Fascinating electronics hobby circuit, Tricky Security Light, presented here is nothing but a motion sensing security light built around one commonly-available passive infrared sensor (PIR) module. A motion-sensing light is one that is activated by a motion sensor rather than by a conventional light switch. Motion-sensing lights are great for lighting areas that you only use briefly, since they only stay on as long as you need. By automatically turning off after a few minutes, they prevent wasted electricity and help conserve energy resources. Besides, motion-sensing lights for your home are great deterrents from burglars, who are startled by the sudden illumination and may believe someone awakes inside the home.
Alas, I cannot offer such great ‘green’ benefits with my design because this is only a funny/tricky gizmo to throw all night-invaders in the deep of trouble. Don’t panic! Please be patient, and let me describe the working of this security light. When powered, it instantly switch on the white light source to illuminate its surroundings, for example the corridor, stairway, backyard, garage, etc (nothing more happens in this standby mode). However, if the motion sensor detects a motion, the light source shuts off gradually, and revert to its initial state within a finite time after the last detected movement. No doubt, the unexpected darkness followed by another sudden illumination will hamper the invader.
As can seen in the circuit diagram, the electronics is very simple and straight forward. In the prototype, only one white LED (3.6V/1W) is used as the light source. After construction, the whole circuit should be housed in a suitable enclosure. The finished unit can be used indoors or outdoors, and can mount to either walls or ceilings of the detection area. The tricky security light can be powered from any external 12-volt dc power source with atleast 500mA output current capacity. Since the input supply is 12-volt, it is possible to connect two – three white LEDs (in series configuration) in lieu of the existing light source. No more changes required here (except in the value of resistor R5), but you should remember to increase the output current of the external 12-volt dc power source upto the actual demand.
In standby mode, output from the PIR sensor module is 0V, the gate of T1 is at 0V level and T1 is off. Capacitor C1 is now charged via R2 and D1, whereupon T2 comes on so that the light source (LED1) is switched on. When PIR sensor module detects motion, T1 whose gate is linked to the PIR sensor module, cuts off T2, so that the light source turns off. However, C1 is discharged fairly slowly through R4, so that the light source is not turned off immediately (goes out slowly). This turn-off delay may be varied by altering the values of C1 and R4.
Note that, in a PIR sensor module, its output goes to high-level (near 3.3V) for a finite duration after the motion detection, and revert to low-level (0V) thereafter. This time period can be changed by adjusting the on-board potentiometer of the PIR sensor module (refer PIR sensor-related articles and tutorials published elsewhere in this website). Finally, resistor R5 limits the operating current of the light source (LED1). Prototype was tested with a 33-Ohm power resistor (33R / 3W) as R5.