Application of NE555 timer IC in the monostable mode of operation.
Concepts related to the working of Light Dependent Resistors.
Implementation of the Bridge rectifier circuit for AC to DC conversions.
Application of the melody generator IC UM66.
The Automatic Night Lamp (with Morning Alarm) serves dual purpose, depending upon intensity of light rays detected. The circuit is capable of detecting darkness and switching ‘ON’ a white LED to act as night lamp. Conversely, it can also sense daylight and play a melodious tune as a morning alarm. The circuit operates off AC supply and the circuit also provides for battery backup, in case of power failure.
No. of ICs used: 3
The Automatic Night Lamp Dimmer with Morning Alarm circuit is powered by a standard 0-9V transformer. Diodes (D1-D4) rectify the AC voltage and the resulting DC voltage is smoothed by a capacitor (C1). Voltage regulator IC (7806) gives regulated 6V DC to the circuit while battery backup is also provided to power the circuit when mains fails. When mains supply is available, a 9V rechargeable battery charges via a diode (D5) and a resistor (R1) with a reasonably constant current. In the event of a power failure, the battery automatically takes up the load without any delay.
The circuit uses light dependent resistors (or LDR) for sensing ambient light conditions. Typically, an LDR offers high resistance in darkness, which reduces to a minimum when bright light is incident on it. In the Automatic Night Lamp Dimmer circuit, one LDR (LDR1) detects darkness while another LDR (LDR2) detects light in the morning. The circuit is designed around the single timer IC NE555 (IC2), which is
configured to work in the monostable mode in the circuit. IC2 is activated by a low pulse applied to its trigger input (pin2). Once triggered, the output pin (pin3) of IC2 goes high and remains in that position until IC2 is triggered again by the input at pin2. When LDR1 is illuminated by the ambient light, its resistance falls to a minimum and pin2 (trigger input) of IC2 remains at a positive potential. This causes the output (pin3) of IC2 to go low and hence, the white LED remains off.
In absolute darkness, the LDR has a resistance in excess of 280 kilo ohms. So, when the resistance of LDR1 increases, a short pulse is applied to trigger input (pin2) of IC2 via a resistor (R2).
This triggers the monostable mode of operation and the output pin of the NE555 (IC2) goes high, causing the white LED to glow. A low-value capacitor (C2) maintains the monostable for continuous operation, eliminating the timer effect. By increasing the value of C2, the ‘on’ time of the white LED can be adjusted. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls. This makes a transistor T1 to start conducting. When T1 conducts, a melody-generator IC UM66 (IC3) gets supply voltage from the emitter of T1 and it starts producing the melody. A single-transistor amplifier (T2) amplifies the musical tone generated by IC3.
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