Creating a wireless transmitter/receiver for off-the-shelf siren

Question

Since I oddly enough haven't been able to find a suitable off-the-shelf product that's reasonable priced (less than 50-60 EUR/USD) and somewhat good looking with Google, I'm about to try a small electronics project as follows:

I have a simple alarm system controller for vehicles that run off 12V with a backup battery. This controller has a siren output with +/- terminals that delivers up to 800 mA when the alarm is triggered. Here's the catch: I want to have the siren wireless so it doesn't have to be mounted in the vehicle, but on a wall some 10 meters away to attract attention. I could run a cable on/in the ground for that distance, but I'd like to avoid that.

I've been googling for weeks for various solutions that doesn't require me to build everything from scratch. My conclusion from these searches is that the neatest solution would be to hook up a wireless (e.g. 433 MHz) transmitter to the alarm controller output and then connect a corresponding receiver to the siren. However, I haven't found a suitable transmitter/receiver combination pre-built, so I'm looking for options to build them.

The closest thing I've found are a couple of Chinese products: the transmitter part e.g. https://www.aliexpress.com/item/433MHz-Wireless-Transmitter-Module-For-Our-Wireless-Siren-For-Our-Home-Burglar-Security-Alarm-System/32917159619.html but those requires the vendor's own siren which is branded (in an ugly way) such as https://www.aliexpress.com/item/NEW-High-Quality-Wireless-Waterproof-Strobe-Siren-433MHz-For-Home-GSM-Alarm-System-Security/32813027446.html.

The question is this: could you please recommend how I would go about building a wireless transmitter like the one above that runs on 12V and max 800 mA and a receiver that can (ideally) be battery powered? I've been trying to find similar threads here, but most people ask for more complicated things like transmitting more than on/off or actually building building the sending/receiving part from scratch.

What I'm looking for is something that powers up when the alarm goes off and sends an ON signal to the receiver. Upon receiving the ON, the receiver then powers up which makes the siren sound. The receiver should then drop the power to its output (siren) after either a fixed or variable amount of time depending on how much complexity to the build either option adds. In any event, there needs to be some kind of safeguard so the siren doesn't sound until the battery is drained.

The siren I'm thinking I could connect a receiver to is something like this: https://www.aliexpress.com/item/12V-103-Mini-Wired-Strobe-Flashing-Red-Light-Siren-Durable-Alarm-Siren-Home-Security-GSM-Alarm/32858844694.html. Runs on 12V with 300 mA current. The beauty of the receiver should be that it just has a 12V output, so that I can connect another siren if needed.

Bonus: the sender and receiver should be linked to each other, to avoid anyone just sending a ON message on the siren and have it sound. Not needed though, I could rely on security through obscurity if it reduces complexity for the build.

I'm ideally looking for something that doesn't require me to create a PCB, and create PIC code etc. E.g. Arduino is fine I think, but I don't have a clue on how to program it. Soldering I can learn if needed, possibly the programming part as well. I'm looking for the best combination of low price and easiness to build here, rather than using expensive Z-wave controllers (expensive) on one hand or build everything from the ground up on the other (difficult).

Any tips or pointers greatly appreciated!

Answer 1

Radio isn't that simple. Imagine your siren built into a box with a radio receiver (just one of those cheap ebay radios that cost a few dollars). The radio will be constantly producing a noisy data output because it is a high-gain system and is designed to try and extract pico watts of power from the "ether" so that a weak radio broadcast can be adequately received and decoded.

A slightly more expensive receiver may have an RSSI pin - this will give you some Indication about the level of the Received Signal Strength. You could attached an op-amp or comparator so that you gate the noisy data output to your siren only when the signal strength is fairly high. This would help you exclude all the mushy noise that your receiver would naturally pick up in the absense of a strong signal.

However, the strong signal you receive may still be from other equipment so your siren will falsely go-off quite often.

What proper EEs do is transmit a message that is equivalent to "on" and this is usually formed from: -

• A preamble of several bytes that allows the receiver to adjust its receiver in time for the next part of the message
• An address byte so that there is a nod towards trying to make the "system" have some level of integrity/security
• A bit that might mean "on" or "off" (your payload data).
• A two byte error check so that even more integrity is afforded.
• Maybe send it twice and, only act on two consecutive transmissions being correctly received.

This is a minimum system and won't guarantee against false alarms every week or so but will be reasonably reliable. You may think that this answer is aimed at those folk wanting to send data messages as per this: -

but most people ask for more complicated things like transmitting more than on/off

I'm sorry to disappoint you but sending one bit of information (i.e. on or off) requires signal integrity to be achieved by transmitting a message as per the bullet points above.

Just think about a music radio receiver tuned to an unused part of the spectrum - all you hear is noise and that noise is converted to thousands of ones and zeros in order to extract data - the whole point about a data transmission with integrity, is that that chances of noise exactly matching the random ones and zeros produced by a good transmission is quite slim so, you package your single bit (on or off) into a frame that has high integrity. This means that the receiver can detect good transmissions relatively easily AND bad "transmissions" (due to noise) only correspond with the frame protocal once in a while.

Transmitting 1 or 0 by radio isn't as simple as you think (and for good reasons).