Simple timer circuit operating unexpectedly

Thread Starter

HughesBombs

Joined Jan 4, 2019
6
New to the forum, first post. Also new to electronics, as you might notice :D

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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dl324

Joined Mar 30, 2015
10,536
Welcome to AAC!

The first thing you're doing wrong is that you're heavily loading the counter outputs while also trying to use them as inputs to other gates.

74HC will sink/source about 4mA at Vcc=5V. You're trying to source about 3X that. That's just asking for problems.

What's the purpose of the diodes on the outputs of the AND gates. You're already asking for problems by mixing 74LS and 74HC because TTL outputs aren't guaranteed to drive CMOS inputs without pull-up resistors and you're exacerbating the problem by adding a diode drop.

The symbol you're using for the AND gates just obfuscates circuit intent. I'll redraw it later.
 

Thread Starter

HughesBombs

Joined Jan 4, 2019
6
Thanks, dl324, for the reply!

Ignore the diodes. They were something I was trying. I meant to remove them in the diagram. Also, the schematic shows only the one 7493 and 8 LEDs, the actual circuit is just like that but there are two 7493’s and 16 lamps. I promise to do better with the schematics next time.

As far as overloading the outputs, I was kinda afraid I might be pushing it. The original circuit I was working on involved 7447s and 7-segments. That was a huge mess. I thought I’d try a simpler method to get the logic straight, then move on to the original design later.

I was unaware of the CMOS vs TTL mixing issue. Still have lots of learning to do, huh? But why would all other “digits” work but not the first one?
 

dl324

Joined Mar 30, 2015
10,536
Redrawn schematic:
upload_2019-1-5_21-0-8.png
Normally we draw so the flow is left to right and top to bottom, but since you're displaying binary output, it makes more sense this way.

I don't see off hand why you're seeing problems with the first counter.

74HC393 is a ripple counter, so it's possible to get some glitches when decoding the outputs. The standard way to get around that is to AND the reset signal with the clock.

Do you have decoupling caps around each IC? I don't usually bother with them, but I add some when I have some unexpected behavior.
 

Sensacell

Joined Jun 19, 2012
2,567
What is the purpose of the diodes?

They actually leave the inputs FLOATING when the output driving them is LOW.

This would leave the nodes undefined.

(EDIT- missed the previous posts)
 

Thread Starter

HughesBombs

Joined Jan 4, 2019
6
Yes I have 100nF decouplers at each IC.

I was under the impression that pull up resistors were used only on unused inputs so they’re not floating? I may need to hit the books a bit more first huh?
 

dl324

Joined Mar 30, 2015
10,536
I was under the impression that pull up resistors were used only on unused inputs so they’re not floating? I may need to hit the books a bit more first huh?
High output for 74LS is guaranteed to be around 2.7V. High level input for 74HC is about 3.2V, worst case. Typical parts will be compatible, but you can't guarantee your part is typical without testing.

Unload the counter outputs by increasing the current limiting resistors to 2k. You don't need the LEDS to be very bright to monitor output state.
 

Thread Starter

HughesBombs

Joined Jan 4, 2019
6
High output for 74LS is guaranteed to be around 2.7V. High level input for 74HC is about 3.2V, worst case. Typical parts will be compatible, but you can't guarantee your part is typical without testing.

Unload the counter outputs by increasing the current limiting resistors to 2k. You don't need the LEDS to be very bright to monitor output state.
Makes sense. This all may explain the wacky behavior when using the 7447s before. Anyway, thanks for the info. I’ll give it a try after while.
 

dl324

Joined Mar 30, 2015
10,536
This one of the best TTL to CMOS interfacing descriptions I've come across:
1980 RCA COS/MOS Databook, page 625
upload_2019-1-6_13-25-51.png
 

Thread Starter

HughesBombs

Joined Jan 4, 2019
6
This one of the best TTL to CMOS interfacing descriptions I've come across:
1980 RCA COS/MOS Databook, page 625
You’re right that IS a good description. I’ll apply this info to the circuit and may even be able to get it to run better with the 7447s like I wanted originally. (And I’ll add that entire book to the library!) I greatly appreciate your help and insight and look forward to using AAC.
 
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