Well, when a problem comes up like this, it's time to solve it! Having several people throw out different ideas helps stimulate more ideas.you guys are bouncing ideas off each other like its playing cards!
I'll take a stab at it. By the way, Zener diodes were named after Clarence Melvin Zener, who discovered the effect - so Zener is a proper name, and should be capitalized.Who would be willing to explain the Zener diode config for me? I had already read, and just reread the Zener diode portion on this site, but SgtWookie has me all turned around (not difficult to do).
The Zener breaks down at it's rated voltage, ±a few percent. It acts as a voltage clamp. Once the Zener voltage has been reached, the diode will start conducting in order to keep the voltage from rising higher.If not reversed biased, the Zener acts like a reg. diode (-.7V) across. When reversed biased, assuming the supply voltage exceeds the Zener voltage, the zener only allows the zener voltage through.
In this case, the order does not matter. Basically, all we're looking for is a way to make up the difference between the 23.5v where the PNP transistors' base will be in cutoff, and the 9v where the DSx outputs of the 4553 will be. 23.5v-9v = 14.5v. The 5.1v and 12v Zeners together are 17.1v; so we have a 2.6v margin for error - which is good.So, in SgtWookies circuit, the 5.1 zener comes before the 12 zener correct?
Yes.And they are both reverse biased based on the 24V supply.
No, the 5.1v Zener drops ~5.1v across itself, so 5.1v is simply subtracted from the 24v. Same thing with the 12v Zener; 12v is subtracted.So wouldn't the 5.1 reduce the voltage that the 12 sees down to 5.1V, therefore causing the 12V zener to act like a regular diode (-.7V)? Am I all horsed up or is that the desired effect?
I second this request if I may. If I'm reading SgtWookie's circuit correctly, R2 is a pull-up resistor designed to keep the PNP transistor based pulled high and thus off whenever the DS output is high (off).Who would be willing to explain the zener diode config for me? I had already read, and just reread the zener diode portion on this site, but SgtWookie has me all turned around (not difficult to do).
Yes, that's correct.I second this request if I may. If I'm reading SgtWookie's circuit correctly, R2 is a pull-up resistor designed to keep the PNP transistor based pulled high and thus off whenever the DS output is high (off).
Yes, very good.After reading the zener diode section here as well, I assume putting two zener diodes in series is equivalent to putting two resistors in series - you add the breakdown voltages together. So, as shown, there will be 12 + 5.1V = 17.1V between the junction of ZD1-R1 and ground when the DS output is low. Subtracting this from the 24V connected to the base of the PNP through R2, we get 24 - 17.1 = 6.9V. Alternately, if we use a single 16V zener, we get 24 - 16 = 8V.
When the DSx output is high, there would be very little current flowing into the 4553's DSx pin. When the DSx output is low, the current sunk depends largely on the Zeners and R1, and somewhat on R2. This is assuming that the 24v and 9v supplies are as advertised; always their respective voltages.This is the voltage left that would pass through the 4553, correct?
It's not to protect the 4553; it's so that the PNP transistors' base can rise to be within 0.5v of 24v, allowing it to turn off. Otherwise, the PNP transistors would be stuck "ON" all the time.Currently, the logic is powered with 9VDC. So using the zeners as shown drops the voltage from 24V to 6.9-8V to fall just below the supplied 9V logic, thereby preventing damage to the 4553. Is this close or am I way off base?
Yes.I assume R1 is a current-limiting resistor.
Yes.Alternately, if the DS output goes low, 17.1V is dropped across the zener diodes and 6.9V passes through the 4553 via the DS pin to ground, pulling the base of the PNP low and allowing the digit to receive power.
Ahhh, "safe from the 24v" isn't how I'd put it. There would be very little current flow through the Zeners, as 15v is less than 17.1v, so neither of the Zeners would be in breakdown. There would be a small amount of current, but R2 would still return the base to nearly 24v.When DS is high, the output is approximately 9V. R2 is pulling the PNP base high, thus providing 24V. This 24V still goes through the zeners to DS. There is a voltage difference between the DS output (9V) and the voltage at the base of the PNP (24V), thus 24-9 = 15V. However, with the addition of the zeners, there must be a voltage greater than 17.1V before they will conduct. Thus 17.1V > 15V and the DS pin is safe from the 24V when high.
As mentioned already, it's really not protection-oriented.Okay, so if I'm not totally off base in my understanding, are the Zeners used to protect the DS pin and thus the 4553 when high or low or both?
Good point. I meant to say protect the 4553 from 24V (over-voltage condition and potential damage to the IC). I guess that is not the concern so much as:Voltage doesn't "pass through" things; voltage is dropped across things. Current passes through things.
Excellent! This makes sense to me. Thank you!It's not to protect the 4553; it's so that the PNP transistors' base can rise to be within 0.5v of 24v, allowing it to turn off. Otherwise, the PNP transistors would be stuck "ON" all the time.The idea is to allow the current flow through R1/R2 to fall low enough that R2 can return the base to nearly 24v.
Done - Started doing it then got lazy. Back on the straight and narrow path now!So CoachKalk, you'll want to:
1) Add designators to all of your parts in your circuit (R1, C1, U1, etc.) as Bill mentioned earlier.
I am waiting patiently for my components. Emailed shipping info almost a week ago, now I am like a kid @ Christmas time. I have already noted the possible changes to the Zener's and resistors when it is GO TIME!2) Tweak the values of the zener diodes if you end up using the 20VDC power supply you mentioned earlier and/or if you change the logic to something other than 9V, such as 12V or 15V.
Fixed - Not sure when I horsed that up, maybe wrong/typo from the beginning.3) The transistors in your latest circuit should be labeled 2N2907 not 2N3907.
Your suggestion for a debouncer is the second time I have read that. The last poster mentioned it should work without it, but I may have to hit the button twice? I was even more overwhelmed then than I am now, so I figured hitting it twice was doable. Now that I am slowly putting the picture together, I would be very interested in a debouncer circuit.4) You may want to add a switch debouncer between your switches and the CD4013. I'd suggest using a CD4093 with a couple of resistors and capacitors. I can provide an example if you'd like.
I have noted this point. I have been printing all Datasheets for the IC's I am using so I will take a look and add the resistor (after of course I check with you guys here to make sure I understood the datasheet correctly).5) Add a 10kΩ pull down resistor between pin 12 of the CD4060 and ground - assuming the 4013 outputs a high signal to start the CD4060. If it outputs a low signal to start the CD4060, then put the resistor between pin 12 and Vcc.
I quickly realized pins were sometimes left off and/or placed in differing spots. I do have the datasheets, but I will look at adding pin notations as well. I think (crossed fingers) I have all the necessary bypass capacitors inplace. I am trying to get my head around the placement of resistors (the ones that all of you "know" should be placed between inputs and ground) but I don't even think about them until it is noted that I missed them. One day, one day I say ... I will get it!6) I recommend labeling all of your IC pins with their respective pin numbers. When you go to troubleshoot your circuit, you won't know what pin LE is, you'll need to know what pin number you need to look at.
7) You mentioned earlier that many of the schematics you've come across don't show Vcc or ground. Many don't include filter capacitors as well. This is not a standard practice so much as an attempt to keep things looking neat and clean (or laziness, but we'll give them the benefit of the doubt). For the same reason, people often draw their ICs, as you've done below, with the pin locations all around the IC, not as you would see them on the actual part. It is up to you how you wish to draw your schematics, but I always try to draw the ICs to match the physical part and always include everything I put on my circuit, including Vcc, grounds, filter capacitors, etc. The schematic doesn't always look as pretty as others, but you have everything in front of you and because the ICs drawings match the physical ICs, I can more readily see how I need to lay out my board be it a bread board, protoboard or custom PCB. Just my two cents on the matter.
Like Bill said, He did it. I am just a good old-fashioned copy-catter!I'll have to look at the start and stop functions as shown in your diagram for my own benefit. I'm not familar enough with the 4013 to say if this will work or not. Has Bill blessed this portion?
You shouldn't have to hit a button twice. Because switches are mechanical, they can have "bounce", meaning a bunch of high and low signals (noise) is generated as you press the switch due to mechanical action. ICs operate faster than we can blink, so having a bunch of high and low transistions can "fool" the IC into thinking it is receiving a lot of pulses in short amount of time even though you're just pressing the switch once. The result is your display may appear to jump to some random number when you press the switch instead of just incrementing by one. You can use a resistor and capacitor combo (better than nothing) or an IC such as the CD4093 with a 0.01uF cap and 10kΩ resistor.Your suggestion for a debouncer is the second time I have read that. The last poster mentioned it should work without it, but I may have to hit the button twice? I was even more overwhelmed then than I am now, so I figured hitting it twice was doable. Now that I am slowly putting the picture together, I would be very interested in a debouncer circuit.
Ooo, I didn't know a flip flop could work as a debouncer - nice.The flip flop will change states the moment the button makes contact, and stay that way.
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