RF transmitter connected directly to sensor
It's a bit more complex than you think: -



You probably need to send a preamble of data first to lock the receiver into your transmission. Before the preamble is sent your receiver is trying to lock into any old noise and glitches coming thru the ether. Halfway along the horizontal in the picture above is where a transmitter starts sending a preamble - it might take several milliseconds for the receiver to "lock-in" - see the blue curve and note that after a short time the receiver is reproducing the data preamble.

This means you need to recognize the preamble and then at some point expect to receive a valid header byte - this can simply be an address that is expected i.e. 100 from sensor 1 and 200 from sensor 2 - the numbers don't matter just the fact that they are different and recognizable.

Then you transmit your data then you append an error check.

That's how a lot of devices will work. Without sending an error check byte(s) you cannot possibly know that what you have received is a reasonable representation of what you sent. You could repeat the data several times and compare numerically the sent data - if it's the same number, then some level of confidence can be gained.

If you want my advice - throw out the 555 ideas and use a small micro.