages of the light-activated switch include the following.

  • These circuits do not need manual operation.
  • This circuit consumes less power.
  • This circuit is very simple to design using fewer components.
  • The light-dark resistance ratio of LDR in the circuit is high.

The disadvantages of light-activated switch include the following.

  • While operating this circuit, caution must be taken.
  • The spectral response of LDR in this circuit is narrow.
  • The temperature stability of this circuit is low.
  • The variation of the resistance value of LDR has delay, because if it goes from dark to light or from light to dark then it restricts the LDR usage wherever the signal of light changes quickly.


The applications of light-activated switch include the following.

  • The light-activated switch circuit is used in security applications where there is darkness above the light-dependent resistor then it stops lighting.
  • This circuit is very helpful in turning ON the light whenever the light-dependent resistor is light up. The LDR will have maximum resistance within darkness and within the light, it will have low resistance.
  • These circuits are used in garden lamps to automatically turn ON at night.
  • These circuits are used in drawer alarms that sound once a dark drawer is opened.
  • This circuit is used to turn OFF a particular lamp (or) group of lamps in response to the differing levels of ambient light.
  • This circuit can also be used as an automatic street light controller system.