Electrical Circuit Implementation
There are two basic parts to the schematic:
- The circuit which is designed to sense failure and shut off water flow
- The circuit which will indicate that a failure has occured
The failure sensing circuit is designed to be able to sense low UV output (from aging bulb to brown out), power outage, and a broken bulb. This is achieved with a photodiode which senses the UV output of the bulb. This is sent into an transimpedance op amp which will gain up the small current out from the photodiode and output a workable voltage. This output from the op amp is fed into a voltage comparator which is referenced to a specific voltage. If the output of the op amp drops below this reference, the output goes to 0. This will cut power to a relay. The normally closed contacts of the relay are connected to the solenoid which is used to trigger the flow stopping mechanism. When current is cut from the relay coil, the contacts switch so that power is cut off from the solenoid, causing the solenoid valve to trigger and block the flow of water. The output of the voltage comparator also is brought out to the warn operator circuit.
The warn operator circuit is designed to pulse a speaker and LED when there has been either a power failure or low UV output has been detected. This is accomplished by having a 555 timer biased off by the presence of a line voltage provided by the failure sensing circuit at pin 7. When there has been a failure, this voltage is removed and the 9V battery powers the circuit to pulse a speaker and LED at a realitivly high frequency to warn the operator that there has been a failure. When the operator realizes there has been a failure, he flips a switch to turn off the warn failure circuit and activate the warn reset system circuit. This switch is a double-pole-double-throw(DPDT)switch that connects the battery and line voltage to the warn failure circuit. When flipped, the switch will disconnect the battery and connect the line voltage to the warn reset system circuit. When the problem has been fixed, the warn reset system circuit will sound at a realitivly low frequency notifying the operator to reset the system.