HC-SR04 Distance Sensor Trigger Signal on 2.5V Voltage level gives no reading
Like most CMOS except 74HCTxx series the input threshold is Vdd/2 +/- 40% over entire temp on some units.

This requires a level shifter.

You can choose many options : 


an IC to levelshift inverting or not
an RC:R pullup network to shift from 1V to 4 V with some T=RC > pulse width using R’s between 100k and 10M to choose ratio for the “0” output level and C to boost the pulse by 3V thus shifted.
it depends on you power consumption specs 


Added to prove my assumptions

Often many poor Chinese specs and others say TTL input to mean 5V TTL logic compatible, because the null current static CMOS load will increase the signal swing of TTL rated VOh,Vol limits.    But 5V CMOS is what they actually have so the input bias threshold is Vdd/2 +/-6.7% nom 25'C upto  40% for -40 to 85'C (NXP)

So I looked for the schematic to confirm my assumptions and found they were valid.