New to the forum, first post. Also new to electronics, as you might notice
I am trying to get this simple timer/clock circuit working properly. A 555 generates the pulse, two 74HC393’s do the counting, and a 73LS08 sorts out the counting logic. I’m using LEDs tied directly to the Q pins via 250 Ohm resistors as the display. The timer displays seconds and minutes up to 20 minutes. (Of course you can look at the schematic and see all that.)
The circuit mostly works, except the 1’s column of the seconds resets at 8 instead of 10 as expected. Most of the time. Every couple of minutes it goes all the way to ten, rarely more than once in a row. I’ve tried swapping out the 7493’s and the 7408 to no avail. I’ve also tried swapping jumper wires, different holes in the breadboard. No change.
Each IC gets verified 5V (4.98V on farthest 7498), each Q pin gives me ~40mA (~20mA with LED and resistor in place). That’s all I’ve measured.
I didn’t get this circuit from anywhere, I just made it. It doesn’t seem like I’m missing anything, but clearly I am.
Anyone have any idea?
Thanks,
John
I am trying to get this simple timer/clock circuit working properly. A 555 generates the pulse, two 74HC393’s do the counting, and a 73LS08 sorts out the counting logic. I’m using LEDs tied directly to the Q pins via 250 Ohm resistors as the display. The timer displays seconds and minutes up to 20 minutes. (Of course you can look at the schematic and see all that.)
The circuit mostly works, except the 1’s column of the seconds resets at 8 instead of 10 as expected. Most of the time. Every couple of minutes it goes all the way to ten, rarely more than once in a row. I’ve tried swapping out the 7493’s and the 7408 to no avail. I’ve also tried swapping jumper wires, different holes in the breadboard. No change.
Each IC gets verified 5V (4.98V on farthest 7498), each Q pin gives me ~40mA (~20mA with LED and resistor in place). That’s all I’ve measured.
I didn’t get this circuit from anywhere, I just made it. It doesn’t seem like I’m missing anything, but clearly I am.
Anyone have any idea?
Thanks,
John
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