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Wednesday, 11 March 2009

Traffic Light Control Circuit (20 Output Sequencing Circuit) This page features a circuit that has twenty open collector outputs that turn on one

Traffic Light Control Circuit

(20 Output Sequencing Circuit)

This page features a circuit that has twenty open collector outputs that turn on one at a time in a continuous sequential manner. The circuit make use of the 74LSxx family of TTL integrated logic devices. The circuits are designed to drive light emitting diodes or low current, low voltage incandescent lights but can also drive other loads of up to 80 milliamps.

As logic circuits go, the 20 Step circuit is fairly simple but due to the high speed nature of the TTL Logic devices used, care must be taken when wiring these circuits. Simply put; The neater the wiring the better.

If you would like to make use of these circuits, please take the time to find and read at least the first 2 pages of the manufactures data sheets for the integrated circuits. Using Google, search for "74ls(part number)" in the first box and "PDF" in the second box on the advanced search page.

A printed circuit board and parts are available for this circuit.

20 Output Sequencing Circuit

The following schematic is for the 20 Output Sequencing Circuit on the circuitboard shown above.

Basic Circuit Operation

  • The circuit is stepped through the sequence by an adjustable LM555 astable oscillator.

  • The Oscillators output is divided by a 74LS90 divider into a 10 step BCD weighted output.

  • The BCD output then drives two 74LS145 - 1 of 10 decoders (See Notes) that are used to produce a 1 of 20 step output sequence.


  • The circuit does not drive the 74LS145's directly but uses a 74LS107 JK Flip-Flop and four 74LS32 dual input OR gates to control to the inputs to the two 74LS145 output drivers. The 74LS107 and 74LS32 are used to create disallowed states in the output drivers alternately. The disallowed states prevent any of the ten outputs on that particular device from being turned ON while the other 74LS145 is in counting to ten.

  • This produces a system where only one of the 74LS145's is able to produce a LOW output state at a time. In essence the circuit counts to 10 twice in succession rather than counting to 20 in a single cycle.

  • This may seem like an unusual method but it allows the circuit to make economical use of the open collector outputs of the 74LS145s decoder/drivers rather using output buffer ICs driven by 74LS138 logic devices.

  • The TTL logic devices in the circuit require a regulated 5 volt supply and draw approximately 60 miliamps. The outputs of the 74LS145's can be supplied from up to 15 Volts with a maximum current of 80 milliamps.

  • The circuit above is shown in a continuous running mode. The circuit can also be externally stopped and reset as shown in later diagrams.

74LS145 Equivalent Output Circuit

Parts List

The following is a parts list for use with the 20 Output Sequencing Circuit. Mouser Electronics part numbers are shown but the parts may be available from other sources as well. Suppliers that handle 'NTE' components should be able to get the ICs.

Part Number Mouser Description Mouser Part # QTY
VR 1 - Voltage Regulators TO-92 5.0V 0.1A - 511-L78L05ABZ - 1
IC 1 - Timers DIP-8 Single Timer - 512-LM555CN - 1
IC 2 - DECADE COUNTER DIP14 - 526-NTE74LS90 - 1
IC 3 - DUAL J-K F/F DIP-14 - 526-NTE74LS107 - 1
IC 4 - QUAD 2-IN OR DIP-14 - 526-NTE74LS32 - 1
IC 5, 6 - BCD-DEC DECODER DP16 - 595-SN74LS145N - 2
R1 - 100K ohm / 1/4 Watt Carbon Resistor - 660-CF1/4C104J - 1
R2 - Trimmer Potentiometers 1Mohms 6mm - 531-PT6KV-1M - 1
R3 - 470 ohm / 1/4 Watt Carbon Resistor - 660-CF1/4C471J - 1
C1, C3 - Radial Electrolytic Capacitors 25V 10uF - 140-XRL25V10 - 2
C2 - Radial Electrolytic Capacitors 25V 1.0uF - 140-XRL25V1.0 - 1
D1 - Green 3mm LED - 859-LTL-4231 - 1
- - 2 Position Terminal Block - 5mm - 651-1729018 - 1

The 1N4148 diodes for the traffic signals shown below are Mouser part number 78-1N4148.

NOTE: The LEDS for the traffic signal lights must be selected by the user as their size and style depend on the mounting.

20 Output Circuit - PCB Parts Placement Diagram

Save and print this diagram to aid in assembling the circuitboard.

Circuit Board Parts Placement Diagram


20 Output Sequencing Circuit PCB And Parts

20 Output Sequencing Circuit PCB - Assembled Example

The printed circuit board is 2.9 inches square and has been commercially made.

The picture shows the circuit board wired for continuous running for the Traffic Light Control circuit. Other modes of operation will be shown in diagrams lower on the page.

The price of the 20 Output Sequencing Circuit circuit boards is: $10.50 US each plus postage. (Each additional board is 10.00 dollars.)

All of the parts in the list above are available for this circuit.

The price for one (1) - 20 Output Sequencing Circuit kit is: 20.00 dollars US.

The price for one (1) - 20 Output Sequencing Circuit assembled is: 25.00 dollars US plus postage.

NOTE: Some of the components supplied with the kit are not from Mouser Electronics.

If you are interested in printed circuit boards and parts for this circuit, please send an email to the following address:

Your message will be answered as soon as possible.

20 Step LED Circuit

The next diagram shows a simple 20 LED driver circuit. Only one current limiting resistor is needed as only one LED can be on at a time.

Single Traffic Light Circuit

If you only need to operate a single set of traffic lights for a display, refer to Section 24 of the 555 Timer page at this site.

20 Step Traffic Light Schematic

The next diagram shows a traffic Light Control circuit.

As shown, the traffic light circuit allows the lights in one direction to be GREEN for 7 steps of the counter, Yellow for 2 steps and RED for 1 step before the light turns GREEN in the opposite direction. Other light sequence steps can be created by shifting the circuits outputs as long as the total equals twenty.

The RED signals of one direction are slaved to the GREEN, YELLOW and RED of the other direction though diodes.

Advanced GREEN lighting could be added but the flashing would have to be done externally to the PCB circuit.

The next circuit has the same function as the one above but can drive higher current lamps such as the #1157 automotive bulb.

Manual Control Circuits

Stop, Start and Reset

The next diagram and image shows external controls that can be used to Start or Stop and Reset the circuit. When the circuit is reset the 555 clock will stop and the number 1 output will go to a LOW state.

If the RESET terminal is held LOW the circuit will run continuously. The RUN terminal has limitations (CLOCK input of the 74LS107) that are described on the data sheet for the device.

The next photo shows the location of the RUN and RESET connections on the circuit board. A jumper normally between the RUN connection and the circuit common must be removed first. Also shown are 5 volt and common connections that can be used to power external circuitry.

If the 555 timer is removed an external clock could be used to step the circuit. Alternately the circuit's 555 clock could provide an output to and external circuit.

Manual Step

The next diagram shows external controls that can be used to Step the circuit manually. When the push button is closed the output of the 555 clock will go to a LOW state and the output of the circuit will advance by one step.

Resistors R1 and R2 have been removed from the circuitboard.

Other Circuits

Shortened Sequence Length

The number of steps in the sequence can be reduced by connecting an external resetting circuit to one of the outputs of the circuit.

The resetting circuit uses a 556 timer to provide complimentary HIGH and LOW outputs that are connected to the 'RESET' and 'RUN' terminals of the circuit board. In the example shown the reset pulse is approximately 0.1 seconds long but could be of any length as set by resistor R-R and capacitor C-R.

Holes and pads are already on the main circuitboard to facilitate the reset connections.

NOTE: Due to the nature of 555 timers, after a reset, the first clock pulse from IC 1will be slightly longer than the normal clock pulses.

The reset circuit's input is shown connected to output 1 - 5 but can be connected to any of the 20 outputs.

Switch S1 disables the shortened cycle.

Shortened Sequence Length For Paralleled Outputs

The next diagram illustrates how to connect the 556 resetting circuit when outputs of the 20 Step circuit are connected in parallel.

The diode at output 1 - 3 isolates the resetting circuit's input from the other outputs in the group.

Stopping The Output Cycle

The cycle can be stopped at a particular output by connecting that output to pin 7 of timer IC 1. When output 1 - 5 goes LOW it will connect pin 7 of IC 1 to ground and stop the timer.

When the cycle is stopped, the output of IC 1 will be forced HIGH and LED D1 will turn OFF.

The diode prevents the voltage at the output of the circuit from being fed back to IC 1 when the circuit is running normally.

Manual controls are used to reset and restart the circuit.

The Original 10 Step Circuit

The next diagram shows the 10 Step traffic light circuit from which the 20 Step circuit was developed.

A 40 Step Circuit

It is possible to have longer step cycles by using Shift Registers instead of the JK Flip-Flop as in the 20 step circuit. The following diagram is an example of a circuit with a 40 step sequence.

NOTE: This circuit is not complete and is presented for information purposes only.

Return to the Main Page

Please Read Before Using These Circuit Ideas

The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.


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