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CIRCUIT
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We have been using disco
lights, using alternately
blinking red and green LEDs, for more than fifteen years in our homes, cars and markets. To make the LEDs blink, we normally employ additional components such as capacitors, resistors, transistors and ICs. Circuits of some low-cost disco lights are presented here which, besides a resistor and LEDs, use none of the other above-mentioned components. During optimum brilliance, the forward voltage drop of a red and a green LED is 1.6 and 2.2 volts, respectively. Consider the basic circuit of Fig.1 using red and green LEDs, a switch and a current limiting resistor. When switch S1 is open, the green LED lights with a voltage drop of 2.2V across points A and B. When switch S1 is closed, the red LED lights with a voltage drop of 1.6V across points A and B, which is insufficient to force a current through the green LED which, thus, does not light. In practice, the green LED does light but quite faintly. However, if a diode is added in series with the green LED arm as shown in Fig.2, the voltage required to force current through this arm, increases by about 0.6V to 2.8V (0.6V +2.2V). Thus when switch S1 is closed the red LED lights as before but green LED is fully off. By alternately closing and opening switch S1 manually or electronically, we can have a disco effect. But fortunately blinking red LEDs (known as master LEDs) are available in the market (costing Rs 3 to 5) which flash at the rate of approximately one flash per second. Thus we can easily replace the arm containing red LED and the switch with a blinking red LED to serve the function of red LED and an electronic switch (refer Fig.3). When blinking red LED is on, the green LED will be off and when blinking red LED goes off, the green LED will light. A voltage as low as 3V is adequate for this circuit. All the LEDs used in these circuits should be high brilliance transparent LEDs for better disco effect. Figs 4 and 5 depict as to how a number of red and green LEDs can be connected for different supply voltages. The main thing to remember is that the total forward voltage drop, in the arm containing the blinking red LED, is quite below the minimum forward voltage drop of the other arm (during conduction). In the figures where two values of current limiting resistors are shown, the lower value resistance will provide higher brightness and vice versa. Readers may connect LEDs in similar way for other voltages. Value of current limiting resistor can also be calculated using Ohm's law for the arm containing blinking red LED. If the green LEDs do not blink properly, the fault can be rectified by either increasing the value of current limiting resistor or by connecting additional diode (1N4001) in the green LED arm. |
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