Basic Phototransistor Detector dIAGRAM

This is a Phototransistor Detector circuit. In this circuit, when the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.

The only critical part of this circuit is the value of resistor R1 which in most cases can be 470K ohms but may have to be increase if the room is dark or decreased if the room is well lit.

Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the phototransistor.

There are a number of phototransistors sizes and case styles. The smaller cases will be easier to hide but connecting wires may be more difficult. 

Laser Controlled ON OFF switch

This Laser Controlled ON/OFF switch schema is built around a 555 timer using very few components. Since the schema is very simple, even a novice can easily build it and use it as a controlling device. A laser pointer, now easily available in the market, can be used to operate this device. This schema has been tested in operational conditions from a distance of 500 meters and was found to work satisfactorily,though it can be controlled from still longer distances.
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Laser Controlled ON/OFF switch Circuit diagram :

Laser Controlled ON/OFF Switch Circuit Diagram

Aiming (aligning) the laser beam exactly on to the LDR is a practical problem. The schema is very useful in switching on/off a fan at night without getting off the bed. It can also be used for controlling a variety of other devices like radio or music system. The limitation is that the schema is operational only in dark or dull lit environments.

By focusing the laser beam on LDR1 the connected gadget can be activated through the relay, whereas by focusing laser beam on LDR2 we can switch off the gadget. The timer is configured to operate in bitable mode. The laser pointers are available for less than Rs 150 in the market. The cost of the actual schema is less than Rs 50. 

Author :Dr K.P. Rao  - Copyright : EFY