The bi-directional sequencer uses a 4 bit binary up/down counter (CD4516)
and two "1 of 8 line decoders" (74HC138 or 74HCT138) to generate the
popular "Night Rider" display. A Schmitt Trigger oscillator provides
the clock signal for the counter and the rate can be adjusted with the
500K pot. Two additional Schmitt Trigger inverters are used as a SET/RESET
latch to control the counting direction (up or down). Be sure to use the
74HC14 and not the 74HCT14, the 74HCT14 may not work due to the low
TTL input trigger level. When the highest count is reached (1111) the
low output at pin 7 sets the latch so that the UP/DOWN input to the counter
goes low and causes the counter to begin decrementing.
When the lowest
count is reached (0000) the latch is reset (high) so that the counter will
begin incrementing on the next rising clock edge. The three lowest counter
bits (Q0, Q1, Q2) are connected to both decoders in parallel and the highest
bit Q3 is used to select the appropriate decoder. The circuit can be used to
drive 12 volt/25 watt lamps with the addition of two transistors per lamp as
shown below in the section below titled "Interfacing 5 volt CMOS to 12 volt
loads"
