Courtesy Light with Time Delay Switch Aspect

Figure 1 

Parts List

The circuit was constructed to provide a time delay feature when switching OFF a light to give the person enough time to reach the bed before the lamp completely turns OFF.

• TS555 – a single CMOS timer which offers very low power consumption and high frequency where the timing remains very accurate in monostable or astable mode while providing reduced supply current spikes during output transitions, which enables the use of lower decoupling capacitors and with added features of high output current capability, high input impedance, wide voltage range, and output compatible with TTL, CMOS & Logic MOS
• BZX79C10 – a 500mW hermetically sealed silicon glass Zener voltage regulator Diode with features such as solder hot tip lead finish, polarity band indicated diode, RoHS compliant, corrosion resistant external surfaces, readily solderable leads, compression bonded construction, through-hole device type mounting, DO-35 package, and Zener voltage range from 2.0 to 75 Volts
• TIC206M – a silicon triac with 400 V to 800 V off-state voltage, glass passivated wafer, sensitive gate triacs, 4 A RMS, and 5 mA maximum gate triac current

The circuit is being supplied by 230 VAC mains without the use of a transformer. This is because of the low current consumption during the operation. The supplied voltage will be reduced to 10 VDC by the circuit combination of two diode rectifier cell D1 & D2, the reactance from capacitor C1, and the Zener diode with 10 V avalanche voltage.

The 555 timer as denoted by IC1 is operating at monostable mode wherein the IC produces a single or one shot pulse upon receiving a trigger signal. It is responsible for providing 15 seconds of time delay which can be set with the aid of R3 & C4. The output Pin 3 of IC1 will be permanently ON when the switch SW1 is closed. This will drive the triac D4 that will then feed the lamp. The monostable operation will start at the opening of SW1 where the Pin3 of IC1 will go low thereby switching OFF the lamp after 15 seconds.

Although the current drain is almost negligible, the circuit is wired permanently to the mains supply. There will be no heat generation due to the absence of a transformer that is why low gate-current triac is recommended. The increase in the value of R3, with the same value of C4, the timing will have a ratio of 100K Ohms per second, since the delay time can only be adjusted with the change in values of R3 and/or C4. Since the circuit is connected to 230 VAC mains, some components in the circuit board may be prone to damage. If any short circuit or loose connection happens, avoid touching the circuit. It should be enclosed in a plastic box while the switch should be a well insulated mains-type.

The courtesy light is suitable to be used as bedroom lamps but can also find different applications for this circuit as long as it suits their needs. Some courtesy lights are utilized in automobiles for turning ON automatically upon opening the door. Others are suitably designed to highlight any architectural creation for indoor or outdoor areas.

Source:redcircuits.com/Page27.htm