Motion Detecting Night Light with Low Battery Indicator

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   The operations in this project are done without a microcontroller. Transistor and Op-Amp are used for control and comparison. 

Components List:  

  1. 9V Battery
  2. Switches
  3. 5.1V Zener diode
  4. Resistors
  5. Op-Amp (u741)
  6. LEDs
  7. LDR (Light dependent resistor)
  8. NPN Transistor (2N3904)
  9. PIR Sensor
Motion detection: 

Infrared radiation is emitted by humans as heat. PIR can sense motion by detecting this infrared radiation. Now to detect motion only during night time, the PIR sensor is activated only if it is dark around. The sensor has 3 pins: Vcc, Gnd and output. From the below circuit diagram see that the Vcc pin is connected in parallel to the transistor to make sure that the PIR receives voltage only if the transistor is OFF.

The transistor is controlled using LDR which responds to change in light by varying its resistance. A voltage divider network is formed using LDR and a resistor. When there is no light, no current flows through LDR (LDR resistance will be in Mohms), hence no voltage is dropped across the resistor which is parallel to the transistor base. So the transistor is OFF (i.e. cut-off region, Vce = Vcc) and Collector-Emitter is open circuited. Connected in parallel to Vce, PIR receives voltage and can sense motion up to 7m.

If there is light, large amount of current follows through the divider network (where LDR resistance will be a negligible value) and the voltage drop across resistor connected in ||el to transistor base will be almost Vcc, turning transistor fully ON (i.e. saturation region, Vce = 0V). As Vce is grounded, PIR receives no voltage in turn motion detection will not be active.

Once motion is detected PIR outputs 3.3V which is connected to series of LEDs for indication via a transistor for current amplification because current output from PIR sensor will be in uA (micro Amp).

Low Battery Indicator:

Zener diode in reverse bias can maintain a constant voltage when provided with voltage greater than its Zener breakdown. A voltage divider is formed with Zener diode having a suitable voltage rating and a resistor to provide a constant reference voltage to one input of Op-Amp (-ve input).

The battery voltage is given to the other input of Op-Amp (+ve input) in a voltage divider network, providing a ratio of it. This battery voltage is compared with a constant reference voltage using an Op-Amp comparator (Op-Amp without feedback form a comparator or else specific comparator IC's can be used). 

The Op-Amp output's HIGH (Vcc) until ratio of battery voltage is greater than reference voltage, once the battery voltage drops below reference (lower threshold) Op-Amp output will be 0V, turning ON the indicating LED.  

Circuit Control:

Switch S1 controls the whole circuit and Switch S2 is used to turn On/Off the battery indicator circuit.

Note: 9V battery has lesser Ah capacity, instead use Li-ion / AAA / AA batteries based on current requirement.

Circuit diagram:







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