Proteus schematic, help needed please

[the_dynamo]

Hi,
I have made a circuit in proteus, but i ned a little help in understanding it and making it work. It is a simple 2x seven segment display counter with 2x 555 circuits. I need help controlling the speed it counts. Im not sure if there is a problem with my poor math.

If for example I needed to make my displays count at one count per second, how would I achieve this? I believe it would be by adjusting the R1 and R4 resistor values. I have tried this but can not work out the relationship between them and the timing.

I have hooked up an oscilloscope (image included) and I am not sure what I am looking at. Would I be correct to say that ChA (pin3 on U1) controls the period, and ChB (pin3 on U2) controls the frequency? If this is correct i'm still not sure how this affects the counting and how I would work out my resistor values to achieve a certain speed.

I have included two images, one of my circuit and one of the oscilloscope. Thanks in advance if you can enlighten me of the math so I can adjust my timing circuit. Thats all I need, just a bit of help understanding and/or the equation so I can vary its speed. Let me know if you can do this? Thanks loads guys.

 

[ravindragudi]

You have rigged up 555 IC in monoshot configuration which generates a single pulse. Thats what you are seeing on the scope.
You need to use the astable configuration for 555 timer IC to generate the clock.
Use this link so you can get started quickly - http://www.ohmslawcalculator.com/555_astable.php

 

[Audioguru]

Your schematic will be MUCH easier to see if you turn off the (*&%$#@) grid lines.

 

[Allen6502]

I redrawn the top half of the schematic and I think it is easier to see what is going on.

The first 555 is configured as astable oscillator running at 73.698Hz with H=12.7ms and L=84uS.

The second 555 is also astable but only runs when the reset pin (4) is high. At the frequency of 129 Hz before I changed R4 from 500K to 250K. The H=7.7ms and L=33uS.

As for the monostables 74LS123, there is nothing coming from their outputs. So I have no idea what they are used for.

From the formula Tw=0.45 x Cext (pF) x Rext (k), I get 67,5 uS. So there must be some wiring mistakes.

Allen

 

[ravindragudi]

I redrawn the top half of the schematic and I think it is easier to see what is going on.

The first 555 is configured as astable oscillator running at 73.698Hz with H=12.7ms and L=84uS.

The second 555 is also astable but only runs when the reset pin (4) is high. At the frequency of 129 Hz before I changed R4 from 500K to 250K. The H=7.7ms and L=33uS.

As for the monostables 74LS123, there is nothing coming from their outputs. So I have no idea what they are used for.

From the formula Tw=0.45 x Cext (pF) x Rext (k), I get 67,5 uS. So there must be some wiring mistakes.

Allen

You missed out two more connections. Connect pin 14, 6 of 74LS123 to GND. The circuit will then be complete.

 

[Allen6502]

Yes, I did connect the 2 pins to GND at first, but the result was the same and so I removed them.

I checked with the 74LS123 application notes and data sheet, And it says:

The Cext terminal of the LS122 and LS123 is an internal
connection to ground, however for the best system performance
Cext should be hard-wired to ground.

see P.2. of the motorola datasheet attached...

Allen

 

[ravindragudi]

So did you try simulation for the 'best performance condition' as given in the DS?

 

[Allen6502]

I'll let the OP to find out.:twisted:

Allen

 

[the_dynamo]

Thanks. Could you explain that last bt to me in more detail please?

"From the formula Tw=0.45 x Cext (pF) x Rext (k), I get 67,5 uS. So there must be some wiring mistakes."

What is Tw and Cext and Rext? I shall check my wiring. Thanks for helping an beginner.

 

[Allen6502]

So, you didn't even read the datasheets of 74LS123 when designing your circuit?

Read page 2 of the datasheet that I attached, carefully and you'll know what I meant...

Ask any question you dont understand after reading the datasheet.:?:

Allen