Siren Driving with positive outout

[TEZARM]

Hi Guys

Hi need to get a Picaxe to drive a siren with a positive output. It must put (+) positive to the siren to drive it. I don't want it to gnd the siren through a transistor as I need the sirens earth to be permanently grounded for this application. It's just a normal 12v oscillating car alarm siren (NOT piezo).
What would be the best way and simplest way to get the picaxe to drive this. I just want the component part of circuit., not the code or anything like that. Thanks

 

[SD2100]

You can do it like this

http://www.picaxe.orcon.net.nz/npnamp.gif

or have a look at picaxe_manual3.pdf (page 7)


Edited by - Phil75 on 18/10/2006 13:06:35

 

[Michael 2727]

You could try this -
<A href='http://www.porepunkahps.vic.edu.au/home/jef01/files/display/MOSFET-Delay-from-Picaxe-Output.htm' Target=_Blank>External Web Link</a>
http://www.porepunkahps.vic.edu.au/home/jef01/files/display/MOSFET-Delay-from-Picaxe-Output.htm
Don't know what the yahoo link is - Unsubscribed.
The 2N7000 MOSFET will only handle 200mA
but you van swap it for an IRF530 etc.
You could also drive the siren from the
Low Side rather than the High side of the
MOSFET.


Edited by - Michael 2727 on 18/10/2006 13:15:33

 

[jodicalhon]

Is this any good? <A href='http://i11.tinypic.com/4cwwehf.gif' Target=_Blank>External Web Link</a> Or have I missed the point entirely? Choose a transistor to suit the current requirements of the siren.

Cheers

Edit: Sorry, forgot to put in the picaxe positive supply of 5V.



Edited by - Jo C on 18/10/2006 13:56:33

 

[Dippy]

I reckon you got the point spot on JoC. Jo C's is the RIGHT one to go for (High sided switch). The other two are low-sided switches - which is specifically what TERZAN didn't want.

 

[xstamp]

I agree with Jo C but you could do it with one less component, by moving the resistor from the the base of the BC548 to its emitter and eliminating the collector resistor. Because the single resistor between the BC548 emitter and ground would limit current through the base of both transistors.

 

[SD2100]

&quot;It must put (+) positive to the siren to drive it&quot;

Relay looks ok to me, it doesn't matter if the transistor is switching the low side of the relay, +12v is connected to one relay contact and the siren to the other, the siren is then grounded. A high output from the picaxe will put +12v on the siren. This will give the desired result. Correct me if i'm wrong, I am sometimes.


Edited by - Phil75 on 18/10/2006 16:51:22

 

[BeanieBots]

I'd also suggest the relay option. A siren for a car probably pulls quite a bit of current and is probably very inductive. A relay would solve many problems that transistor drive might encounter.

 

[TEZARM]

Cool. Thanks Guys. Yes JO C is an excellent example of what I was tryimg to decide. But BeanieBots mentions problems that could occur with this method. I have a little problem trying JO C method at the moment. Each time the siren chirps on and then off you can hear it clicking after it turns off (Chirp, Click, Chirp, Click). I can fix this problem with 1uf across the siren but probably not very professional this way. Could this be one problem BeanieBots? If using a relay would CONTACTS NOISE not be a problem aswell? Thanks

 

[BeanieBots]

To be honest, I don't know what is/could be causing that issue. My concerns with using a transistor relate purely to back emf and what current rating the transistor would need to be. The relay option would completely isolate the PICAXE control and thus eliminate any emf/wiring issues that MAY have given rise to other &quot;strange&quot; effects. There will certainly be &quot;noise&quot; at the relay contacts which will lead to premature contact failure if not suppressed with a snubber but this is only likely after many thousands (rather than millions) of operations. A large back emf spike would kill the transistor after the first use if not caught by a diode and/or snubber circuit.
The relay option also offers electrical isolation from incomming leads which is always an issue in automotive applications to prevent undesired spikes from entering your micro board. The transistor method is fine (and preferred outside automotive use). It just requires a little more attention to detail.

 

[TEZARM]

Huh? Snubber? Whats that mean?
Oh ,an, I don't know which is best to do. Someone tell me please. Anyone else done this and can recommend the best way to do this.
Anyone know where to find a professional car alarm schematic so I can see how they do it. They use transistors I guess. How do they do it? Anyone here ever designed a car alarm before? The siren pulls a little over 2 amps when blasting it's head off!!!

 

[Dippy]

I tend to agree with BB.
Back EMF, power spikes and oscillations are always to be taken into account when driving electromechanical/inductive things using a transistor. All these spikes can cause all sorts of side-effects varying from upsetting/resetting software, triggering transistors, nasty oscillations etc to blowing things up.

As suggested, a relay is probably the safe bet. Though you may still have to add something to reduce the siren electrical noise getting to your sensitive parts.

If you have an oscilloscope you could have a look.

Postscript.
I wouldn't get to tied up with the term snubber. BB probably meant it in the usage as in 'suppressor' - though snubbers is whole different subject and there are many types. So, for now, think of 'snubber' as 'spike suppression' as in 'dissipative snubber'.

First, if you wish to use transistors, what transistors are you using? What is their current spec?
If your chosen transistor is man enough, consider, as suggested previously, a diode across the siren as you would if you were switching a relay. Perhaps you'll need a capacitor too.

The 2 amps you mention is obv steady state but if you could measure the switch-on transient it would give a clue as to the level of back emf at switch off.

Maybe play safe and go a relay?


Edited by - Dippy on 19/10/2006 11:23:33

 

[BeanieBots]

As general rule, most things in a commercial car environment are switched on with relays.
A &quot;snubber&quot; is an electrical circuit designed to reduce spikes caused by switching inductive loads. The most common is an R and a C in series across the switching contacts. The C absorbs the energy while the R limits the current that a fully charged C would discharge into the contacts when they make. For relay contacts, the values are not very critical. A low R (a few ohms) and a moderate C (a few hundred nF) should be OK. The values are more critical when used in things such as switchmode power supplies where the term &quot;snubber&quot; is more common. A &quot;catch diode&quot; or &quot;free wheeling diode&quot; is a very simple form of &quot;snubber&quot;. Snubbers can even consist of active components such as transistors when very fancy ultra high efficiency is required. ie, don't dump the unwanted energy into a resistor but feed it back into the power supply to be used constructively.
Anyway, a simple RC across the contacts will do you nicely.

 

[jodicalhon]

I agree that a relay is probably the best way to go. 2 amps would kill my transistors deader than a doornail! If not deader.

(Doh! Not only forgot the picaxe pos supply, I forgot to add a snubbing diode, too! Too late at night - more sleep needed for Joey.)

Cheers.

 

[Guest]

&quot;a simple RC across the contacts will do you nicely.&quot;

Do you mean a resistor and capacitor in series across com and n/o contacts of relay? Sorry guys, I'm not to bright here.

Dippy, i would use tip41c and tip42c transistors. I think there 4a to 6a rated.

 

[TEZARM]

Thanks. Good Night Joey.

 

[SD2100]

Can't go past a good ol relay for turning automotive things ON, no need to worry about
nasty spikes etc, a general purpose auto relay is good for about 30A so this will handle your sirens 2amps with no problems at all and will last for years. Just give the picaxe some protection from the electrical system nasties.

 

[jodicalhon]

Not knowing much about relays, but ever ready to make a twit of myself in the hope of learning something, I've been drawing again.<A href='http://i11.tinypic.com/2gv39f4.gif' Target=_Blank>External Web Link</a>.

All comments welcome!

 

[BeanieBots]

Jo C,
The RC snubber goes across the relay contacts to reduce the spark generated when the contacts open. This is to help reduce contact errosion from the sparks. The diode (which you have correctly shown across the coil) catches the high voltage generated by the relay coil when it is de-energised. This is to protect the transistor from over voltage.

 

[jodicalhon]

Ah, thank you, BeanieBots. I knew I'd learn something.

Tezarm: <i>Do you mean a resistor and capacitor in series across com and n/o contacts of relay? Sorry guys, I'm not to bright here. </i>

Seems you're brighter than me, Tezarm!

Cheers.

 

[jodicalhon]

Any better? <A href='http://i11.tinypic.com/48333bm.gif' Target=_Blank>External Web Link</a>.

I'll stop this now. Last one. I promise.

 

[BeanieBots]

Looks good. No harm in leaving the diode in place that you originally had on the siren. Belt &amp; Braces.
I agree. I think we've flogged this one to death now so probaly not worth re-posting an updated diagram. I'm sure everyone has got the idea now.

 

[TEZARM]

So should I also put that diode in aswell yeh?
Wheres that artist gone? JO C, would ya mind doing the honours one last time.
This is great, all we need is BeanieBots Brain and JO C's fantastic artistic skills and not so clever Tezarm understands instantly. What a team you two make. I understand so much better when I can see what I am trying to do rather than read. This forum needed a good artist, your hired JO C.

 

[jodicalhon]

Ah, you're too kind, TEZARM. (I went to bed, by the way). I would do another schematic, but I promised!

As for my artistic skills: what you do is find a nice schematic, (Wilf's always look good - that's his transistor - thanks Wilf!), and then cut and paste in the Paint program that comes with Windows.

I must say I do draw a nice rectangle, though.

 

[TEZARM]

Shhh! An artist should never give away his secrets. What a friendly fellow you sound like.
Glad to have a Picasso like yourself on this forum. Goodnight. zzz zzz zzzzzzzzzzzzz