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20 Output Sequencing Circuit by IC digital

2015-01-10 14:07  
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I to see this circuit in web http://home.cogeco.ca/~rpaisley4/Traffic145.html
good idea circuit.

20 Output Sequencing Circuit by IC digital

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 JK Flip-Flop (74LS107) and four – dual input OR gates (74LS32) to control to the inputs to the two output drivers (74LS145) This arrangement alternately creates disallowed states in these devices. 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 LOW output states 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 economically make use of the open collector outputs of the 74LS145s decoder/drivers rather than having to use output buffer ICs.The TTL logic devices requires a regulated 5 volt supply but 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.

Source: http://home.cogeco.ca/~rpaisley4/Traffic145.html

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