A switch will be used to turn the solenoid valve on and off. For this project a flip-flop will be used to allow current through a relay to power the solenoid valve. The flip-flop chosen for this project is a SN74LS74AN. It is a chip that will output 5 V DC when activated and when deactivated it will send no voltage.
The start button for the system will be hooked directly to this chip and a voltage will come from a OR chip to deactivate the output voltage. Figure 1 shows the flip-flop and the pins that will be used for this application.
As seen in figure 1, the start button will be used to connect pin 4 (set) to ground and turn on the output from pin 5 (Q). Pin 5 will be wired to the relay, which when closed will let 24 VAC to flow through the relay and power the solenoid valve. When a low signal is seen at pin 1 (clear) the output at pin 5 (Q) will stop and the relay will open, therefore turning off the solenoid valve and the water to the sprinkler. In order to get a low voltage, an inverter will be used to change the 5 VDC from the photo eye, the stop switch, or the timer to a low voltage. (This same inverter has several applications in the project.) The inverter that will be used is a 74LS04. This inverter has several inputs and outputs.
Figure 2 shows a schematic of the inverter with all of the inputs and outputs that are used for the sprinkler control system, including the flip-flop.
As seen in figure 1, the signal from the OR chip is sent into pin 3 and the opposite of that signal is sent out from pin 4, which is connected to pin 1 (clear) on the flip-flop. This is the signal that stops the signal coming from pin 5 (Q) on the flip-flop and shuts of the water to the sprinkler.