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The following schematic shows the circuit for supplying power to one of the solenoid valves.
The power to the solenoid comes from the 12V power supply circuit. The relay and the transistor are powerd from the 5V circuit, which is fed from the Arduino.
The solenoid is controlled by a cascade-switched circuit. A digital output pin of the Arduino is connected to the base of a transitor which controls the current to a normally open, SPST relay. When the relay coil is energized, it closes the contacts, which allows current from the 12V supply to flow through the solenoid. When the solenoid coil is energized, the valve opens, allowing water to flow from the reservoir into the fish tank.
The following photograph shows coil-side view of three relays. Two of the relays control separate solenoid. The third relay controls the heater. The wiring for the three relays is identical. The short red and green jumpers connect the relay circuit to the 5V power rail along the nearest edge of the breadboard.
The brown, yellow and blue wires are connected to digital I/O pins on the Arduino. Each of those wires is connected to an NPN transistor by a 220 Ω resistor. The transistors are on the ground side of the relay coil. Power diodes are in parallel with the relay coil act as snubbers to the charge stored in the coil. When the current to the coil is switched off, the snubber diodes (a.k.a. flyback diodes) allow the electro-magnetic field in the coil to safely dissipate through the coil windings.
The following photograph shows the contact side of the relay circuit. There are three sets of long read and black leads that connect the relays either to one of the two solenoid valves or to the heater. Two of the three relays have red indicator LEDs to provide visual confirmation that the contact is closed. The rightmost relay circuit has its LED circuit removed to make the connection to the solenoid leads more clear.
Copyright 2013 © Gerald Recktenwald
This material can be freely distributed and remixed as long as the original source of the material is cited.
Read more: Wiring of the Solenoid Valves