By combining a digital counter, decoder, drivers, and relays,the author 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.
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