Schematic Diagram

These are actually running flashing or chasing light circuit. Road diversions and the like are then clearly visible. The circuit described here provides a similar effect but for use in model roadways for instance. The speed of the running’ row of LEDs is determined by the frequency of the clock generator N1. Depending on the type of IC used, this frequency will be of the order of 6 Hz t 30% when potentiometer P1 is in mid position. The output V of the clock generator is fed to the - Johnson counter lC1. The outputs of this counter become logic 1 in sequence. The counter is reset to the start when O4 goes to logic 1. This explains the link between pins 15and 10of lC1.0utputs OO. . . O3 are connected to tour monostable multivibrator circuits consisting of ll2 . . . N5. The multivibrators are triggered by the negative going edge of the square wave outputs of O0 . . . O3 and the pulse period g can be preset with potentiometers P2 . . . P5 which of course determine how long each group of LEDs will light. These periods need to be more or less equal to ensure smooth running of the lights. The circuit uses four groups of four LEDs each. The LEDs in each group will light simultaneously. Figure 2 shows how the LEDs should be connected for road markings in a bend: LEDs D16, D15, D14 and D13 light first,followed by D12, D11, D10 and D9, and so on. Schmitt triggers N2 . . . N5 are not capable of supplying sufficient cur- rent for the LEDs and therefore the buffers T1 . . . T4 are included. The current through the LEDs is about 30 mA during each flash;the average current taken by the circuit operating at the highest frequency is of the order of 30 mA. When, however, the flash period is longer than the running period, the current con- sumption may rise to a `maximum of 100 mA. The printed circuit board (see figure 3) is fairly compact. The preset potentiometers are neatly grouped together and altered on the circuit diagram and the board.



Parts list

Resistors:
R1= 47 k
R2...R5,R10...R13=10k
R6..°.R9=22k
R14...R17=47.Q
P1 . . . P5 = 1 M preset potentiometer
Capacitors:
C1 = 330 n
C2...C5=100n
C6 = 10 p/16 V
Semi-conductors:
‘ T1 ...T4=BC547B
D1 . . . D16 = LED yellow
IC1=4017
lC2=40106

There are many occasions when a switching delay is required. One way of achieving this is to use an RC network and an inverter (see figure 1).
This is quite practical and obvious as there are nearly always some gates Ieft over in a circuit. Unfortunately, every electronic component has a definite tolerance and so it is virtually impossible to determine the delay precisely in advance. However a considerable improvement can be achieved by connecting two inverter/ RC networks in series as shown in figure 3. The nominal threshold voltage of the inverter in figure ‘l is half the supply voltage and has a tolerance of 1 30%. Figure 2 shows the signal input to 1 the gate. lf this input is between UC 0.35 Ub and UC = 0.65 Ub the inverter may consider it either logic _ 0’ or ll These voltages occur when a capacitor is charged through a resistor after a period of 0.43 ·r and I l.05r respectively. (r is the time constant of the circuit and is equal to R x C). The nominal threshold voltage UC = 0.5 Ub is reached after a time of t = 0.69 r. lf the two inverters and RC networks of figure 3 are used, each RC net· work must produce the same delay, equal to half the total value of figure l.

The total delay will then be % x 0.43 1*+% x 1.05 *r= O.74 Tat its worst case! This is a lot closer to the nominal value of 0.69 1: The foregoing should make it clear why the circuit of figure 4 gives such consistently reproducible results. However, for really satisfactory operation, CMOS inverters must be used. The reason is that these gates have a threshold value of about half the supply voltage. Further, their output will always be either zero or the supply voltage. Schmitt triggers should not be used! If the delay times using 4000 series CMOS are found to be too long the new 74HCXX series can be used. These are pin and function compatible to the 74LSXX series and just as fast!