Overall Purpose: Takes TTL output from a vibration sensor and delivers it to a Sensaphone 400 (TM). A pseudo-opto-isolator is simulated for the N.C. contact characteristics of the monitoring device.
Circuit Operation: The idea is that if the sensor no longer feels vibration (say a running motor), a warning is raised. However, since vibrations are irregular and instantaneous, we need to wait a little bit before declaring an emergency. The real vibrations are many 10's per second. Waiting 10 secs then ensures that the motor has actually stopped.
At the circuit's heart is an RC network to count down/charge up. The Schottky Inverters (74AHCT14's) are to reduce some of the noise. Retriggering occurs when the left NMOS dumps the capacitor charge every time an instantaneous vibration is felt. The right NMOS emulates the opto-isolator that pulls the N.C. zone port open.
Reality vs. Simulation: On breadboard the pulse lengthening is actually between 0.5s and 13s. In addition, the two diodes are included to raise the trigger voltage (0.7V each) to the last Schottky Inverter to provide even more delay; however, that effect does not simulate well here.
PCB Device Indicators:
GREEN LED - power applied
YELLOW LED - activity indicator
RED LED - Fault on Sensaphone connected zone port (not enough vibrations in the time period.)
