For my final project, we are building an ultrasound range finder, without using a microcontroller. I have been able to build a block diagram (attached document). In the circuit, I'm using a 555 timer to modulate the output signal for the transducer, and another 555 timers to control the 4-bit counters.
How do I trigger both of the 555 timers at the same time, and stop the second 555 timer when a signal is received by the ultrasound receiver?
IF - what you want is for the start of an output pulse to enable the 2nd 555 to start clocking the counter, and the reception of a reflected pulse to inhibit the 2nd 555 ...
THEN - place a set-reset flipflop in front of the 2nd 555. Set it with the output pulse, reset it with the received pulse, and its output goes to the 555 enable.
NOTE - Coming out of the reset state or power-on, the first half-cycle of a 555 astable circuit output is longer than all of the succeeding cycles and half cycles. This is because for that first half-cycle only, the timing cap is being charged up from 0 V; in all following cycles it is being charged up from Vcc/3 V.
Using the 555 is a pretty bad choice here.
Your first 555 would be easily replaced with a crystal oscillator, producing a more stable tone; for maximum efficiency and sensitivity, you should low pass filter, no matter how you produce a square or sawtooth wave, both the tone you transmit, and what your receiver picks up, so that you only get a harmonic signal at the fundamental frequency.
The comparator alone doesn't make too much sense? That looks like you half-understood a PLL or a correlator, but then didn't really do the integration.
Now, your system is supposed to measure times very accurately. The 555 is really not suitable at all here, as it's wildly inaccurate in timing. Time measurement is really the domain of crystal oscillators, and it would be much wiser to do it with a crystal oscillator.
So, I think, the honest answer here is: Drop both your 555. They do nothing for you that something else couldn't do just as well, but probably easier and more accurate.
Generally, I don't think your timing methodology makes too much sense; I'd recommend drawing your measurement procedure as finite state machine, and building exactly that from digital gates.
PS: There's a reason why microcontrollers are so popular in the last 40 years; making you do this project without one is not really worthy of a final year project, where you're supposed to show you're able to solve a real-world problem with real-world methods on your own.
