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Traffic Light Controller


The circuit given here is substitute of old mechanical traffic-light controllers which are not reliable. The circuit’s timing and sequential operation are done by two CMOS ICs (IC1 and IC2) while the actual power switching is done by triacs,
A 10V negative power supply is obtained directly from the mains my means of D1, R1, D2, and C1. Gates N1 through N6 constitute IC2 while IC1 is a Johnson counter. N1 – N3 are wired as an astable multivibrator whose time period can be adjusted between 1 second and 10 seconds with VR1. The decade outputs of IC1 are wired such that when Q0 and Q5 is high, the output of N5 goes low. Similarly, the outputs of N4 and N6 become low when Q1 to Q4 and Q6 to Q9 become low respectively. Since we have negative supply, a low output of any of the hates N4 to N6 cause the respective triac to fire.
Thus, the ratios of the time periods for the lamps in the sequence O:G:O:R are 1:4:1:4.
Resistor R10 to R12 and capacitor C4 and C6 are absolutely necessary, these avoid spurious triggering of the triacs which may hamper traffic flow.

circuit diagram of traffic light controller

PARTS LIST
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 5 KΩ/5W
R2, R3, R4 = 22 KΩ
R5 = 100 KΩ
R6 = 1 MΩ
R7, R8, R9 = 1 KΩ
R10, R11, R12 = 100 Ω/1W
VR1 = 1 MΩ
Capacitors
C1 = 1000 µF/16V
C2, C3 = 22 µF/16V
C4, C5, C6 = 00.1 µF/400V
Semiconductors
IC1 = CD4017
IC2(N1 – N6) = CD4049
D1 = BY127

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