Comparatively this circuit employees some different type of light sensor, and also a different type of transistor. It has a similar three-stage system diagram as shown below:
The Input is made to the light-dependent resistor (LDR). This is constructed from a semiconductor material such as cadmium sulphide. The resistance of this substance varies according the amount of light that falling on it. The typical range of resistance of an LDR is about 100 Ω when it is in bright light to about 1 MΩ in darkness.
Processing make use of a voltage divider (R1/R2) that usually supplies a varying voltage to the bipolar junction transistor, Q1. The voltage level at the point between the resistors is low (about 10 mV) in very bright light, but goes up to about 5 V in darkness. With LDRs otherwise than the ORP12, voltages will be different and it might be mandatory to change the value of R1 or re-place it with a variable resistor. Output is a good frequency of sound that will be from an audible warning device, for example, a siren.
Input is accepted by a light-dependent resistor (or LDR). This is made from a semiconductor material such as cadmium sulphide. The resistance of this substance varies according the amount of light falling on it. The resistance of an LDR ranges between 100 Ω in bright light to about 1 MΩ in darkness.
The main function of this circuit is to detect an intruder passing between the sensor and a source of light(local), for example, a street light. When the shadow of intruder falls on the sensor, its resistance increases, causing the voltage to rise at the junction of the resistors. The result is increased current flows through R3 to the base of Q1. This turns Q1 on and the siren begins to sound.
This circuit normally uses a BJT; almost any N-P-N type will do, provided that it is capable to carry the enough amount current required by the audible warning device to work, the device in our example is a piezo electric siren. It is appropriate to use ultra-loud (105 dB) warbling siren in a security system that needs just about 160 mA when it runs on 12 V. The BC337 transistor is rated at 500 mA.
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