The projects “Automatic Night Lamp Circuit” is a very simple and build around four major components i.e. TRIAC, SCR, LDR and Resistor. The lamp used in this project is porch light which would turn on automatically in dark (night time) and turn off in light (day time).
Circuit Description of Automatic Night Lamp Circuit
The circuit of Automatic Night Lamp Circuit is used shown in figure 1. The circuit is very simple and straightforward that anyone having very little knowledge about TRIAC and SCR.
SCR (Silicon Controlled Rectifier) is a PNPN silicon device with end p-terminal forming the anode A, end n-terminal forming the cathode k and a gate, called the cathode gate Ge, at the p-layer next to the cathode. SCR has two stable state namely ON state and OFF states. No intermediate state is possible. Thus, with the suitable forward voltage and gate current, SCR switches to ON state and draws heavy current limited only by the resistor in the external circuit. It remains in the ON state indefinitely. Only the SCR fires i.e. comes in the ON state, gate loses all control. SCR can be turned OFF by reducing the supply voltage below the holding voltage VH i.e. reducing the device current below the holding current IH. This function of SCR is similar to that of a mechanical switch and hence SCR truly forms an electronic switch.
Like SCR, TRIAC is also essentially a four-layer device and has three terminals only namely anode, cathode and gate. But unlike SCR, triac is a bidirectional device. It can conduct is either direction and can block voltage of either polarity. A voltage of either polarity can switch a triac from OFF state to ON state. Hence the function served by a TRIAC is essentially that of two SCRs connected in parallel but in opposite directions. The gate terminal G makes ohmic contacts with both the n and p materials. This permits trigger pulse of either polarity to start flow of current.
LDR (Light Dependent Resistor) is basically a semiconductor device whose resistance change according to the intensity of light fall on it. The resistance will increase in absence of light on LDR and resistance will decrease in presence of light on LDR.
Working of Automatic Night Lamp Circuit
At day time when an adequate amount of light fall on the LDR its resistance is very low. Let the switch SW2 is in position 1 (i.e. connected to SCR) and switch SW1 is closed. AC input voltage is rises to 330 volts. The voltage is divided into two voltages because resistor R1 and LDR are connected in series. At day time, the peak voltage is less than breakdown voltage the SCR will remains off. When light on LDR become low the resistance of LDR become high as a result SCR gate is triggered and lap start to glow. We know that SCR is only operated in positive half cycle as a result the intensity of bulb will reduce but it will boost the lifespan of bulb.
In order to reduce this limitation a TRIAC is used for this switch SW2 is move to position 2. The brightness of lamp will distinct increases.
PARTS LIST OF AUTOMATIC NIGHT LAMP CIRCUIT
|Resistors (all ¼-watt, ± 5% Carbon)|
|R1 = 10 MΩ|
|SCR1 = OE206 or SN206 or 2N2601 or TY6004 or any 2A, 600V SVR
TRIAC1 = SPT6M or 2N5757 or any 2A, 600V TRIAC
|SW1 = ON/OFF Switch
SW2 = SPDT switch
L1 = 6W Lamp
F1 = 3A rating Fuse