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
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.
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.
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.
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.
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.
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:
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.