clear, unmistakable
agreed, prearranged
conflicting, confusing, contradictory, mixed
wrong
Laughing when you should be crying sends out the wrong signals to people.

alarm, danger, distress, warning
hand, non-verbal, smoke, visual
turn (AmE)
busy (AmE) (engaged tone in BrE)
All I get is a busy signal when I dial his number.

buy, sell (both business)
A strong buy signal was issued to traders.

give (sb), make, send, send out
When I give the signal, run!

interpret, read
The brain interprets the signals from the retina as light.

interpret sth as
The remark was interpreted as a signal that their government was ready to return to the peace talks.

pick up, respond to
Interviewers quickly learn to pick up non-verbal signals.

act as
The insect's yellow spots act as a warning signal to its predators.

come from sth
Try to read the signals coming from the patient.

indicate sth
the signals that can indicate danger

at a signal, on a signal
At a prearranged signal, everyone started cheering.

signal for
She made a signal for the car to stop.

signal from, signal to
Wait for the signal from the leader of your group.

railroad (AmE), railway (BrE), traffic

operate

be on red/green, be red/green
The traffic signals were on red.

fail

box (BrE)
failure (BrE)

faint, weak
strong
high-frequency, low-frequency
input, output
acoustic, analogue/analog, audio, chemical, digital, electrical, electronic, GPS, light, radar, radio, satellite, sonar, sound, television, TV, video, wireless

carry, pass, relay
The nerves carry these signals to the brain.

amplify, boost
convert (sth into), scramble
The signal is scrambled into code before it is sent.

decode, encode
emit, generate, produce, send, transmit
detect, pick up, receive, respond to
This equipment can detect very low-frequency signals.

block, jam
It is possible to jam GPS signals in battle.

travel
The digital signal travels down wires to the server.
A light signal can travel well over 16 km before it halves in intensity.

fade

intensity, strength

signal from
a faint signal from the satellite

signal to

frantically
I saw her signal frantically to us.

for
He raised his hand to signal for the waiter.

to
She tried to signal to the bus driver to stop.

clearly
These changes clearly signal the end of the welfare state as we know it.

effectively (esp. BrE)
A change of mind effectively signals a change in overall policy.

not necessarily
A fall in demand does not necessarily signal the death of the industry.

appear to, seem to
These events appeared to signal the end of an era.

try to
be intended to, be meant to
This address was meant to signal a change in policy.

Signal is used with these nouns as the subject:
bell

Signal is used with these nouns as the object:
approach, arrival, beginning, dawn, desire, determination, emergence, end, move, onset, readiness, return, shift, start, turn, turning point, willingness