?

If you click through to that post, it tells you

 

On 21/05/2017 at 13:58, Pumatron said:

Yes they are the alarm ultrasonic sensors, one is a sender and one is a receiver. It basically measures the time it takes to send and receive, if it takes to long then something must be in its ways (i.e. A car thief etc) and sets the siren off. 

Thanks!

I think that both of them might well be TXRX units, ie both transmitting and receiving, but there again I could be wrong!