Transistors to replace relays (Alarm application)

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Hi Everybody, I currently use a small relay to change the NC status of a PIR passive to power a relay switch to sound a buzzer. Now the relay eats about 20mA at 12V, but I want loads of these, so soon I'll be drawing massive amps. I know a transistor can do what a relay does, but can anyone point me to the right resources where I find out how to replace the relay with transistors?

I am looking in particular for transistor specs, i.e. how many mA can the transistor drive, and what volatage can it handle. Thanks for your help.
 

beenthere

Joined Apr 20, 2004
15,819
You might want to look up FET's, as they work on an electric field and consume less current than a transistor. What are your requirements? Be nice to have a schematic to work from, too.
 

bloguetronica

Joined Apr 27, 2007
1,372
A transistor can work many ways when a relay is often used in one manner. The implementation of a transistor depends pretty much on the rest of the circuit. A circuit diagram of the old circuit would be helpful, so we can see how we should connect the transistor.
 

beenthere

Joined Apr 20, 2004
15,819
Your buzzer appears to be using AC to operate. A transistor will not be able to replace the relay, as transistors need DC voltage to operate. It might be possible to use a triac to pass the AC. As a less time-consuming alternative, look at some "solid state relays". They will pass the AC, and use somewhat less current to operate.
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Thanks for your reply. I am sorry for not being clear about the buzzer. The buzzer is a piazzo that operates on 12V DC. Now I have established that what I apparantly need is a 'NOT gate'. I attach PIR3.jpg where I have drawn how I think this works.

pir3.jpg

The questions I still have relate to appropriate components. I have looked at one local electronics supplier. Most transistors seem to handle upto 300V. My circuit is for 12 V. Is there such a thing as a 12V transistor that can handle say 50mA or can I use a 300V one?

I believe the values of 1K and 10K would suit 5V. Perhaps a silly question, but can the resistors R1 and R2 be dispensed with entirely and if not, why not?
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
The coil in my first drawing was meant to show a relay, sorry. I can see now that looks like a buzzer. I hope my second drawing is clear now.

Just to explain what this does, there is a passive infrared sensor. When it detects a person walking across it, the normally closed contact (N/C) opens for a few seconds. This sounds a buzzer. So my relay converts the N/C "open" signal to close the current for the buzzer.

If I have many units running off a 12V battery, then I need to supply quite a lot of current. I am now aware there is a chip 7404 (NOT gate) that can do this, but it needs 5V. I also see there is a 7805 chip that might take the voltage down. I have a 12V supply.

All being said, ideally, I'd like to keep this simple. So I'd like simple transistors to do this job. My questions again, what are suitable transistors and are the resistors really necessary?
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Okay, I can see dispensing with R1 would short the battery, my question was silly, but how about R2. Please direct me to resources where I can find out what components to choose exactly, e.g. detailed specs (mA load, mA trigger and V) of commonly available transistors.
 

Voltboy

Joined Jan 10, 2007
197
Isn't there NO PIR because if having the circuit always in NC could be a waste of energy, why don't just get one NO PIR so when its detect a person walking, it close the circuit and sound the buzzer.
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Isn't there NO PIR because if having the circuit always in NC could be a waste of energy, why don't just get one NO PIR so when its detect a person walking, it close the circuit and sound the buzzer.
Yes, thanks for the point Yoda, that would be nice if my PIR's came in N/O (normally open) types and would solve it. I find these are very rare (I currently can't source any). The best PIR's I can find are N/C types. So I have to use the best N/C PIR's I can get.
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Don't most PIRs come with form-C relays?:confused:
Thanks for your question. Yes, many PIRS do come with relays built in, however the ones I am trying to use can switch a small current only, say upto 50mA, but then this is N/C (normally closed) where I would need N/O (normally open).

Of course I am showing the buzzer for illustration only. I may replace the buzzer with a power relay to drive a high amp motor, e.g. a siren.
 

Thread Starter

hobbybuilder

Joined Mar 24, 2007
16
Yes, thanks for the point Yoda, that would be nice if my PIR's came in N/O (normally open) types and would solve it. I find these are very rare (I currently can't source any). The best PIR's I can find are N/C types. So I have to use the best N/C PIR's I can get.

Thanks for your question, Thingmaker3. Yes, many PIRS do come with relays built in, however the ones I am trying to use can switch a small current only, say upto 50mA, but then this is N/C (normally closed) where I would need N/O (normally open).

Of course I am showing the buzzer for illustration only. I may want to replace the buzzer with a power relay to drive a high amp motor, e.g. a siren.
 

thingmaker3

Joined May 16, 2005
5,084
The forum software starts a new page when a thread gets long enough. Its normal.

The resistors are absolutely necessary. http://www.allaboutcircuits.com/vol_3/chpt_4/index.html explains why.

If you want to make this happen with only one transistor, try this:
PIRBuzzer.jpg
Choosing a transistor appropriate to your needs will depend on the voltage and current needed by the buzzer, relay, or other load. 12V we know, but how much current? If you will be using different loads, what range of current draw do you expect?

For a small piezo buzzer or relay coil, a 2N2222 will probably suffice. They can handle around 3/4 Amp. For a contactor or horn, you'll need something beefier.
 
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