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CD4528 Power On issue

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electrookie

New Member
Hi all. I am using a CD4528 Dual Monostable Multivibrator and when it is first powered on, it starts the 1st clock. I want to get it to start up and NOT start the clock. It is being used as a 2 stage timer (4 stages shown in schem.) where the 1st timer must end (go low) before the 2nd timer starts. The attached schematic shows 2 4528's in use for 4 stage cascade timers.

2 problems exist. As stated above, at power on, both (all 4 actually) outputs are high and not necessarily for the designed time frame. I can open pins 3 & 13 and the outputs stay low at power up, but then the cascade will not start when triggered. So, I did this, then after power was on, I tied pins 3 & 13 to +V and the cascade starts when triggered. So what I need to do is replicate this sequence, bring pins 3 & 13 to +V after the rest of the circuit is powered up. So some kind of simple delay "should" work, but I can't seem to get it, with various ideas and designs I have tried so far.... HELP...

Now the 2nd issue is this: when the 4528's are built into the rest of the shown circuit, all the timers should NOT all start at the same time. They are supposed to cascade and NOT be re-startable till all timers are done. I'm getting confused trying to chase around the 1's & 0's to figure out why this is. Am I missing something here??? This circuit was originally shown in a Popular Electronics Mag. circa. 1995-1996. It look's like it should do as shown, but I have built it 3 times and know I have it wired properly, getting the same result every time. So I either built it wrong 3 times, or there is something else not properly shown in the design.

As always, I am not committed to the CD4528, I will use 2 555's (a 556) if it works. I just found this circuit and thought it was very cool and doable. Any other suggestions would be appreciated.

And I know that PIC's are the way to go for most of what I want to do, but I do not know PIC's (YET), and I want to get this all working now, not after I learn PIC's. So for now, NO PIC's Please..:D

Thanks guys....
 

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dougy83

Well-Known Member
I've never used the 4528, but in the truth table in the datasheet, I get the impression that the A input should be held low, and the B input should be used as the cascade timer start pin. This should give a timed positive going pulse.
 

mesafloyd

New Member
Pin 3 and 13 are neg true resets... you have them tied to +12... in that state they are non operational.. try tieing them together to a 1m res to +12 and add a .1uf (or whatever to Gnd... that will force the outputs to a known state (low) when power is turned on... then when this new cap fully charges it will release the timers to operate..

Floyd
 

electrookie

New Member
Thanks for the help, trying these is the next step...

Thank you dougy83. I guess that's like reading the instructions before trying to build that cabinet from Ikea... :) I did not review the truth table closely to notice this, nor did it occur to me to try pins 3 & 13 in different conditions. I will give it a shot and report back what happens.

Thank you mesafloyd. I did try something like that with each pin (3 & 13) but not combined. I will give this a try as well.

If these things work, excellent. If they both work, then I really have learned something new, and that is what makes a day a good day!

This leaves me with the other situation, which may be resolved by one of the fix's you guys advised me on. I will report back on this as well.

Thanks again fellows...:)
 

electrookie

New Member
Issues resolved about 4528

OK, here are the results of suggestions.

dougy83, putting pin 3 to gnd was what I had tried before, just not the way you pointed out. The result was the same, the output would stay off at power on, but I was not able to trigger the clock. Same thing visaversa.

mesafloyd, your recommendation was the ticket. I had tried the same thing with each pin (3 & 13) separately and it did not work. Doing it your way works like it is supposed to, including the second issue of both clocks starting together. I wonder why it did not work separately? But I will not stress over that because your advice works.

I sure appreciate this forum. I learn much from you all and get my ideas working. I would be scratching my head for days if not for your help.

I'll be back with the next issue when it comes up, and I am sure it will come up, whatever it is. Thanks all....
 

mesafloyd

New Member
While you were using the same RC valuse on both timers, one probably came out of reset at a different time... and started the sequence before the other. (Different RC Values due to tolerances.)
On any IC, when there are multiple gates/timers etc.. the thresholds will be the same on that chip. So putting a single RC on the two resets means they both will come out of reset at the same time...cux the thresholds are the same ;-)
You're probably OK to put all 4 of the resets on both IC's to the same single RC in your case.
Lots of luck to you amigo.

Floyd
 

mesafloyd

New Member
One last and final point... anytime you have a latch that could be in any state... if you do not control it, you cannot determine what state it will be in at power up.
Sooo, you need to have a power on reset(POR) to guarantee the state when power is turned on to properly begin your sequence.... else your are at the mercy of the planets alignment... ;-)
Floyd
 
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