2-Digit Counter Circcuit

Thread Starter

aac044210

Joined Nov 19, 2019
178
Hi:

I have now graduated to a 2-digit counter circuit. It works fine, except when I first apply the power, it doesn't
necessarily start at 0. Not sure if some circuitry is required to create a smooth transition on power-up. I am using
74LS90s and 74LS47s.

Thanks
 

dl324

Joined Mar 30, 2015
12,871
Schematic
Terrible schematic. I had to look up the pin functions.
clipimage.jpg
Since it's LS TTL, you should use a Schmitt inverter to hold the reset to 0 pins high when power is applied.
clipimage.jpg
When power is first applied, the capacitor will be discharged and will charge towards 5V through R1. The output of the inverter will be HIGH until C1 charges to the upper threshold voltage; at which point the output will be LOW.
 

Thread Starter

aac044210

Joined Nov 19, 2019
178
Terrible schematic. I had to look up the pin functions.
View attachment 223101
Since it's LS TTL, you should use a Schmitt inverter to hold the reset to 0 pins high when power is applied.
View attachment 223103
When power is first applied, the capacitor will be discharged and will charge towards 5V through R1. The output of the inverter will be HIGH until C1 charges to the upper threshold voltage; at which point the output will be LOW.
Thanks Dennis!
 

AnalogKid

Joined Aug 1, 2013
9,248
I think the Schmitt gate can be eliminated. There is no Reset input maximum pulse width or risetime in the datasheet. This makes sense since the Reset input goes to one input of an unclocked set-reset flipflop in each stage that has its own positive feedback. And I've never experienced a problem with hanging an R-C power-on-reset directly on the Reset input (without the Schmitt gate). To do this, reverse the positions of R1 and C1, and connect the R-C node directly to pins 2 and 3.

For LS-TTL, R1 can be 1K or less. For standard TTL, R1 can be 470 ohms or less. A 330 ohm resistor and 1 uF ceramic cap will work well for either logic type.

ak
 

Thread Starter

aac044210

Joined Nov 19, 2019
178
I think the Schmitt gate can be eliminated. There is no Reset input maximum pulse width or risetime in the datasheet. This makes sense since the Reset input goes to one input of an unclocked set-reset flipflop in each stage that has its own positive feedback. And I've never experienced a problem with hanging an R-C power-on-reset directly on the Reset input (without the Schmitt gate). To do this, reverse the positions of R1 and C1, and connect the R-C node directly to pins 2 and 3.

For LS-TTL, R1 can be 1K or less. For standard TTL, R1 can be 470 ohms or less. A 330 ohm resistor and 1 uF ceramic cap will work well for either logic type.

ak
Thanks Dennis and AnalogKid. I applied this circuitry to the 74LS90 for the MSB and it starts counting at "1".
Should the same be applied to the LSB 74LS90?
 

AnalogKid

Joined Aug 1, 2013
9,248
Yes. As in the post #5 schematic, one R-C network can drive both Reset pins. For standard TTL, decrease the resistor to 220 ohms.

ak
 

Thread Starter

aac044210

Joined Nov 19, 2019
178
Hi:

I have attached a schematic for a 4-Digit Counter Circuit for anyone that might
be interested. An improvement would be to use multiplexing but the package
count would be pretty much the same. however, the wiring would be simplified
and only 7 current-limiting resistors would be required as opposed to 28.

1606169984347.png
 

Attachments

AnalogKid

Joined Aug 1, 2013
9,248
Maxim used to have a bunch of multi-digit counters and display drivers.

AND - consider the CD4553. 3-digit counter and display mux logic in one package. Add a BCD-to-7-digit decoder and you've got 3 digits with only two chips.

ak
 

Thread Starter

aac044210

Joined Nov 19, 2019
178
Works great if you have them. I bought my stock in the 70's.
Hi Dennis. But they are hard to find and a Newbie might just as well work
with something that is readily available. I am using chips that I bought back in
the 70s during my university days. Now that I am retired, I am finally using
them. Actually, back in the 70s we had very few electronic parts suppliers in my
area but now we have a fantastic supplier with good prices.
 
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