hi, i am designing an electronic alarm, i origionaly had 3x 2 input or gates feeding into a NOT gate which fed into a 555 timer which then went to a siren gen to a speaker... i was told it had too many IC's in it (need a PIC for a seperate keypad circuit aswel) so i tried using various logic gated to try and eliminate 1 chip.... i did it in the end by using 3x or gates and 3 NAND gates (1 to use as a not gate and 2 for the latch to replace the timer) i then found out that it was illegal because it can only go off for a certain length of time so i thaught instead of doing a logic latch with 2 NAND gates, i could do an astable timer with them, but i cant find how to do it

i want it so that when a switch is pressesd (or released, either is good) the alarm sounds for a set time even if that switch returns to its original state so i need a constant power source AND an input and a reset aswell.

 

Please post the schematic, because I don't know what you're talking about.

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Who invented the stupid term, "NOT gate"? It isn't a gate. A logic inverter is a logic inverter, isn't it?

Go to any manufacturer's site and search for a "hex NOT gate", you won't find it. They have hex inverters instead.

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Uncle $crooge

 

Perhapps it's because they couldn't spell any words longer than four letters.

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I also post at the following sites:
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Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

hi, sorry i do not have a schematic, i have just used an alternative.... i have used 3 or gates, an inverter, a 555 timer and a siren gen and i am not using the keypad (i am using a key switch instead) so i have cut out one chip.

Thanks
Mike

 

Since you won't sketch a simple schematic then I guess you won't get any help.

I use Microsoft Paint program to make schematics. Straight lines are made with the Shift key down. I copy and paste parts of other schematics or from datasheets into the sketch. Why can't you do it?

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Uncle $crooge

 

I have the choice of computer software (circuit maker 6 student, its free), paint, or by hand. I nearly always do them by hand, possibly not the neatest, but much quicker (to me anyhow. the software often has parts missing too). A scanner or decent camera should get it onto your computer.

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Angry!? I'm absolutely electrolytic!

Will have to make do with myspace now I guess...

 

hi, i wasnt going to use that circuit but i have changed my mind again, i want to use it so there is a time delay when the front door is opened (i will not need another chip if i can do this using 3 or less NAND gates) as i am now using a keyswitch, it should be easy to de-activate it.

Here is a schematic...

(p.s. it doesnt work, the output Q wont change state... that is all i want, a simple on/off time delay with a set button)

Attached Files

untitled6.zip (5.1 KB, 8 views)

 

You made a schematic with a BMP file type that uses more than 500k of memory space. Its space is so huge that you had to zip it to attach it.
Next time, save it as a PNG file type and it will be 1/100th the size and won't need zipping. Here is my corrected PNG of your schematic.

Do you want a time delay so the door switch can open but the alarm is delayed from sounding until the person keys in a correct code?
Your circuit won't do anything when the switch is opened because the logic is backwards. Swap the 100k resistor and switch positions.

A time delay is roughly equal to the resistance times the capacitance. So your delay is only 1k x 1nF= 1us. If you want 10 seconds then use a 4.7uF film capacitor (an electrolytic has too high a leakage current) and a 2.2M resistor. Also, add a 22M resistor in series with the input of gate C to isolate its input protection diode.

Attached Images

untitled6.PNG (5.0 KB, 18 views)

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Uncle $crooge

 

Thanks for the tip

I am not using a keypad now, i am using a key switch to de-activate the alarm (i know it is less secure but our teacher said it will be too complicated with too many chips)

i just put those values in to see if the circuit would work.

I will try your sugestion now, thanks
Mike

 

Sorry, my circuit doesn't make a delay. Instead it makes a 10 seconds long output pulse. Re-arrange its parts to make a delay.

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Uncle $crooge

 

hi, i thaught that was the monostable circuit??

i did that circuit on crocodile tech. and nothing happened (Q still didnt change state)

 

In my circuit, when the switch is closed then it makes the input of gate B high. The 2.2M resistor makes the input of gate C low so its output is high. Gate B then has both inputs high so its output is low and the capacitor remains uncharged. Because the output of gate C is high then the output of gate D is low.

When the switch is opened then the input of gate B goes low, therefore its output goes high. Therefore the input of gate C also goes high due to coupling from the uncharged capacitor. With the input of gate C high, then its output goes low which makes the input of gate B also low and also makes the output of gate D go high.

The capacitor charges through the 2.2M resistor until the input of gate C is low, causing its output to go high and also the inputs of gates B and D. Therefore the output of gate D goes low.

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Uncle $crooge

 

so swap the 100k and switch around?

even if i do this, gate D doesnt change state (the changes stop at B)

 

This circuit has a low output which goes high about 10 seconds after the switch is opened and stays high even if the switch becomes closed.
Reset it by shorting the capacitor with a switch.

Attached Images

untitled6.PNG (5.8 KB, 7 views)

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Uncle $crooge