Need some help modifying alarm

I purchased a radioshack alarm, It comes with small, not loud siren, I tested the voltage coming from the siren it is 1.5 volts. I purchased a louder siren but it requires 12v.

I was thinking of using a relay to switch the 1.5v then run a 12v power source on the switched side.
However I cannot find a relay the will switch with only 1.5v and handle 12v.

The other option i was thinking of is just replacing the transformer with a larger one. Any Ideas or help would be great.
Thanks.

I tested the voltage coming from the siren it is 1.5 volts.

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That seems odd. Are you sure the "siren" is not just a speaker and you are measuring the DC voltage superimposed on the AC waveform? What is the power supply voltage to the alarm? Model number?

Here is the link:
https://www.radioshack.com/product/index.jsp?productId=2036292&cp=&pg=2&origkw=alarm+system&kw=alarm+system&parentPage=search

I am pretty sure it is a siren, I bought a siren from Radio Shack and hooked it up the the two wires, it made the same noise, just not very loud. I will try to get some pictures of the board up.

I am pretty sure it is a siren, I puchased a 6vdc - 12vdc siren from radio shack, plugged it in and it made noise, just not very loud. I will see if I can get a schematic or some pictures of the board.

Here is the link to the alarm i purchased:
https://www.radioshack.com/product/...larm+system&kw=alarm+system&parentPage=search

You could always use a transistor to drive the siren. A transistor will turn on at 0.7V, so all you need is a resistor on the base to limit the current from your alarm. But I would need to know how your existing alarm works and how much current your new siren draws, before I could give you detailed instructions.

You might try connecting a AA battery to your existing siren and see what it does. If your alarm is truly supplying 1.5 volts DC to your siren it should sound the same with a 1.5V battery connected to it.

I'm with kchriste on this one, most of these types of alarms use speakers instead of sirens and have an a/c signal on the alarm output and not DC voltage.

Another option would be to use another output the alarm system may have. Some alarms will have outputs for switching on lights or other external indicators. You might be able to wire up to one of these outputs or maybe wire up a relay to one. If it's an output that pulses, like one that would flash a lamp for instance, you'd need to wire the relay to latch on so you'd get a constant siren output.

Do you have a schematic or manual for the alarm you could link us to? That would be helpful as well.

The specs on it show that it uses a 8VDC power supply. But I would still try qsiguy's suggestion of appling a 1.5V battery to the original siren first and see what happens.

I have tried the battery already, it works. I hooked up my dvom it is definetly 1.6 volts. I am positive it is a siren. There is already a little transistor on the board that has 4 wires 2 go to the board and 2 go to the siren. I will try and take some pictures of the board. I was worried if i use a larger transistor that the current going back to the board would be to much, could i maybe tie one of these to ground elsewere? or use a resistor?

I was worried if i use a larger transistor that the current going back to the board would be to much, could i maybe tie one of these to ground elsewere?

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If you are going to use a separate battery or power supply for your 12V siren you don't have to worry about this. The only problem would be if you want to use the SAME battery to power your alarm and the new siren. Below is a diagram of a circuit that uses a separate battery for the new siren. It can be done using one battery for both but you need to know which power lead of the original siren is switched by your alarm.

Perfect, exactly what I needed!

Sorry One more question, Not sure on all these symbols,
I am assuming that R1 and R2 are resistors. 15r and 100r must be there ratings. but what is T1 Tip 31/ the symbol to the left with the arrow, is this a diode? Kind of new to this stuff. Thanks in advance

15R and 100R are their resistances in ohms ohm.
T1 is a transistor.

If you need to ask these sorts of questions then I think you need more experiance before attempting this.

ya all of you say this, I figured it out. made this and it is working great.
thanks for everyones help.

kchriste said:

If you are going to use a separate battery or power supply for your 12V siren you don't have to worry about this. The only problem would be if you want to use the SAME battery to power your alarm and the new siren. Below is a diagram of a circuit that uses a separate battery for the new siren. It can be done using one battery for both but you need to know which power lead of the original siren is switched by your alarm.

Click to expand...

how u have decided the value of resistors?