Published:2009/7/13 20:24:00 Author:May | From:SeekIC
This circuit produces a string that alternates between red and green at a steady 1-s rate, making it an attractive addition to any holiday display. Two neon bulbs, NE1 and NE2, are connected in a relaxation-oscillator circuit that switches back and forth at about a 1-Hz rate. The values of R1, R2, and C3 set the switching rate. Dc power for the neon oscillator circuit is supplied via C2 and Dl. No dc filter capacitor is used or needed. When NE1 turns on, current flows through the LED in U1, an optocoupler, turning on the SCR and grounding the anode of D2. Only the positive half cycle of the ac waveform passes to the LEDs, which lights only the red ones (LED1 to LED15). After about 1 s, NE1 turns off and NE2 turns on, causing U2 to ground the cathode of D3. Now only the negative half cycle of the ac waveform passes to the LEDs, which lights only the green ones. If you'd like a change rate of less than 1 s, lower the values of R1 and R2 ; to lengthen the rate, increase the value of capacitor C3.
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