Single Digit LED Counter issues

[Mnye00]

Hi All,

As a bit of background, I am a total newbie trying to build what would seem like a simple project - a single digit LED counter (counting from 0 to9) with a reset switch. I have breadboarded it all and can get the LED to display 0 but can't get it to count up. The schematic is in the attached GIF and I was wondering if someone could take a few minutes and post how to wire the U1 (7805) and U2 (555) portions of this. I think that my issue is somewhere in here.

Thanks in advance for your assistance.

 

[tollinjose]

connections in the circuit shown in your picture is clear and simple. where you got confused ?

Have you test this circuit using any simulator ? if no try that. Ensuring that you will get confidence to try in real hardware.

 

[(*steve*)]

Are you using actual TTL chips (7490)? Or are you using CMOS (74HC90)?

What are the *exact* part numbers of the chips?

 

[davenn]

show us a photo, sharp and well lit so we can see if you have made a wiring mistake

cheers
Dave

 

[Mnye00]

Thanks, I will post a pic when I get home tonight.

I believe that the chips are 74LS47N and 74LS90N but I will confirm later.

 

[(*steve*)]

OK, LS is fine, and long as it's not HC, C, HCT.. anything with a "C" in it.

 

[Mnye00]

Pics posted

OK, so my partial parts list is as follows:

1. MC7805ACTG
2. SN74LSA47N
3. SN74LS90N
4. LM555CN (CMOS?? - could this be the problem)

I am getting the LED to illuminate but don't get any response from the switch intended to initiate the count so maybe the 555 is the issue?

I have attached some pics of the build in stages. Thanks in advance for any help.

 

[mursal]

I think pins 2,3,6 and 7 have to be pulled low on the 7490 to get it running? Click
But I cant find the data sheet to set the count up for module 10

Also are the LED resistors in series with the LED's or are they the resistors shorted out?
Be careful with switch bounce, that you don't get multiple edges for one switch closure
And 7447 driver is common anode (+) Click

If all else fails, break the circuit up and fault find the individual parts. For example, if you pull out the 7447 and ground (0V) each output going to the 7 segment display will the LED's light up?

 

[Mnye00]

OK, I did as suggested with the 2,3,6,7 on the 7490 and now I can get the LED to show different digits but they are random and no pulses from S2 change the display.

The LED resistors are in series with the LED, yes.

I will check into the bounce issue.

Thanks

 

[Arouse1973]

You are trying to run a BCD to seven segment display who input to the BCD IC is in binary counting mode. I think you might need to connect pin 12 and 14 BCD mode and input goes into pin 1.
Thanks
Adam

 

[mursal]

That's good news ........

You could disconnect from the counter and hard wire the inputs to the LED Driver chip, to check that the chip/display is displaying the correct number

0 0 0 0 = 0 All LED's on except g the middle one
0 0 0 1 = 1 led b and c turned on to display a 1
0 0 1 0 = 2 should be dispayed
ect ect all the way up to 9

If that works, reconnect and see what binary is coming out on the counter

 

[Mnye00]

Ok, so I did the check and the output to the display is correct. I reconnected and the display shows 0. Does that help? Still no counting up happening when depressing the switch.

 

[Arouse1973]

The only other thing that I can think of if everything else is as it should be. So if your input are changing then I would suggest you have damaged the chip somehow or you don't actual have a physical connection from the output of the counter to the BCD inputs.

 

[mursal]

That a step in the right direction

Disconnect pin 14 on the 7490 from the 555 first
Check the output pins on the 7490 to see if they are counting? Check pin 12 goes high then low, when you toggle (high/low) the clock input to the 7490 (Pin 14)? You can put LED's on each of the 4 outputs to watch the binary count if you want. Don't forget the LED resistors.

You will have to be careful not to get multiple edges (switch bounce) when you connect the flying lead from pin 14 to 0volts, or you will jump digits on the count. Pin 14 will float high when unconnected, but for the test, you can put a 1000 ohm (1K) resistor across pin 14 to the + rail if you want. Then just connect the flying test lead to the 0V rail to increment the output count.