Homemade alarm problems

[plumber1969]

Hello im a newbie on here so please go easy

I have manage to build a motorhome alarm using car parking sensors

I have link 5 car parking sensor together giving me a total of 16 sensor place around the motorhome

Each sensor has a buzzer wire positive and negative going to a piezo buzzer the voltage at the piezo is betweeen 6volts and 11.78volts at different stages of detection

I have removed the buzzers to all of them by one {this is to warn me inside the motorhome }and have used all the wires as triggers

I have wired all 5 postives together after putting blocking doides on each line this then goes to the gate on a mosfet irf540

the neutrals go to the source

I then have 16 x 1 watt leds in a ring going to the drain via the postive of a 12 volt battery {this battery is not linked to the alternator }
The neutral going to the source

I have had it up and working fine then i ask a elecronic guy to put a mosfet into a box to make the design look better and different one irf640 and when i fitted it all the parking sensor blow up ie there is now a constant bleep from all of the sensor units when you are not in detection range of the sensor dam

So my question
1 if the mosfet was at fault how can i protect this circuit ie the positive and negative coming from the a reverse polarity or breakdown of the mosfet?

2 I belive I have protected the postive side of the buzzer wire from reverse polarity with the use of the diodes ?

3 how can I or do I need to protect the neutal side of the buzzer wire from reverse polarity?


Thankyou for any help what so ever !

 

[ronv]

The problem may be with the 6 to 11.78 volts from the sensors. The 6 volts may be enough to turn on the FET. What you might try is adding a 3 to 5k ohm resistor from gate to source of the FET as it could be the sensors have nothing to make them go to ground when off.

 

[plumber1969]

homemade alarm

Thankyou for the reply Ronv

Do i simple connect it direct to gate and source pronges of the mosfet


just to add the negative of the buzzer wire is connected to negative of the 12 volt battery

I will post a drawing which might explain how i have wired it up better and hopefully you can see the flaws in my design

thankyou for your help Tony

 

[ronv]

A drawing would be good.

 

[plumber1969]

Hello Ronv thankyou again

What does the resistor do when used in this way ? is it draining any power in the mosfet closing the gate ?

Thanks Tony
ps i have sent you a drawing via PM

 

[ronv]

Drawing

Hi Tony,

I'll post your drawing here for you so we can get everyones input.



Yes, just add the resistor from gate to source (pin 1 to pin 3). This may not fix it because we don't know exactly how the alarm works. The resistor will just try to make the gate go to ground which turns the FET off. With 6 volts on the gate it is on.

The other problem is that you have 16 leds all in series across 12 volts. Depending on what kind of leds they are they will drop about 2 to 3.6 volts or so for each one. So for them to work you might need as much as 57 volts to light them. The other problem with how you have them shown is that if the voltage were high enough there is nothing to limit the current through the leds so they would burn out. To fix this we can put a few (depending on what kind you have) in series with a resistor to limit the current. Then put several like that in parallel with the first so they all work. Post a link to your leds if you can or at least tell us the color and we can guess.

 

[plumber1969]

Thankyou again ronv

My mistake for not explaining the LEDs are one watt each and have resistors
Built in the design of the glass they are produce for a 12 volt system

I have had them all working together nice and bright off the one 12 volt battery

My main concern was if the MOSFET failed would the voltage from the 12 volt battery
go through the buzzer wires?

Im still trying to work out how the damage MOSFET could of fried them in the first place
To prevent this happening again

So I suppose I'm asking if you could tell me is there enough protection on the two buzzer wires to
Prevent any possible current / voltage going through them to the parking sensor

Thank you again for your kind help Tony

Ps do you design or make circuit boards and assemble . I'm also looking
for someone to create another ideal I have

 

[ronv]

Ahh, I see. You could add another diode at the battery to keep things safe if the wires to the battery can be reversed.
I supose it is possible for the FET to short to ground and draw to much current from the alarms. You could add a resistor in series with the diodes to keep the current low in the alarm in case of a mistake.



Maybe on the last question. Give me a pm.

 

[plumber1969]

Thanks Ronv
You have been most helpful

If there was a short to ground as you say would this short go back through the neural of the buzzer wire to cause me problems if so what protection if any could I put on the neutrals of the buzzer wire .I will put the diodes and resistor in place as you suggest

I have attached another picture of the circuit board for the alarm if this helps .Facing the board bottom left the first ys3 is for 12 volt battery power the second ys2 the buzzer wire connections and the 8 x sy2 are for the sensors.What i think has happen before is that maybe a short to earth has gone back through the buzzer wires i think it might of damage the little controller but my skills are lacking to repair this.

**broken link removed**

before i continue with making this set up again , by using the mosfet in this way to simply try and use it as a swith to turn on 12volt leds by a 6-11 volt trigger is the best and safest way to give protection the the car alarm circuit bord or is there another method ?

Thankyou most kndly for your help Tony

 

[plumber1969]

Hello Ronv

I have answered one of my own questions regarding protecting the neutrals from the buzzer wires by simply putting blocking diodes on them .I have tested the circuit at it works .

thanks tony

 

[plumber1969]

Hello Ronv

Just discovered i think why the mosfet may of damage?

Can you check for me if im thinking correctly

As said there are 5 main parking units each can give out between 6-11volts to the buzzer wire whilst under detection

I have wired all 5 up to one gate of a mosfet irf540 which i believe had a vgs of + _ 20v

It is possible for all of this units to trigger at once if someone was to walk around all the sensors

So should i wire a mosfet for each parking unit i will atached a picture

Thanks Tony

 

[plumber1969]

Hello Ronv here is a picture of my paln can you let me know if im thinking correctly and that i do need to do this .As im not sure ,after reading the data sheet for the irf540 n channel mosfet im still not realyy sure what im reading thanks again

Tony
**broken link removed**

 

[ronv]

That shouldn't be necessay as the voltages don't add.
The circuit worked at one time the way it was?
If so do you have any idea what might have happened to blow the sensor board?
Not knowing what is inside the sensor board makes it difficult.

 

[plumber1969]

Hello Ronv

Thankyou I was not sure if they added at the gate {lack of knowledge }so as they dont add the reason for them blowing up I assume is that there must of been a short to ground somehow which travelled back through the neutral wires of the buzzer into the unit as I had no diodes on that neutral side of the buzzer wire circuit but had blocking diodes on the positive side

So now I can now assume with blocking diodes on both positive and negatives no voltage can travel back to the parking unit via the buzzer wire which is now my only concern

With regards to the parking unit i belive just after the socket for the buzzer wire connection there is a small controller which i have been told before dosent like any reverse current /voltage i havent investigated that any futher as it is beyond me and easier to buy new units as they are very cheap in the uk about £5

So my main aim now is to try and simply prevent any voltage going back through these wires


Tony

 

[plumber1969]

got that wrong



Well i got that wrong

I have just wired up my new parking unit to a car battery.

I have wired up the positive of the buzzer wire to a 1xn005 diode and the neutal to a 1nx005 diode but the diode on the neutral is preventing the circuit to turn on the mosfet i think this is because of the forward 0.7 voltage

can anyone please tell me if there is any sort of diode that is design to go on a neutral wire to complete a circuit but will block any voltage

Thanks in advance Tony

 

[ronv]

The way they were wired originally should have worked ok.
Try it like this. The resistors should protect the alarms even if there is a short. The 100K is to make sure the FET turns off.

If you can, post a link to your alarm board.

 

[plumber1969]

home made alarm

Thanks Again Ronv that diagram is excellent thankyou

I will post a picture as soon as i tidy it all up if i can i will try and post a video

I have followed what you have said and rewired it up at home and it works

Just to check as im learning which is hard going .But its fun at times

What is the reason for the resistors before the diodes

Can you explain

I know I must seem to be like a stuck record with trying to protect the negative side of the buzzer wire, maybe it cant be done
but I now think when I put the wires to the mosfet I must of connected the negative wire from the buzzer direct to the positive of the battery by mistake

Hence my fear of damaging them again ?

Thanks Tony

 

[ronv]

I have followed what you have said and rewired it up at home and it works

Good.

Just to check as im learning which is hard going .But its fun at times

What is the reason for the resistors before the diodes

Can you explain

The resistor is just to limit the current in case there is a short on the positive line.

I know I must seem to be like a stuck record with trying to protect the negative side of the buzzer wire, maybe it cant be done

You should be able to add a diode as shown.

but I now think when I put the wires to the mosfet I must of connected the negative wire from the buzzer direct to the positive of the battery by mistake

That might blow it.

Hence my fear of damaging them again ?

If you have it all wired up now and it is working, I wouldn't take a chance and add more protection to it. But if you want to the diode should work.

 

[plumber1969]

homemade alarm problem

Thankyou Ronv

I cant thank you enough for the support and help you have given me ,I truly appreciate your help and time you have spent helping me.

What is the square circuit is that a just connecting wires

Thankyou Again Tony

 

[ronv]

I just put that there to show the buzzer.