Flash Away, Flash Away, Flash Away All!

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.

Holidazzle tree lights flasher parts list
Amt Part Description Allied Part #
1 Voltage regulator, 5V 568-3101
1 555 Timer 263-0133
1 8-bit binary counter 735-1899
1 Decade decoder 568-6567
3 Dual driver 735-2237
5 SPDT Relay 686-0013
5 1N4148 Diode 263-1538
5 Green LED 263-1170
1 Heat sink 619-0160
1 Breadboard 761-0010
1 Power supply 653-0136
Additional parts required: Various capacitors and resistors.

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 000 2 and 111 2. (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 000 2.

Be sure to include 0.1 muF 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 = t 1 + t 2

Frequency = 1/P

For the components shown in the schematic diagram:


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