Please help

Thread Starter

soda

Joined Dec 7, 2008
177
Hi to all members,

This is a mono stable timer for a alarm system. When the PIR relay is closed, pin 3 of the NE555 must stay low and when the PIR relay open the output at pin 3 must turn on or go high.

Please look at my schematic to see if it's correctly wired.
 

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edgetrigger

Joined Dec 19, 2010
133
Hi to you,

Your description points to a bistable operation. In mono stable the output will go high and reset back after a time delay set by RC.

The circuit as regards mono stable is correct and will work. The second transistor is used to pull the trigger pin low for closed PIR relay.
 

Thread Starter

soda

Joined Dec 7, 2008
177
edgetrigger,

It's not so much about the mono stable but it's more about the trigger pin 2 of the ne555 and the PIR relay contacts.

When the PIR relay contact open, only then i want pin2 to be triggerd. So, when the PIR relay contacts is close, nothing must happen.

You say that the 2nd transistor will trigger pin2, but i want just the opposite of that action.

Do i need a PNP for the 2nd transistor?
 

edgetrigger

Joined Dec 19, 2010
133
Sorry, i wrongly analysed. When the PIR switch is closed first transistors o/p is low and the second transistor's o/p will be high. When the PIR switch is open first transistor's o/p is high and the second will output low.

So that is how you wanted and you have built it properly. No need of a pnp here infact.


Your ckt should work. let me update you tomorrow, it is 2.10am at my place got to sleep!
 

Thread Starter

soda

Joined Dec 7, 2008
177
Ok edgetrigger,i also see it the way you see it, thanks for your help. i'm going to make a pcb and build it.
 

CDRIVE

Joined Jul 1, 2008
2,219
Ok edgetrigger,i also see it the way you see it, thanks for your help. i'm going to make a pcb and build it.
No, that's bassackward. SOP dictates breadboard first, if OK.. then make the PCB. This includes perforated copper clad experimenter boards. This greatly reduces the "Oh Sh_t!" factor later on. One of the first things that an electronics enthusiast/experimenter should have on his bench is a Proto-Board. No solder - no fuss plug and play. You'll never regret it. ;)
 

CDRIVE

Joined Jul 1, 2008
2,219
Why don't you semplify your circuit as per the attached? When pir contact opens, pin 2 will be pulled up by the resistor R2 . You will avoid to assemble two useless transitors.

Cheers

Alberto
This would be OK if his relay has N/O contacts but not as drawn (N/C contacts) in your print. The 555 triggers on the negative going edge. If the Trigger pin is held low, as you show it, the output will remain high forever.
 

CDRIVE

Joined Jul 1, 2008
2,219
This will work with N/C or N/O relay contacts. Read the notes on my print. BTW; why do you need a relay to trigger the 555? A FET or BJT should be sufficient to pull the Trigger pin low.
 

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Last edited:

Thread Starter

soda

Joined Dec 7, 2008
177
Hi cdrive,

I allays test all my diagrams on breadboard before i make a pcb.I already did this and everything was working the way i wanted it to be.That was after i got help from edgetrigger.

The PIR (programmable infrared detector) relay work in such a way that when someone crosses the beams the relay open and when it's in a resting position, the relay is close.

I could use a cmos nand or, nor-gate instead, but that would then be more complicated.

Thanks for all your reply's
 

Thread Starter

soda

Joined Dec 7, 2008
177
CDRIVE,

I know you try to help me, but the relay you talking about is 5mm by 2mm in size.

I'm definitely not going to replace that, it's just too small. It has the size of mother board components.
 

Thread Starter

soda

Joined Dec 7, 2008
177
CDRIVE,
I cant see any reason why i must replace the relay because the relay is part of the PIR. The PIR is a complete unit jou buy at any elect. shop.

Do you know what a pir is. It's a infra-red sensor you use for a motion detector and when someone crosses the beams it sets off the alarm.

On the other hand, I've already made the PCB and soldered the components to the board and using it, so i don't know why you keep on trying to help me. It's over and done.
 

CDRIVE

Joined Jul 1, 2008
2,219
CDRIVE,
I cant see any reason why i must replace the relay because the relay is part of the PIR. The PIR is a complete unit jou buy at any elect. shop.

Do you know what a pir is. It's a infra-red sensor you use for a motion detector and when someone crosses the beams it sets off the alarm.

On the other hand, I've already made the PCB and soldered the components to the board and using it, so i don't know why you keep on trying to help me. It's over and done.
My last post on this topic was addressed to Alberto, not you. This may be your topic but you don't own the thread or the board. BTW, thank you for explaining what a PIR is, I hadn't a clue! And if you find the time do a dictionary search for the term "ingrate".:rolleyes:
 
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