Small Signal Generation/Detection

I would like a device (sensor) that can generate a distinguishable signal; the signal attributes are not that important, but the signal must be detectable from a distance of max 100 feet and through walls. The signal only needs to be sent once (per event) and it is desirable to run it from a single AA cell or similar small power supply.

I've thought about a simple ultrasonic tone pulse and using the tone frequency as the unique attribute. I also thought of something like a spark gap transmission of a few pulses or other type of RF signal (like a single transistor FM transmitter with a single tone modulation circuit) or other EMP. I've not sorted out how to do these though. Another idea was an active RFID tag, but the design is getting too complicated and expensive.

Any ideas of the simplest device? I'm hoping to have about 5 devices that I can tell the difference between which one is signaling.

I also thought of something like a spark gap transmission

Forget it.

What type of walls? That can change your requirements dramatically.

Are you trying only to detect the presence of a signal (IE only a Binary signal, such that you are only detecting that there is either A signal or Not a Signal, and not trying to transmit any more data?)

I would probably not do something with a spark gap unless you are desperate, those tend to be noisy and cause problems in surrounding electronics.

Also, adding the requirement of distinguishing between pulses makes it much more difficult. The FM idea isn't bad though, since it tends to work through walls, though that will involve a relatively high power requirement per device.

Simple 433 MHz transmitters will work just fine on 3.7-4.2V from a LiPo battery.
Drive each with a 3.3V/8MHz promini, sending a simple 1 byte message using virtualwire library.

I guess an analogy could be a birds chirp, which is unique to the bird and easily heard over the distance through walls.

I thought of the spark gap such as an electric spark ignition from a lighter, just connect the HV line to a wire antenna. Power could come from the mechanical force of opening the door. The burst could be detected with a high end receiver - it might be too difficult, besides potentially being illegal.

I also thought of a simple RLC oscillating circuit that would run off a charged capacitor for a second or so, then recharge the cap from a battery. This could be an ultrasonic or RF signal, like a simple beacon - most beacon plans I’ve seen are again too complex or require too much power. I thought initially it would be easy to find a small RF beacon, something in the unlicensed low frequency band.

I’m leaning a bit toward a simple AM or FM transmitter (which would be simpler and use less power?) and using a commercial receiver (AM/FM radio) to detect and demodulate the signal, and feed the tone to Arduino. Kind of like this “simplest bug”: Spy Circuits. <— how could I get a modulated tone there?

It’s too much to have the complexity of a micro at each sensor location to drive a tx, and power is an issue.

Power - not a problem.
Use a trimmed down '328 circuit, go into power down sleep mode in between transmissions.
Transmitter goes into sleep also with no changes on its data line.
Interrupt the '328 to wake it up & send a 1 byte message out.
3.3V/8MHz 328P will last a month or more on 1000mAH LiPo.
Run it slower, will run even longer.
This one uses MAX1811 (goes in the left hand socket) for battery charge from 5V source.

Well, the keypad and RF module you have there are two projects I've been wanting to get to...