Traffic Light Control Circuit
20 Output Sequencing Circuit
As logic circuits go, the 20 Step circuit is fairly simple but due to the nature of the TTL Logic devices used, care must be taken when wiring these circuits. Simply put; The neater the wiring the better.
NOTE: The 20 Step Circuit does not work in circuit simulation programs. The likely cause is that this circuit uses input states that force the outputs of the 74LS145 drivers to be turned off. These states would normally not be used with these devices and are probably not programmed into the simulator's software.
If you would like to make use of these circuits, take the time to find and read at least the first 2 pages of the manufactures datasheets 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.
20 Output Sequencing Circuit
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 is an unusual logic scheme but it allows the circuit to make economical use of the open collector outputs of the 74LS145s decoder/drivers rather using output buffer ICs that are driven by 74LS138 logic devices which have eight steps.
- The TTL family devices used 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 stopped and reset externally as shown in later diagrams.
74LS145 Equivalent Output Circuit
20 Output Circuit - PCB Parts Placement Diagram
Circuit Board Parts Placement Diagram
20 Output Sequencing Circuit PCB And Parts
20 Output Sequencing Circuit PCB - Assembled Example
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 ICs for the 20 Step circuit board are not available at this time.
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20 Step LED Circuit
Single Traffic Light Control Circuit
20 Step Traffic Light Schematic
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
If the RESET terminal is held LOW the circuit will run continuously. The RUN terminal has limitations (CLOCK input of the 74LS107) that are explained on the data sheet for the device.
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 And Reset
When push button S1 is closed the output of the 555 clock will go LOW and the output of the circuit will advance by one step. When DPDT switch S2 is moved to the right the circuit is reset and counting cannot advance.
NOTE: The variable resistor R2 has been removed from the circuitboard.
Additional 20 Step Circuits
Shortened Sequence Length
The resetting circuit uses an external 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.
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
Stopping The Output Cycle
When the cycle is stopped, the output of IC 1 will be forced HIGH and LED D1 will turn OFF.
Manual controls are used to reset and restart the circuit.
10 and 40 Step Circuits
10 Step Circuit
40 Step Circuit
NOTE: This circuit is not complete and is presented for information purposes only.
Please Read Before Using These Circuit Ideas
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.