SIMPLE CIRCUITS

SIMPLE CIRCUITS WITH SENSORS

                                           Fig: single transistor light sensor

Description: When light falls on the LDR, its resistance is low and when light doesn’t fall on the LDR, its resistance is high so the transistor is switched off and the led D1 stops glowing, indicating darkness and glows when a light is falling on it. R3 is variable. . The circuit works as a light sensor which switches off the LED whenever light falling on LDR is blocked.

                                                  Fig: Two transistor dark sensor

Description: Similar to above single transistor sensor except for the addition of another transistor Q2 where it improves the sensing capability and accuracy, reducing the effect of noise. When it is exposed to light, Q1 conducts while Q2 doesn’t and when it is dark, Q2 conducts hence enabling the led.

                                                Fig: dark sensor using 555 timer

Description: The above circuit works as a dark sensor. If the light falling on LDR is blocked, it switches on the LED D1. It has been configured in monostable mode of 555 timer which means that the circuit switches on the LED D1 for a specific time. The switching time is determined by the combination of resistance R1 and capacitor C1.

In this circuit, time (T) = 1.1xR1xC1 = 1.1x47x10 3 x100x10 -6 = 5.17sec.

 Pin 3 is the output of the 555 timer. It greatly reduces the noise interference. C3 is ceramic while C1 is electrolytic capacitor.

                                             Fig: Touch sensor with 555 timer

Description: Similar to light sensor except that ldr is removed and a touch plate (any sensitive conductor) is placed. Note that the electrode wire connected to capacitor C2 should be around 2-5cm long. As we touch on the wire, the circuit triggers the LED for a specific time as determined by the resistor R1 and capacitor C1. You can connect the wire to a small metallic touch plate using an alligator clip.

                                     Fig: Clap switch using 555 timer and transistors

Description: A condenser mic which is readily available at any electronics store is placed before the transistor Q1 at its base. When you clap, the 555 timer is activated and the led D1 indicates it.