Help Identifying Elements of Door Ajar FM Transmitter

Hello everyone this is my first post on this forum and I come to you with questions about a circuit in my door jam that senses whether or not the door is open via a magnet. There are some strange details that make me wonder if there is more than meets the eye....

First off, it has no physical connections whatsoever to other devices or power and runs off of a 5.6V battery. After a bit of research it appears to be an FM transmitter (although I'm not entirely sure). The DIP in the middle is just a bank of transistors and I believe the potentiometers are for controlling the frequency band of the FM output. I've labeled in one of the pictures what I think is the antenna? I also found a nail in the door jam adjacent to the detector with a wire wrapped around (not sure if it is significant).

The thing that seems strange about this circuit to me is that it has no wired connections whatsoever. Here's my confusion:

So it's a circuit that detectors if a door is ajar or not. What are the practical applications of this? Perhaps an interrupt circuit to make sure the heat isn't on when the door is ajar? Maybe just a simple status LED for security reasons. I do rent this apartment from someone, the first explanation would be advantageous for them, but why would you not tie it to power in the wall? What good is this circuit if it needs a new battery all the time? I'm assuming it would be active in the state when the door is ajar since the door is apt to be closed a whooole lot more than it is open. And why the FM transmitter? I actually fired up this bad boy and attached it to the 5v line on my arduino just to see if it had continuity by testing random points further in the circuit, around the IC, etc. and it appeared to be in working order. What I'm really paranoid about is that there is something shady like a microphone on board, or the circuit is being used to trigger something as a microphone or camera. Whether or not it makes sense to use the door as a trigger for a recording is somewhat questionable to me since I can't really see the significance of a door opening. I suppose you would know when people are coming and going but that's about as far as I can think. I looked up the ID no. and "Model No. FMT1" but didn't find really find much.

I've attached some pictures of the circuit for you to check out. I would really appreciate it if I could get a few glances. I'd really like to know whether or not the transmitter is communicating with another device or if there are any onboard components besides a simple magnet activated switch sending an "on" or "off" state signal to a receiver somewhere.

Well, the CA part is just a bunch of transistors: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CFAQFjAE&url=https://eeshop.unl.edu/pdf/CA3086.pdf&ei=AQQxU4-oCeTF0QG-pICADg&usg=AFQjCNGsZkctZdJXFDMGTo0-g_IM0i5pug&sig2=R2Q16M_N0tEqQqoXX54Z-A&bvm=bv.63587204,d.dmQ

best guess is a capacitive wireless (because of FCC part 15) proximity sensor which would transmit when the door is ajar. It could be part of an alarm system, door prop alarm or someone wanting to know when someone is leaving or coming. I don't think it's from this era. Probably more like the 70's to 80's technology.

Your antenna labeled thing is an inductor and it could be the antenna. Ferrite antennas were quite common on AM radios, but they were much longer. Look for PCB loop patterns on the back of the PCB as well.

The funny looking nail/spring combo is fairly common when screen door stops **broken link removed** are in use. A light, somewhat long, extension spring attached to the top of the jam keeps the chain from hitting the glass. Got one at home if interested in a pic.

Thanks for the reply.

I know exactly what you're referring to with the nail + spring and I do have one of those installed. I did identify the bank of transistors in the DIP. Your guess on the era is interesting...

What is it about part 15 of FCC regulations that narrows down the possible devices? Does the fact that it runs off of a battery seem a bit impractical to you? What could be said about the power consumption of this device? Unless it could run a long time it seems like bad design and makes me wonder if somebody put it there covertly.

e.g. https://www.google.com/patents/WO2006029345A2?cl=en&dq=wireless capacitive door alarm sensors&hl=en&sa=X&ei=kBIxU_ruIsbm0gGWr4GIDA&ved=0CEwQ6AEwAw

The RCA parts were some of the first IC's available. I used a CA3059 for my aquarium and someone used CA3140, CA3150 and CA3160 at work in the 80's. So, both the IC and the construction puts it in the 80's or so.

There might be some other clues such as:
The date code on the IC. It's usually a 4 digit number. like 4082 or so, meaning 40th week of 1982.

Part 15 sort of says that the device must take interference and except it with undesirable operation possible.

Also see: https://www.google.com/patents/US39...a=X&ei=hhYxU9WVL8Xp0gGdz4GoBw&ved=0CDoQ6AEwAA

I can't examine the exposed end. A wire to a disc would suffice.

More info might be on the model tag.

I agree the rca chip as far as I know is an op amp, a very old one.
I dont see a mic on the board, the antenna you suspect could be, or it could be a loading coil for the antenna.
Also I'd agree that its an old alarm system wireless door open transmitter.
The fact theres no battery fitted means that its not going to give you any trouble or snoop on you, but if your worried about it remove the pcb and fit it back in the door so it looks as it should.