Transmit Time Limiter
Posted on Feb 3, 2013 10260
This circuit prevents making transmissions that are too long, which `time out` repeaters and/or tie up a communicatio
ns channel too long. On transmit, the PTT (push to talk line) from the microphone is at ground. This causes one input of the AND gate (4081) to go high. The 555 is held in the reset mode. An MC1453B monostable multivibrator generates a pulse to the 555 and causes it to produce a gate of length approximately l.liC, where R is selected for desired time delay and C is 100 Â¥. The output of pin 3 of the 555 causes the 4081 gate to go high, turns on Ql, and keys the transmitter. At the end of the cycle (1.1/C), the AND gate will lose one input, which turns off Ql and unkeys the transmitter.