Help: Word Display Circuit

[Cyborg86]

Hi all,

I would like to build a circuit that displays a 'HELLO' word randomly on five 7-segment LED displays. for example letter L shows first, then H, then O, then L then E, finally O, and next time the letters will show up in different order. I know that i have to use PRBS (Pseudo-Random Binary Sequence) generator to feed the circuit with random logical values. But i don't know how to design the whole circuit.

Is there anyone who has an idea about how to design this circuit ?

Thanks all,

 

[Marks256]

Use a PIC....

 

[Cyborg86]

I am a first year student, and I don't have background about PIC. my assignment is to desgin this circuit same as i described above.

So do u have any idea about how to design it like tht ?

Thnks,

 

[Nigel Goodwin]

I am a first year student, and I don't have background about PIC. my assignment is to desgin this circuit same as i described above.

So do u have any idea about how to design it like tht ?

If you're a first year student then it looks like you're in the wrong classroom?.

It's a simple job for a micro-controller, but NOT to do in hardware.

 

[Marks256]

Funny how in the first thread he said that he would like to design this circuit, and in his second post, the states that this is an assignment, so theoretically he has to design the circuit...

 

[Cyborg86]

I didn't say any thing funny.
i think it is the same if it i have to build it or i would like to build it. I asked for help and i described my problem.

Thanks anyway,

 

[Cyborg86]

Hi Nigel Goodwin,
Can you tell me please which class i should go to ?

Thanks.

 

[Marks256]

I think that may have been a rhetorical comment...

 

[k7elp60]

In my opinion it can be done with out a microprocessor. I have a circuit that I built using a hex inverter as 3 RC oscillators driving a 74AC138 1 of 8 data selector. I will be happy to help modifiy this to do as you would like. My circuit randomly flashes 8 LED's. The circuit could easily be modified to randomly flash the LETTERS H E L L O on 7 segment displays.

 

[Nigel Goodwin]

Hi Nigel Goodwin,
Can you tell me please which class i should go to ?

The one that gives out first year assignments!.

 

[grim]

so you need 1 of 5 outputs to come on, then second, the third etc. then clear, then to start again, but with the 4th then the 2nd then the 5th etc


so how about a clock running at a very high speed, connecting to a 4017 or some other counter, . you then need to stop the clock at varying times, to get a different number of outputs and hold it in a latch.

now if the display was powered from a dropping resistor, which generates heat, then a ptc or some other heat sensitive device would give a varying resistance....... you can also add a light sensing device, or any other components that will give a slight variation in resitance (or capacitance, or inductane, or voltage etc)

if the clock's running at 1MHz, then you won't need much of a percentage variation.

if you manage to get some variation into the high speed clock as well as the slow one, then it's even more randomised.

infcat your trying to design an unrelaibel circuit. which we all know is incredibly easy to do. inadvertantly

 

[Cyborg86]

Thanks Grim for ur comments.

k7elp60, i would be grateful if u can help me on this task. So, can u tell me how you are thinking to design my circuit ?

 

[k7elp60]

Thanks Grim for ur comments.

k7elp60, i would be grateful if u can help me on this task. So, can u tell me how you are thinking to design my circuit ?

I will help you with it. The first thing is to determne the segments of the displays you are going to use. I would recommend using common anode displays. For the letter H you would use segements B,C,E,F and G.
You figgure out which segments for the other letters and then tell me what the Vf is for the segments. The drive circuits will be easy.

 

[Cyborg86]

All right I will figure out the segments for the other letters. but wht does Vf stand for ?

 

[k7elp60]

All right I will figure out the segments for the other letters. but wht does Vf stand for ?

Vf means to forward voltage of the LED segment. It would be wise to select the displays. The displays should be common anode that are not multiplexed.
The Vf will determine the value of the drive voltage and the current limiting
resistors.

 

[Cyborg86]

I figured out the segments for the letter and the vf of the display. All details are included in the document attached.

What is next ?

 

[k7elp60]

I figured out the segments for the letter and the vf of the display. All details are included in the document attached.

What is next ?

The next thing is to determine the current limiting resistors for the displays.
I used this formula to calculate the following resistors. Vcc=9V Vf=2.5
and If = 3.33Ma and If = 10Ma.
R=(Vcc-Vf)/If. The closest standard values are 2.0k and 680 ohms.
This test will determine the drive circuit for the displays. My circuit as it stands will drive 20 Ma. It is possible that the displays will be bright enough with only 3 Ma per segement, but if not then the 10 Ma per segment should be adequate as per the spec sheet.
What you need to do is get 6 ea 2K ohm and 6 ea 680 ohm 1/4 watt resistors and connect up one display to a 9V battery as shown on the attached schematic. Then tell me if the 2.0k resistors are okey or you need to use the 680 ohm resistors.

 

[Cyborg86]

I've done the test for the display with two values of resistors. 680 ohm resistors are suitable for my 7-segment LED display.

I already know how to build the 555 timer to feed the circuit with clock pulses. I only need to know how I can make the binary numbers count randomly so that the message 'Hello' appears in random order.

Thanks,

 

[Cyborg86]

I will use TTL ICs for this circuit, and as far as I know TTL ICs need 5 volts to operate. SO, if i want to use 9 v battery i have to connect the ICs through resistors before i connect them to the battery in order to to drop the voltage down to 5. And that also applies to the 555 timer chip. Am I right ?

 

[Nigel Goodwin]

No, use a 7805 5V regulator to get 5V from your 9V supply. But if you use CMOS chips you can use the 9V supply directly, and they will be cheaper then TTL and use MUCH less power.