how can I create a positive logic pulse from a steady high input?

[Skidood]

Hi

As some of you may have seen, I am building this circuit,

http://www.engineers-excel.com/Apps/Alarm/Description.htm

The block with the UP arrow creates a positive pulse output for a fraction of a second when the input goes high and stays high.

How can I create the positive pulse?

I do have an oscillator (555) running on the same circuit board..it produces a 2 Hz clock pulse, (yes, that's 2 Hz) maybe I can tie into that somehow and use it in conjunction with a flip flop?

THANKS in advance.......

 

[Old Steve]

This is one method:-

 

[Old Steve]

I forgot to label the gate - it's a non-inverting Schmitt trigger buffer, one buffer from a 74HC7014, for example.

If you use this method, tie the inputs of the unused buffers to ground. Leave the outputs of the unused buffers open.

In reality, you could probably even omit the buffer, but it's the 'correct' way of doing it.

 

[Skidood]

Hey Old Steve, not only do you kinda look like a wizard....you are one! ..thanks so very much.....I get why you would want to use the buffer...

Just for the helluvit, I want to post the projects I have done over the last 30 years:

-7-band stereo equalizer for girlfriends car (my design, using bandpass filter op-amps and slide pots, worked so-so, ended up being stolen)
-radar detector (using the legs of a capacitor as the antenna, this little project was in Popular Electronics back around 1985)
-about 10 guitar effects pedals
-several audio circuits esp small amps
-5 pairs of speakers (the last and best pair are pictured below...they are about 5 feet tall)
I am sure I am forgetting something....anyway this current project is the first one that has logic gates and flip flops

 

[Old Steve]

Hey Old Steve, not only do you kinda look like a wizard....you are one! ..thanks so very much.....I get why you would want to use the buffer...

Just for the helluvit, I want to post the projects I have done over the last 30 years:

-7-band stereo equalizer for girlfriends car (my design, using bandpass filter op-amps and slide pots, worked so-so, ended up being stolen)
-radar detector (using the legs of a capacitor as the antenna, this little project was in Popular Electronics back around 1985)
-about 10 guitar effects pedals
-several audio circuits esp small amps
-5 pairs of speakers (the last and best pair are pictured below...they are about 5 feet tall)
I am sure I am forgetting something....anyway this current project is the first one that has logic gates and flip flops

Not bad looking speakers. As big as me. (I'm 5'4" tall.)
And it sounds like you've been busy. I don't do much discrete logic stuff any more - I cheat and use a microcontroller for most projects. Haven't done anything for some years until the last few weeks, either. I've been busy with my other hobby - woodturning.

I don't look so much like a wizard now, either, I got sick of the beard and shaved it off last week.

 

[Skidood]

I was looking at the micro controllers in consideration of this project, as I may need to produce several of these things (I am self employed and this is a potential "product") Its a wall mounted alarm module that runs off a 9V DC adaptor and monitors 3 different pressure switches mounted on various steam and water lines. I am eliminating the "alarm acknowledge" portion of the circuit. Just using the silence button....The flashing light will actually be a piezo beeper. I also am using LEDS which will turn from green to red to provide a visual indication as to which supply line has gone down.
I briefly looked at the Arduino product ....but I dont know yet if they are suitable and I dont really want to learn the programming, I think I can solder up a few boards in less time once I get a working reliable circuit......I have a cursory knowledge of PLC programming and having looked further into that was like encountering a bear in the woods....if I could use an Arduino or something like that, and have someone write the program for me, I would be willing to compensate them....anyway.... BTW I was just listening to "Land Down Under" about 3 hours ago, lol...

 

[AnalogKid]

The alarm system you describe is not very complex in its logic requirements, and can be done easily with CMOS logic, or LM339 comparators, or opamps, or plain old transistors. And, of course, a PIC, Arduino, or whatever. But all that typing...

ak

 

[Old Steve]

I was looking at the micro controllers in consideration of this project, as I may need to produce several of these things (I am self employed and this is a potential "product") Its a wall mounted alarm module that runs off a 9V DC adaptor and monitors 3 different pressure switches mounted on various steam and water lines. I am eliminating the "alarm acknowledge" portion of the circuit. Just using the silence button....The flashing light will actually be a piezo beeper. I also am using LEDS which will turn from green to red to provide a visual indication as to which supply line has gone down.
I briefly looked at the Arduino product ....but I dont know yet if they are suitable and I dont really want to learn the programming, I think I can solder up a few boards in less time once I get a working reliable circuit......I have a cursory knowledge of PLC programming and having looked further into that was like encountering a bear in the woods....if I could use an Arduino or something like that, and have someone write the program for me, I would be willing to compensate them....anyway.... BTW I was just listening to "Land Down Under" about 3 hours ago, lol...

A micro would definitely be the best/easiest way to implement it, and it would take up the least board space, but the programming is a bit of a learning curve, and the initial cost of a good compiler is reasonably high. Raw microcontrollers themselves, PIC etc, are cheap and a programmer can be bought from eBay for about $10. Mine is the the K150 programmer, with free burning software from the web. I use MicroEngineeringLabs PICBasic Pro compiler, from inside the free MPLAB IDE available from Microchip, (the maker of PIC chips). PICBasic has a lot of commands for LCD control, serial communications etc so it's very easy to use. For this project, a simple and cheap PIC16F84A, (about $5 each), would do nicely, it has 13 programmable IO pins.
Still, it's not that complicated, so discrete logic is a good way to go about it.

 

[Old Steve]

Just in case you feel like having a look, here's a link to the PICBasic Pro manual:-
http://melabs.com/resources/pbpmanual/

 

[AnalogKid]

Two alarm logic questions:

1. If a supply line goes down and trips the alarm, then goes back up before the alarm is acknowledged, do you want the alarm to continue until it is reset, clear itself automatically, or silence itself but keep a visual indication that something happened, like the red/green LED turning yellow?

2. What are the voltage and polarity of the signals coming in from the sensors? Low on fail? High on fail? Open collector high (or low) on fail (alarm system supplies pull up resistor)? Etc.

Right now it looks like the logic can be done with two CD4093 quad NAND gate chips and some signal diodes for $1 or so. Plus 4 red/green LEDs for input status and a bright red critter to go with the piezo beeper.

ak

 

[Skidood]

Thanks Old Steve. I will check that out.

 

[Skidood]

Two alarm logic questions:

1. If a supply line goes down and trips the alarm, then goes back up before the alarm is acknowledged, do you want the alarm to continue until it is reset, clear itself automatically, or silence itself but keep a visual indication that something happened, like the red/green LED turning yellow?

2. What are the voltage and polarity of the signals coming in from the sensors? Low on fail? High on fail? Open collector high (or low) on fail (alarm system supplies pull up resistor)? Etc.

ak

1. Total reset...no need to press any silence button or acknowledge (I am not including the acknowledge part of the circuit anyway) It is not needed to have a residual indication that there was a problem which has now disappeared
2. 9 volts DC...not cast in stone at all....logic LOW upon alarm state...logic HIGH when all is good and the pressure switch is made.... this is "fail-safe methodology"...if someone cuts the signal cable from the switches, or if the switch fails,we get the alarm.

I was going to use AND gates to assimilate all 3 inputs from the pressure switches to create 1 "all is good" or "there is a problem" signal for triggering this alarm circuit...but I will wire the green/red LEDs to the 3 individual lines coming in from the switches prior to the AND gate.

BTW...you must be a RUSH fan?

 

[Skidood]

the inputs would be tied to ground through a 47K resistor, but held high by the pressure switch, so they go low on alarm

 

[Old Steve]

Sounds like AK is on the right track. Instead of the AND, diodes would be fine for joining the multiple inputs together.
And it definitely wouldn't be worth using a micro unless you planned to use them for other projects in the future. Good toys, though.

(And I used to be a bit of a Rush fan, but haven't heard them for many years now. Love "Land Down Under", too, of course. )

 

[Skidood]

Of course...diodes...duh....on that Schmitt trigger circuit, do you think it will work if fed with a positive voltage coming from a logic gate? I will need to use an inverter between the diode outputs and the Schmitt trigger as the signal from the switches should be "high" if all is good...compared to that excel circuit where a high input means alarm..

 

[AnalogKid]

Your latest description wiped out about half of my prelim schematic. So now there is only one latch, and it is used to silence the alarm if the fault does not clear itself before someone gets to the alarm controller. Then, a "shut up" button resets the latch. At this point if an input signal still is in the alarm (low) state, it must go high and then low again to trigger another alarm. Correct?

ak

 

[AnalogKid]

BTW...you must be a RUSH fan?

And an amateur drummer, a great combination.

ak

 

[Old Steve]

Of course...diodes...duh....on that Schmitt trigger circuit, do you think it will work if fed with a positive voltage coming from a logic gate? I will need to use an inverter between the diode outputs and the Schmitt trigger as the signal from the switches should be "high" if all is good...compared to that excel circuit where a high input means alarm..

Youre losing me a bit now, (haven't been following all posts), but yes, it should work with the signal from a logic gate. Of course, you could always use a larger value resistor to reduce the load on the previous gate, and then you could use a smaller value capacitor. (I chose those values for a pulse a little under 1mS long.)
And many Schmitt trigger buffers are inverting, (74HC14 for instance) - just choose the right type for the purpose. I selected the 74HC7014 because it suited the situation as presented.
Also, one gate of AK's 4093 could be used as an inverting Schmitt trigger buffer - just tie the inputs together. (A 4093 has Schmitt trigger inputs.)

 

[AnalogKid]

Almost done. The system is reduced to one magic IC plus a 555. Zero logic gates, as in no CMOS, TTL, ECL, DTL, RTL, BTL, CPLD, FPGA, PIC, etc.

AND, it will run on anything from 5 to 48 V by changing the LED resistors, and can withstand 50 V input transients without protection.

ak

 

[Old Steve]

That would work. To cut the component count still further, you could remove the oscillator and the AND gate, drive an NPN transistor from the S-R flip-flop's Q output and have it switch a buzzer that beeps on and off automatically, something like this one:-
http://www.altronics.com.au/p/s6112-3-15v-pulsating-tone-piezo-chassis-mount-buzzer/

Something I noticed is that in this version, the alarm won't trigger if a sensor wire breaks or becomes disconnected.

You could also replace the OR gate with two diodes followed by a tie-down resistor.

And a single NPN transistor could replace the inverter.

Now the only IC would be the S-R flip-flop.