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Flash Away, Flash Away, Flash Away All!

By Design News Staff -- Design News, January 12, 2004

Tired of bobbing reindeer heads and inflatable snowman displays, Jon decided to add some real flash to his holidays. By combining a digital counter, decoder, drivers, and relays, he built a simple, yet flexible circuit that allows for exciting flashing patterns and sequences for a few or many light strings. Building the "flasher" on a solderless breadboard makes modifications easy.

This circuit sequence through a pattern of Christmas lights to turn on and off each string of lights, one at a time. After sequencing through all strings, the circuit turns the last string off, then turns all strings on, and then off, before repeating the pattern.  Some simple wiring changes will let you set up other patterns.  This circuit provides for five strings of lights, but you can increase or decrease the number, and adjust the timing.

A 555 timer chip supplies about a 0.5 Hz signal to an SN74LS93 4-bit binary counter chip that counts between 000₂ and 111₂. (The circuit uses only the 74LS93's 3-bit counter.)  The counter's outputs connect to an SN74LS42 decade decoder that provides an individual logic-0 output for each counter state. Outputs from the SN74LS42 control drivers within SN75451B chips (two drivers per chip). Finally, each driver controls a relay that turns on or off a string of Christmas lights.

Each driver in an SN75451B chip has two logic inputs, and a logic 0 at either will turn on the driver, thus activating its associated relay.  By connecting the outputs of the decoder to the driver inputs, you can establish the patterns you want.

The schematic diagram above provides the sequence described above for five strings of lights. The relays handle 110V AC.  To operate lights at higher voltages, substitute relays with contacts that will handle the current each light string requires.  This circuit includes an optional light-emitting diode (LED) for each driver so you can test operation before connecting it to line-powered lights. (After testing, remove or disconnect the LEDs to reduce power through the 78M05 regulator.) After power up, the SN74LS93 counter may need two or three pulses from the 555 timer chip to start the proper counter sequence. That's normal--counter chips don't automatically start at 000₂.

Be sure to include 0.1 µF disc ceramic capacitors between +5V and GND throughout the circuit to reduce transients on the power-supply lines.

To adjust the period (P) of the 555 timer, use the following formulas:

t₁ = 0.693 * (R_a + R_b) * C

t₂ = 0.693 * R_b * C

P = t₁ + t₂

Frequency = 1/P

For the components shown in the schematic diagram:

t₁  =  0.693 * (4.70 x 10⁵ + 1.00 x 10⁶) * 1.0 x 10^-6 = 1.02 s

t₂  =  0.693 * (1.00 x 10⁶) * (1.0 x 10^-6) = 0.7 s

P = 1.7 s

F = 1/1.7 = 0.6 Hz

Circuit hints:

Integrated circuits in dual-inline packages (DIPs) come with a marked band or impression at their left end. Position a chip so the pins are horizontal, and from a top view, count pin numbers counter clockwise, starting in the lower-left corner.

See the schematic diagram for the front-view pin designations on the 78M05 voltage regulator.  Use a heat sink on this component.

LEDs have a polarity, so if an LED doesn't operate properly in one configuration, reverse the leads that connect it.

The 1N4148 diodes come marked with a black band. That band corresponds to the flat bar in the diode symbol in the schematic diagram.

Observe the + polarity marked on electrolytic capacitors, and match it to the polarity shown in the schematic diagram.

Make sure you have a common ground among all the digital circuits and the +12V power supply and the 5V power from the regulator.  DO NOT make a common electrical connection to the 110V AC light circuits.  The relays isolate this voltage from the digital circuits.

Resistor markings:

Yellow-violet-yellow = 470K ohms

Brown-black-green = 1M ohms

Red-red-brown = 220 ohms

Room for improvement

You can use other counter types for more or fewer strings of lights:

SN74LS90, divide by 5 or divide by 10

SN74LS92, divide by 6 or divide by 12

SN74LS93, divide by 8 or divide by 16

Adapt the decoding scheme as needed to drive SN75451B chips. If you need a 16-output decoder, use an SN74LS154, and ensure your power source can power any added relays.

If you plan to drive more than a few SN75451B inputs with an output from the SN74LS42 decoder, "buffer" the decoder's output with two SN7404 inverters connected in series. The SN7404 circuits increase the drive capability of the decoder's output so it will reliably controls the SN75451B drivers.

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Talkback

• Save some chips and some money use a CD4017
  - 2007-12-18 15:50:20
• Don't forget to connect +5 to pin 8, and GND to pin 4 on each SN75451B. This is missing in the...
  - 2007-12-18 07:35:22
• A similar circuit but with a pseudorandom pattern can be made by substituting a 4-bit shift...
  - 2006-12-13 12:35:00

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