Need help solving simple problem

Hello,

I used to design this PCB for water circuit

May I have your help please in checking the PCB layout
because I am facing the same old problem

the buzzer is not stopping ones it gets activated

this is happening even before inserting the probes into the water I mean in the air
I thought maybe the circuit is very sensitive
so how I can reduce the sensitivity ?
I checked the components and they are all perfect even the soldering
Please help

So you said that the buzzer turns on but then not off? I assume that means that it isn't on before you put it in the water?

Also a hint of advice for the PCB layouts that I've seen used a lot. Its better to use 45 degree corners as opposed to 90. I can't remember why right now but I recall hearing that somewhere.

problem is solved

I replaced 555IC and the problem is solved
very strange !!!!

now when the probes touchs the water the buzzer start making sound and when I remove it from water it stop after only 2 seconds and the counter counts 1

in fact it should stay making sound for 15 to 20 sec not 2 ??!!

any idea ???

problem solved
I changed C2 10uf cap to 100uf

but when the probes stays in the water for long time
and then when it removed the counter counts not correct number example
if it's 2 then it goes to 5 like that

otherwise it's ok if the probes got removed fast
so it counts normal

any idea ??

What is IC U1?

R1_________________2.2k
R2_________________1k
R3______________1Meg
R4 ________________100k
R5 ________________470
R6 _________________ 500
D1 ________________ 1N914 or 1N4148
C1________________0.01uF (102) ceramic Capacitor
C2________________100uF 16V Electrolytic Capacitor
C5 ________________ 40uF
Q1____________BC108 NPN Transistors
Q2____________BC141 NPN Transistors
IC1_________________ 555 (Lo-Power version)
U1______________ 74LS90 TTL BCD Counter IC
U2______________ 74LS47 TTL Seven Segment Display Driver IC
U3 ______________ 7805 Regulator
DISP1___________ Common Anode 7 Segment LED Display
SW1_______________SPST Switch
BZ1___________Piezo sounder
B1__________________9V cell with holder

How I can reduce the sensitivity of the probs pins ?

What is this circuit supposed to do? It looks like it is supposed to count the number of times the probes have been in and out of water since the power was turned on. It is not designed to be in contact with water for longer than the timeout of your 555 monostable (110 seconds with the values you have). If you remove the probe slowly after that, you can get "contact bounce" (a property of mechanical switches), which means the probe circuit will open and then close again briefly when you are pulling them out of the water. This triggers your monostable again, advancing the counter.
I think the whole point of the monostable is to remove contact bounce (it will be immune to it during its timeout). If you leave the probes in the water longer than that, you are defeating the purpose of the monostable.
What is your actual application? Maybe we can solve your problem if we know more.

cannibal said:

How I can reduce the sensitivity of the probs pins ?

Click to expand...

Changing the value of R1 should be able to adjust the sensitivity. Perhaps a variable pot there would work better?

Lefty

A value of 910M for R3 and a value of 100nF will give you a 10sec delay to switch off after the probe has been removed. However using an NE555 the mininimum threshold current may not be met. In that case replace the NE555 by a CMOS type.

Your chosen values give you a tremendeously long time delay of several thousand seconds if at all.

Its better to use 45 degree corners as opposed to 90.

Click to expand...

1) If etching is poorly controlled, undercutting can occur inside the corner.
2) Above about 2 GHz, the impedance at the corner is better controlled.
3) It's prettier.

Another hint: In most of my designs, I try to fill unused area with ground, to make the etching more uniform (and faster) and to reduce etchant consumption.

Boncuk said:

A value of 910M for R3 and a value of 100nF will give you a 10sec delay to switch off after the probe has been removed. However using an NE555 the mininimum threshold current may not be met. In that case replace the NE555 by a CMOS type.

Your chosen values give you a tremendeously long time delay of several thousand seconds if at all.

Click to expand...

His chosen values of 1Meg and 100uF gives a 110 second timeout.