555 monostable to trigger a 555 astable

Wendy

Joined Mar 24, 2008
22,141
I am a strong believer in try it for yourself. I have a youth group meeting tonight (DeMolay), so I won't be able to get back to it today.

You probably notice I'm trying to teach you how to think and design the circuit. Teaching is as much fun as designing. If you make a mistake it is no harm or foul, I just explain where I see a problem and show a solution. :D This is meant to be fun for everyone.

The only question I have is what I would label U3. A hysteretic circuit such as what is shown in the 555 Schmitt Trigger will have a dead zone between 1/3 and 2/3 volts. If you put the voltage ½ Vcc then it will act like a set/reset flip flop. The only problem is I don't know where it will power up. If it powers up the wrong way then I'll need to add a RC circuit on the reset pin (4) to automatically clear the chip during power up.

I hate adding extra parts, but sometimes you gotta.

I would use 555 chips for protoboarding, but if you plan on making a permanent version I would switch over to a dual 555, a 556, for the permanent copy. That way it would only be 2 chips.

The resistors on the Set FF can be large, as in 100KΩ. The LED I put on it is basically a simple diagnostic that can be eliminated later.
 
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Thread Starter

thedude123

Joined Aug 29, 2011
39
No problem Bill. No rush, I'm just glad we are making progress.
Yes I am learning alot. Honestly though even if you just posted a schematic I would analyze it to pcs.
metering the outputs and watching the results. Yes, I am the guy who takes stuff apart and investigates
And yes, I fight with the wife about throwing anything away without first doing an autopsy.

I do thank you for making me think. I will get more out of this project in doing so.
 

gerty

Joined Aug 30, 2007
1,281
Here's a little .gif file that shows what is going on inside a 555. I hope the animation works.
I am not the creator of this, I only wish I knew who it was so that I could thank him/her. It has helped me to explain to my students what is going on inside a 555.

click the thumbnail to start animation
 

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Wendy

Joined Mar 24, 2008
22,141
Something else came up, so I will have to get back with it tomorrow. If you finish the schematic I will be glad to critique it.
 

Thread Starter

thedude123

Joined Aug 29, 2011
39
Something else came up, so I will have to get back with it tomorrow. If you finish the schematic I will be glad to critique it.
Bill,

I was under the impression that you wanted me to wait until you looked into it some more. I can go ahead and breadboard the latest schem tonight and see what happens.

Could you advise on which type of transistor in the schem? I'll look into them a bit and see if I can figure out a proper one.
And are the diodes your using the typical 4148 switching diode?

I have everything on hand except for the transistor. I can pick it up this evening.

Thanks!!!
 

Wendy

Joined Mar 24, 2008
22,141
I'd get a pack of them, if you stick with this hobby they will come in handy.

I was just promoted to a moderator, which means my plate has extra helpings of spam. Well, not really, but it sounded cool. Actually I'm learning another side of this board. Since I love to write I asked to keep the E-book Developer title up.

Basically I enjoy designing and drawing schematics. I'll be finishing yours this evening. There are some design features I've never used quite the way I've drawn it, such as the 555 flip flop. This came from another idea I came up with about a year ago...

555 Bistable Multivibrator

I will add a RC network to yours to guarantee it powers up the right way, but it may not be needed.
 

Thread Starter

thedude123

Joined Aug 29, 2011
39
Congrats on the promotion to moderator. Yep, your gonna get busier for sure. I too am a moderator on a different forum.

I have a pack of the 4148 diodes so if they will work I'm set there.

As for as the transistor, do you mean get a pack of misc ones or a pack of the most common type? My understanding of the use of transistors is to amplify the current. I will study up on why we need one there.

Sounds like we are both learning a bit due to this circuit requirement,,, me more than you obviously,
that's a good thing!!

Thanks again!!
 

Wendy

Joined Mar 24, 2008
22,141
As far as current goes, a 555 is very robust. It is rated for up to 200ma (0.2A), though I would keep it down to 100ma. One of the reasons I'm going through some of the contortions is to keep parts count down. I only use transistors when I have to.

Now and then I come up with something I think is new, only to find it is old. Well, it was new to me at the time.

I've got to learn to slam spammers better. They are the enemy, I must kill... ban them!
 

SgtWookie

Joined Jul 17, 2007
22,210
Here's a little .gif file that shows what is going on inside a 555. I hope the animation works.
I am not the creator of this, I only wish I knew who it was so that I could thank him/her. It has helped me to explain to my students what is going on inside a 555.

click the thumbnail to start animation
That .gif came from the Williamson Labs 555 tutorial website:
http://www.williamson-labs.com/555-tutorial.htm
If you perform a Google search for "555 tutorial", that page is 3rd on the list.
There are several different speeds of animation available on that page.
 

Wendy

Joined Mar 24, 2008
22,141
OK, final schematic. Took a little longer than planed (all the way around), so I went ahead and calculated the values needed.

 

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thedude123

Joined Aug 29, 2011
39
Bill,
This looks like alot of effort and I thank you for that.

I'm studying it and will surely have a few more questions soon.

Schem layout
U1-top right
U2-top left
U3-bottom left
U4-bottom right

U1-mono
U2-mono
U3-Schmitt trigger (NOT gate)? I think Q1 is the NOT gate....? Double NOT Gate??? I'm confused on this part...)
U4-astable

The operation of U2 (top tight) It looks like it is wired monostable, yet there is a note of 1.5Hz under it... I'm confused

U3 has a note "Set FF" What does this mean?

U4 has a diode, CR4 as used for when <50% duty cycle is needed to bypass R12 during the charging part of the cycle. My calculations put the duty cycle at 50.09%. Is the diode added because the duty cycle is so close to the 50% to safeguard the operation?

I'm concerned that my triggering method of U1 is still a problem, unless the U3 schmitt will counter this. ?

Circuit Operation as I understand it.

Triggering U1 will charge C3 to 2/3 Vcc in 4 sec, then dump to 0v. (the delay).
Then U2 and U3 are triggered simultaneously.
U2 gives a single, one time 0.5 second pulse to D1 and SA1.
U3 Inverts the input signal to discharge C5 to 1/3 Vcc nearly immediatly.
U3 then outputs to U4 through Q1 (NOT GATE) to make U4 operate D3 and SA1 for 1 sec, then delay for 5 min continuing until S1 is opened.

All the outputs will most likely be to the same LEDs and Sonalert. I don't think this is a problem as none of the outputs are operating at the same time. (correct?) If needed I can separate the LEDs if it helps get the circuit working properly, no problem.

Thanks Bill, I'm going to build this today and work with it. I'll let you know how it goes and post a breadboard pic,, (neater this time...) :)
 
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Wendy

Joined Mar 24, 2008
22,141
Doh, yes, I forgot the designations for the chips, when I get a chance I'll add them.

You can do a nice job of printing out schematics like these by saving them as a file (downloading them from the board) and using Windows Image and Fax View, it has a print function.

U2 is a 555 Hysteretic Oscillator, the most basic of 555 oscillators. You may need to tweak the resistor to get best performance, but it will work first time.

Q1 is a basic inverter. I want U4 (a super slow oscillator) to only work with the output low. I had to invert the logic output of U3.

C5 causes U3 to start up with the output high. When the output of U2 goes high the output of U3 goes low, and stays there. Set FF stands for set flip flop. I mentioned this circuit in post #67.

You can not connect the outputs together. One of the LEDs (D2) is a diagnostic, you can eliminate it. In your description it was stated as if it were two LEDs. If you want a common LED then move one of them parallel to the sonalert, it is common and shows the way to connecting two outputs together.
 

Thread Starter

thedude123

Joined Aug 29, 2011
39
Does your schematic address my triggering issue?
"Releasing S2 will trigger U1 pin2" Not just a quick "press and release"

Since a monostable 555 is triggered by a pulse to pin 2 and not just directly grounding it. ie. I can manually trigger the circuit with a wire from pin 2 and just touching it to ground and removing it.

Something in me still thinks I need a component to do this "pulse to ground" for me.
I keep going back to some sort of relay that when energized would just operate from NO/NC and back to NO... providing my quick pulse to ground thus triggering the whole circuit to go.... This seems to be my only problem left in this. Whith your help I'm confidant that everything is will work.

Also, Bill, which of these ICs is providing the 4 second long monostable operation that is the alarm after the 4 sec triggering? I see the operation of the mono 4 sec delay but according to the calculations of each,, I'm unclear of which is the 4 sec long alarm.

Also, on a positive note,,, I wired a transistor between the circuit I have been working with so far. As you may recall, my circuit was blinking the diode for 4 sec, then turning off. I needed the opposite, (Wait 4 seconds then blink for 4 sec, then off) (still working on the 4 sec long blink)
I wired one in as an inverter like in your schem just to try it and it worked!!

This was my first time using a transistor. I was concerned about the 6V max input rating so I made a voltage divider that dropped the voltage to the base to around 3V just to be safe as Iv'e learned that these components are prone to blow if not wired right.

Thanks again,,, onward and upward!!!
 

Wendy

Joined Mar 24, 2008
22,141
If you turn on the circuit without depressing S2 the circuit starts timing, similar to the circuit is post #58.

If you depress S2 and turn on the circuit it does not start timing. It only starts timing when you release it.

Far as I can tell that is what you requested.

The components are labeled, including the 4 second monostable.

If you read 555 Monostable you will see why you need the signal conditioner formed by R1 and C2 is needed. You can't just simply ground pin 2 without potential unintended consequences (which is explained in the article).
 

Wendy

Joined Mar 24, 2008
22,141
OK, these are (hopefully) the final schematics. Pick one. :D





If this does what you want it do you could replace the four 555's with two 556's.
 

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