|
Light
emitting diodes are advan- tageous due to their smaller size, low
current consumption and catchy colours they emit. Here is a
running message display circuit wherein the letters formed by LED
arrangement light up progressively. Once all the letters of the
message have been lit up, the circuit gets reset. The circuit is
built around Johnson decade counter CD4017BC (IC2). One of the IC
CD4017BE’s features is its provision of ten fully decoded outputs,
making the IC ideal for use in a whole range of sequencing
operations. In the circuit only one of the outputs remains high
and the other outputs switch to high state successively on the
arrival of each clock pulse. The timer NE555 (IC1) is wired as a
1Hz astable multivibrator which clocks the IC2 for sequencing
operations. On reset, output pin 3 goes high and drives transistor
T7 to ‘on’ state. The output of transistor T7 is connected to
letter ‘W’ of the LED word array (all LEDs of letter array are
connected in parallel) and thus letter ‘W’ is illuminated. On
arrival of first clock pulse, pin 3 goes low and pin 2 goes high.
Transistor T6 conducts and letter ‘E’ lights up. The preceding
letter ‘W’ also remains lighted because of forward biasing of
transistor T7 via diode D21. In a similar fashion, on the arrival
of each successive pulse, the other letters of the display are
also illuminated and finally the complete word becomes visible. On
the following clock pulse, pin 6 goes to logic 1 and resets the
circuit, and the sequence repeats itself. The frequency of
sequencing operations is controlled with the help of potmeter VR1.
The display can be fixed on a veroboard of suitable size and
connected to ground of a common supply (of 6V to 9V) while the
anodes of LEDs are to be connected to emitters of transistors T1
through T7 as shown in the circuit. The above circuit is very
versatile and can be wired with a large number of LEDs to make an
LED fashion jewellery of any design. With two circuits connected
in a similar fashion, multiplexing of LEDs can be done to give a
moving display effect
|
