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.
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
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
Circuit Board Parts Placement Diagram
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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.
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 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.
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.
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.
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.