Construction

Simple Traffic Light System 

Aniruddh Kumar Sharma

T

raffic lights are used at road intersections to control vehicular traffic without traffic policemen. These allow orderly and uninterrupted movement of vehicles during peak hours. Generally, a traffic signal system has three lights and is run by a microcontroller. The green light on the bottom of the light stack signals the traffic to proceed, the orange light in the middle warns the traffic to slow down and prepare to stop, and the red light on the top signals the traffic to stop. Traditionally, traffic lights use bulbs, which consume a lot of power.

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Here we present an adaptive traffic light system that uses readily available components. As traffic lights are required throughout the day, LED lights have been used. These are more efficient and save energy.

Circuit description Fig. 1 shows the circuit of the simple traffic light system. It is built

Fig. 2: Typical traffic light system on the road crossing

Fig. 1: Circuit of simple traffic light system w w w. e f y m ag . co m

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cONSTRUCTION around an NE555 timer, a decade counter CD4017, AND, NOR and NOT gates, and a few discrete components. NE555 is used in astable mode to generate the basic clock for decade counter CD4017. The clock frequency of NE555 is determined by resistor R1,

Fig. 3: Power supply circuit

preset VR1 and capacitor C1. It can be adjusted by varying preset VR1. Pin 5 of timer NE555 is the control voltage pin that is primarily used for filtering when the timer is used in noisy environments. However, by applying a voltage at this pin, it is possible to vary the calculated period. A 0.01µF capacitor connected to pin 5 of NE555 bypasses any noise from altering the calculated pulse width. Reset

Fig. 4: Actual-size, single-side PCB for the adaptive traffic light system

Fig. 5: Component layout for the PCB 6 2 • A p r i l 2 0 1 0 • e l e c t ro n i c s f o r yo u

pin 4 of NE555 is pulled high through resistor R3. When switch S1 is closed, pin 4 is connected to ground and the astable multivibrator stops clock generation. Decade counter CD4017 is at the heart of the traffic light system. It is a 5-stage Johnson counter having ten decoded outputs. The clock input receives the clock pulse from pin 3 of NE555. Schmitt trigger action in the clock input circuit of CD4017 provides pulse shaping that allows unlimited clock input pulse rise and fall times. These counters advance by one count at the positive clock signal transition if clock-inhibit signal pin 13 is low. A high signal on reset pin 15 clears the counter to zero. Q0 through Q7 outputs of CD4017 control the traffic light signal for four roads as shown in Fig. 2. Q0 and Q1 outputs control the green, orange and red traffic light sequence for a single road (say, road 1). When reset switch S2 is pressed, Q0 output of CD4017 is high and the other outputs are low. The low Q1 output is inverted by NOT gate N13 and fed to pin 13 of AND gate N5. Q0 output is fed directly to pin 12 of gate N5. Both the high inputs of AND gate N5 make its output high and the green LED (LED1) glows. Pin 14 of CD4017 receives a clock pulse and its Q1 output goes high while the other outputs are low. Q1 output is fed directly to pin 8 of gate N6, while the low Q0 output is inverted and fed to pin 9 of gate N6. Both the high inputs of AND gate N6 make its output high and the orange LED (LED3) glows. w w w. e f y m ag . co m

cONSTRUCTION Parts List Semiconductors: IC1 - NE555 timer IC2 - CD4017 decade counter IC3 - CD4001 quad NOR gate IC4, IC5 - CD4081 quad AND gate IC6, IC7 - CD4069 hex inverter IC8 - 7812, 12V regulator T1 - BC547 npn transistor D1-D5 - 1N4148 switching diode D6 - 1N4007 rectifier diode BR1 - 1A bridge rectifier LED1-LED13 - 5mm LED Resistors (all ¼-watt, ±5% carbon): R1 - 15-kilo-ohm R2 - 2.2-kilo-ohm R3, R4 - 10-kilo-ohm R5-R17 - 1-kilo-ohm VR1 - 1-mega-ohm preset VR2 - 10-kilo-ohm preset Capacitors: C1 C2 C3 C4

- 100µF, 35V electrolytic - 0.01µF ceramic disk - 1000µF, 50V electrolytic - 0.1µF ceramic disk

Miscellaneous: X1 - 230V AC primary to 15V, 500mA secondary transformer S1 - On/off switch S2 - Push-to-on switch

Again, pin 14 of CD4017 receives a clock pulse and its Q2 output goes high while the other outputs are low. The low Q0 and Q1 outputs are also connected to input pins 1 and 2 of NOR gate N1, respectively. Both the low inputs of NOR gate N1 make its output high and the red LED (LED2) glows. At the same time, both Q2 and Q3 outputs of CD4017 make the output of AND gate N9 high and the green LED glows (say, for road 2). All the three road lights are red. This light sequence for roads works one after the other as explained above. When Q9 output of CD4017 goes high, counter CD4017 resets and the process restarts. The green and red lights glow for the same time period (say, one minute), which is set by preset VR1. The orange light glows for a few seconds (nearly 10 seconds). When the orange light glows, transistor T1 receives base bias. The high at 6 4 • A p r i l 2 0 1 0 • e l e c t ro n i c s f o r yo u

the base of transistor T1 drives it into saturation and preset VR2 connects to pin 7 of NE555. Resistances of presets VR1 and VR2 come into parallel and the time period becomes small (nearly 10 seconds). When there is a heavy rush on the current road, the traffic policeman can keep the traffic moving with green light by closing switch S1. NE555 doesn’t regenerate clock pulse. As there is no clock signal for counter CD4017, its output does not advance. Open switch S1 for normal operation. Fig. 2 shows the typical traffic light system on the road crossing. Fig. 3 shows the power supply circuit. The 230V, 50Hz AC mains is stepped down by transformer X1 to deliver a secondary output of 15V, 500 mA. The transformer output is rectified by a full-wave rectifier BR1, filtered by capacitor C3 and regulated by IC 7812 (IC8). Capacitor C4 bypasses any ripple present in the regulated 12V supply. LED13 acts as the power-‘on’ indicator. Resistor R17 limits the current. A capacitor above 10 µF is connected across the output of the regulator IC, while diode D6 protects the regulator IC in case its input is short to ground.

Construction An actual-size, single-side PCB for the simple traffic light system is shown in Fig. 4 and its component layout in Fig. 5. Assemble the circuit on a printed circuit board (PCB) to minimise time and assembly errors. Carefully assemble the components and double-check for any overlooked error. Using preset VR1 set the ‘glowing’ time period for red and green lights as one minute. Using preset VR2 set the ‘glowing’ time period for the orange light as ten seconds. Connectors ‘A’ through ‘L’ provided on the PCB allow for extension of LEDs. For higher brightness, you can add more LEDs through relays.  w w w. e f y m ag . co m