Sequential Flasher Circuit

A 555 timer (IC1) controls a 4017 CMOS decade counter. The first four outputs of the 4017 drive a CA3079 zero-voltage switch. Pin 9 of the CA3079 is utilized to inhibit output from pin 4, effectively disabling the stream of pulses that the IC typically generates. These pulses occur every 8.3 ms, corresponding to a frequency of 120 Hz, with a pulse width of 120 µs. The CA3079's operation ensures that the lamps connected to the triacs are turned on and off close to the zero crossing of the AC waveform. This method enhances lamp longevity by minimizing the inrush current that would occur if the lamp were activated near the peak of the AC waveform. Additionally, switching at the zero crossing significantly reduces radio frequency interference (RFI). It is important to exercise caution, as the CA3079s are driven directly from the 117-V AC power line.

The circuit utilizes a 555 timer configured in astable mode to generate a square wave signal at a frequency of 120 Hz. This frequency is determined by the resistor and capacitor values connected to the timer. The output of the 555 timer is fed into the decade counter (4017), which counts the pulses and activates its output pins sequentially. The first four outputs of the 4017 are connected to the CA3079 zero-voltage switches, which are responsible for controlling the triacs that, in turn, manage the power to the connected lamps.

The CA3079 operates by detecting the zero crossing of the AC waveform, allowing it to switch the load on and off at a point where the voltage is nearly zero. This technique is crucial for reducing the stress on the lamps, thereby extending their operational lifespan. The inhibition feature provided by pin 9 of the CA3079 is a critical aspect of the design, as it allows for the suppression of the output pulses from the 4017 when necessary, ensuring that the triacs do not activate the lamps at inappropriate times.

Safety considerations are paramount when dealing with components connected to the AC mains. The CA3079 being directly connected to a 117-V AC power line necessitates the implementation of appropriate isolation and protective measures to prevent electrical hazards. Care should be taken during the design and assembly of the circuit to ensure compliance with safety standards and to mitigate risks associated with high voltage operation.

Overall, this circuit design highlights effective strategies for controlling AC loads with minimal interference and enhanced reliability, making it suitable for various applications where lamp control is required. A 555 timer, IC1, drives a 4017 CMOS decade counter. Each of the 4017s first four outputs drives a CA3079 zero-voltage switch. Pin 9 of the CA3079 is used to inhibit output from pin 4, thereby disabling the string of pulses that the IC normally delivers. Those pulses occur every 8.3 ms, i.e., at a rate of 120 Hz. Each pulse has a width of 120uS. Because of the action of the CA3079, the lamps connected to the triacs turn on and off near the zero crossing of the ac waveform.

Switching at that point increases lamp life by reducing an inrush of current that would happen if the lamp were turned on near the high point of the ac waveform. In addition, switching at the zero crossing reduces radio frequency interference (RFI) considerably. Caution: The CA3079s are driven directly from the 117-Vac power line, so use care. 🔗 External reference