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1 Traffic Light Driver

Single Traffic Light Driver Circuit

  The circuit on this page is designed to control a one – Three Light (Green – Yellow – Red) traffic signal assembly.

The length of time that each light is on is independently adjustable between 4 and 40 seconds for the parts values specified. The length of the ON time can be lengthened by increasing the values of capacitors C1, C2 and C3.

The current rating of the relay contacts is 5 Amps at 30 Volts.


  •   If there is no power to the traffic light circuit the RED light will always be ON.
  •   When power is applied to the circuit the RED light will be ON until the first timer runs out.
  •   Each time the RED light goes ON, the GREEN and YELLOW lights are forced OFF by resetting timers IC 2A and 2B via C7, R11 and Q1. This prevents more than one light being on at the same time.

    See Method 1 in section 10 of the LM555 Timer page at this site for more information.

  •   An optional, external STOP switch can be used to force the RED light ON and the GREEN and YELLOW lights OFF.
  •   The control section of the circuit is designed to operate from a 12 volt power supply and requires 90 milliamps of current.
  • NOTE  The Single Traffic Light Driver circuit circuit cannot control a set of traffic lights at an actual intersection.

    A basic intersection signal scheme would require at least 6 steps in the cycle and still more would be needed to allow the Yellow and Red lights in opposing directions to overlap before the signal for the stopped direction changes to Green.

  •   This circuit is derived from the Three Stage – Cycling Timer Circuit shown in section 24 of the LM555 Timer page at this site.

Parts Lists

The following table shows the parts list for the Single Traffic Light Driver Circuit. The list shows the Digi-Key stock numbers as well as the schematic part numbers

IC 1 LM555 Single Timer LM555CNFS-ND 1
IC 2 LM556 Dual Timer LM556CNFS-ND 1
Q 1 2N3904 2N3904FS 1
D1, 2, 3, 4, 5, 6, 7 Diode 150mA 1N4148DICT-ND 7
R1, 2, 4, 5, 7, 8, 9, 11 100K ohm – 0.25W 100KQBK-ND 11
R3, 6, 10 1M Potentiometer 3306F-105-ND 3
C1, 2, 3 33uF / 35V P5180-ND 2
C4, 5, 6 0.33uF / 63V 495-2548-ND 4
C7 10uF / 50V P5178-ND 2
C8 100uF / 50V P5182-ND 2
Relay 1, 2 Relay 5A SPDT 9VDC Z1641-ND 2
Terminal Blocks 2 Position – 5mm ED1623-ND 4


Parts Placement Diagram


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