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a circuit needs to be checked

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cannibal

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Hello

I am again with my circuits :D

could you please see this circuit and tell me if it will work or not

the idea is when the contacts are wet (water) then the circuit will make sound via buzzer for almost 20 sec by 555, also a number will appear in the 7 segment display for everytime the contacts become wet
 

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  • waterrelaydsd.GIF
    waterrelaydsd.GIF
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Pin 2 of the 555 doesn't have a resistor to the supply voltage for it to be high. Therefore it might always be low. Then the circuit won't work.

Where on earth will you find an old BC108 and BC141?? They were obsolete a long time ago.

The 555 will not time out then the buzzer will beep continuously if the probes are in the water for a long time.

The 9V battery won't last long when it must supply the high current to the old TTL ICs and the LED display.
 
Pin 2 of the 555 doesn't have a resistor to the supply voltage for it to be high. Therefore it might always be low. Then the circuit won't work.

you mean I have to short circuit pin2 of 555 and the + line ???

Where on earth will you find an old BC108 and BC141?? They were obsolete a long time ago.

hehehehehe I got them :D

The 555 will not time out then the buzzer will beep continuously if the probes are in the water for a long time.

this is what I want :)

The 9V battery won't last long when it must supply the high current to the old TTL ICs and the LED display.

this project only for test and showing the idea
 
Pin 2 of the 555 needs to have a resistor from pin 2 to the positive supply, not a short circuit. Then a transistor will pull it low to trigger the 555 and clock the counter. Any value from 10k to 100k is fine.
 
Please conform if eveything is fine now

note: It's ok for me if the buzzer makes noise forever as long as if the contacts in water ;)
 

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  • waterwater.GIF
    waterwater.GIF
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The schematic looks fine now.

The counter will show any random number when the power is turned on. Add a "power-up reset" resistor, capacitor and diode to reset the counter to zero automatically when the power is turned on.
 
Ditch the 9V battery and use a 6V lantern battery.

Ditch the TTL and use LS.

Ditch the 7805 and use and use an LM2940.
 
audioguru said:
The schematic looks fine now.

The counter will show any random number when the power is turned on. Add a "power-up reset" resistor, capacitor and diode to reset the counter to zero automatically when the power is turned on.

WOW this problem I am facing now

when the power is ON a strange number appears

Could you please apply what you said in the attachment because I know my self I will make wrong connections


Hero999
I will try to upgrade my cicruit later in the feature :)

audioguru
I am really waiting your reply
 
No, you draw the connections on the schematic and we'll tell you whether you've done it correctly or not.
 
I really don't know how to connect power-up reset" resistor, capacitor and diode !!!!

If I do know, then I won't be asking questions , rethar then that I will be answering questions !!
 
Look on the datasheet of the 74LS90 at the two Master Reset pins. They are normally grounded but must be high to make the counter reset to zero.

Connect both pins together and use a resistor to ground to make them low when the counter is counting. Select the resistor's value so that the input current of each pin in the resistor does not cause the voltage to be higher than a 0.4V logic low.
Then add a capacitor from the two pins to the positive supply so that it makes the pins high before the resistor charges it.
Connect a diode across the capacitor to quickly discharge it when the power is turned off.
 
Wow I will try that :)

by the way I thought I burned my TTL IC's
when I saw a strange number appearing at the start

thanks I will try it now in the schematic and let you guys check
 
cannibal said:
I really don't know how to connect power-up reset" resistor, capacitor and diode !!!!

If I do know, then I won't be asking questions , rethar then that I will be answering questions !!
You learn more from looking at datasheets and trying to figure it out for yourself rather than just asking us, not only that but you'll feel better about yourself as you've solved the problem for yourself. Even if you get it all wrong we'll tell you that you've done it incorrectly and how to fix it.
 
You have the diode connected so the reset inputs are always high. Then the counter will never count.
The diode must be connected from the reset pins to ground in reverse. Then they do nothing when the circuit is powered but discharge through the diode when the power is turned off. Any little diode is fine (1N914, 1N4148).

Look at the max input low current for each reset pin.
Look at the recommended low input voltage (0.4V).
The R13 resistor value is simply calculated with Ohm's Law (R= V/I).

The capacitor will charge in a time that is 2 x R (ohms) x C (farads). Use a value that charges in 1 second.
 
it's impossible for me to know the current because I didn't have the circuit in bredboard yet

let's say the current was 0.5A
then R=0.4/0.5=0.8 ohms

then C= T/R=1sec/0.8=1.25 F

hope I did good
 

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How could the input current to a logic IC have an input current of 0.5A??!
Simply look on the datasheet.
The maximum input low current into the reset pin is -0.4mA. So the current for both pins is -0.8mA.
Then the resistor to hold it down to +0.4V is 0.4V/0.8mA= 500 ohms or less.
The capacitor to make RC=2 seconds is 4000uF.

If you don't want to use a huge capacitor then amplify the current with a transistor and use a smaller capacitor like this:
 

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  • 74LS90.PNG
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WOW the 4000uF is very huge

**broken link removed**

I will try to use transistor to amplify the current

and I will let you know when I finish
 
A 16V 4000:mu:F capacitor isn't very large, a 100V 4000:mu:F capacitor is very large.

For your reset pin you need something like 100k and 100pf which will give a pulse of about 100:mu:s which is more than long enough to reset the counter.
 
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