7-Seg Countdown Timer Help Needed

[NoOneYouKnow]

Alright, a few months ago I needed help on a count down timer I was working on for 4 digits, and got it working perfectly. Well, now I need to reduce that to two digits, and I really have no clue to reduce this circuit down. I tried a few things on breadboard, but couldn't get it working at all. I just need to remove the seconds digits. I know I could just not connect those two 7-segs, but there's still the 2 74HC192's that are there.

Also, if possible, can it be setup to stop at 00? Right now it stops once it hits 00, but it flips back to the pre-set number. I need it to display 00 until the circuit is reset.

I'd appreciate a quite response if it wouldn't be too much trouble, this is fairly important.

ORIGINAL:

**broken link removed**

 

[HS3]

I am not clear on what you are trying to do. That circuit has two digits for the minutes and they can be preset manually and it has two digist for the seconds which cannot be preset. Which two do you want to get rid of?

A divider will usually work by detecting the last count (zero in this case) and resetting to the original state automatically so it is not trivial to have it stay at zero. Or maybe it is... what you could do instead of resetting everything is not reset the chips but interrupt the clock signal from the 555 oscillator.

 

[ericgibbs]

hi,
If you want to completly disable the seconds count, make the changes in RED.

When the start switch is pressed [momentarily] the LOAD is pulled low which loads the minutes with the BCD switch settings.

It also sets the 1c10/11 latch which enables the 555 count output [ the VR1 pot is set for 60secs]. The output of 1c12/6 is connected to the DW of IC2.

When both the minutes counter BORROWS go low [zero terminal count] then ic10 gate resets the ic10/11 latch, which stops the count at zero.
Ground ic12 pins 13 and 2.

The BCD values will not be reloaded until you press Start.

I would leave the seconds counters in circuit and keep the 555 timer frequency the same ie; 1sec.
Just unplug the two 7seg seconds LED's and move the LOAD wire, this would give the count to zero and wait for Start press.

The problem with removing the seconds dividers and making the 555 a 1 minute rate, is that the count down error could be as much as 59secs as the 555 clock is not in sync with start press. Using the 555 as a 1 second timer, the error would be around 1 second.

Do you follow that.

 

[NoOneYouKnow]

Thank you very much, that worked rather well. Here's how I have it, I completely removed the seconds portion.

**broken link removed**

The 555 timer is in another portion of this circuit, which I have a question about below.



I designed this portion myself, but it doesn't seem to work correctly, most likely because of my lack of knowledge dealing with transistors. Anyhow, here it is. The LEDs in the diagram don't work. Everything else works like it should, but the LEDs don't light. Any ides?

**broken link removed**

 

[ericgibbs]

hi,

It appears that you have TR2 and TR3 connected as a Darlington pair, with the LED's in the emitter line.
At best, at the emitter of TR2 will be about 3V.
Assuming the LED's have a Vforward drop of 2V, this gives about 1V across the 1K9 which gives around 0.5mA thru the LED's.
[Usually small LED's require 10 to 20mA]

Looked thru the 556 cct, what exactly are you expecting the circuit to do.?
Whats the 'Timer' connected too.?
Purpose of the two push switches.?

Is that a set of relay contacts.? [R2/5]

Regards

 

[NoOneYouKnow]

Sorry, I don't know all the specifics about drawing schematics, I just draw them to where I can understand them. Heh

Now I see the problem with the LEDs, I calculated the LED resistors as being from the full 6v, at 20mA.

The way this circuit is suppose to work is like so. The 555A Astable will maintain a constant pulse rate, around 7Hz. When the lower switch is pressed, it activates the 555B Monostable, keeping TR3 high long enough for three pulses from 555A to go to TR2. The "Timer" output between the two is there to drive the first circuit I questioned you about before, as an add-on. TR2 is there solely to drive the LED cluster.

R2/R5 was something I was just fooling with to change the duration of the 555B Monostable to allow either 3 pulses from the 555A, or 1 pulse, but I'm not using the portion of this circuit anymore.

 

[ericgibbs]

I questioned you about before, as an add-on. TR2 is there solely to drive the LED cluster.
.

Hi,
Are you asking me for input on this LED part..

 

[NoOneYouKnow]

No, sorry lol, just explaining how it's all suppose to work. I see what's wrong now since you explained that it's about 3v at the emitter or TR2.

Thanks for all your help. ^_^