Hi,

Can anyone help me. I made a 24-hour clock with 8 seven-segment displays (with 1/100th of seconds) on a breadboard. It works but i noticed a strange effect on my pulse.

I use a 555-timer for 1.6 kHz signal and with a simple binary counter i divide it to 100 Hz. When i put it on my scope i get a nice clean pulse but as soon if i connect it to my 24h clock it starts to wobble. I tried to place capacitors but it doesn't work.

What can be the reason of it? Thanks
Stefan

 

Hi,

Can anyone help me. I made a 24-hour clock with 8 seven-segment displays (with 1/100th of seconds) on a breadboard. It works but i noticed a strange effect on my pulse.

I use a 555-timer for 1.6 kHz signal and with a simple binary counter i divide it to 100 Hz. When i put it on my scope i get a nice clean pulse but as soon if i connect it to my 24h clock it starts to wobble. I tried to place capacitors but it doesn't work.

What can be the reason of it? Thanks
Stefan

Pictures, please. "Wobbly" has no recognized meaning.

 

Unless your circuit is compact the wiring will be picking up a fair bit of mains interference. Mains frequency (50Hz or 60Hz) would beat nicely with 100Hz. That could account for the 'wobble'.

 

Pictures, please. "Wobbly" has no recognized meaning.

I suspect he is alluding to this:



From TTL Cookbook by Don Lancaster.

 

555-timer IC makes for a lousy time-base for a time-of-day clock.

For better accuracy, use the 50Hz signal out of your wall outlet (after proper isolation with a step-down transformer, of course).

 

thanks all.
But i think i found the reason. Set my powersupply to 5 V but with 8 digits i get 600 mAmps. The voltage at my board drops to max 4,1V?? I have 2 different powersupplies. Both of them drops.
Is that normal for a PSU?
When i raise the voltage to 5 V on my board the clockpulse is much, much steadier.

I thought i get 5V if i ask for it. I have a Velleman 3005N and a programmable 3005D

Stefan

 

Is that normal for a PSU?

Not for the output of a good quality one rated to supply more than 600mA. What is the voltage at the supply output?

 

Now i have a 680 mAmps and i get the supply voltage to 6,20V !!! to get a 5V on the board. Not good. I have the possibility to send it back within 30 days. I think i'm gonna do that. For this clock i made for testing i uses a 4511 decoder. When i'm fooling a bit with the output enabled then the voltage raises to 6,20V if i'm turning off all these digits . In that case i burn some other IC's.

 

Now i have a 680 mAmps and i get the supply voltage to 6,20V !!! to get a 5V on the board.

If the supply is 6.2V and the board gets 5V then something between supply and board is dropping 1.2V, implying a resistance of 1.76Ω. Looks like the problem may be there, rather than in the supply.

 

never thought of that. Just a semi - beginner in electronics. thanks. i shall look at it tomorrow. I can enables the digits one by one and see what happens. Thanks Alec

 

The Vellman supply has a current limit adjustment. If it is turned down to less than 600 mA, the output will sag at that current.

With no load on the supply, adjust the voltage to 5.0 V and turn the current limit all the way down.
Apply a dead short to the power supply output.
Adjust the current limit until the output current meter reads 1.0 A.
Remove the short. The supply now will hold at 5.0 V out for all load currents up to 1.0 A.

ak