Last Update: : Preliminary information subject to change.
One of the most common and useful applications for a Decaf project is controlling relays that switch higher AC or DC voltages and currents. This can be as simple as turning on lights, or more complex such as controlling pumps and other equipment as needed based on sensor inputs.
Available PiXCL relay modules have 1, 2, 4, 8 or 16 relays, with the digital output signals from the Decaf board received by optical isolators. The relays coils are switched by a +5Vdc supply (from the Decaf or external). The switched voltages are completely isolated from the logic signals.
The input connector header has active-L inputs, plus GND and a required +5V which provide power to the optical isolators. The +5V supply to the relay coils is handled by a shunt which sets either Decaf or external supply. When a relay control input is set L, a LED is illuminated signaling the selected relay has been activated, and an audible click can be heard.
Decaf Digital Outputs are active-H, and Control Outputs if present are active-L. If Digital Outputs are to be used (the most common situation), the outputs have to be set H before the relay coil 5V is available.
Each relay has three connections: Common, NO and NC. The aim here is to ensure that at project power-off and initial power-on state no devices connected through the relays are active.
Let’s suppose that the controller is a Decaf IOC-M-2 with 8 Digital Outputs and 2 Control Outputs and Relay-8 and Relay-1 boards are to be controlled. When the Decaf board app starts, all Digital Outputs 0-7 are set high (i.e. none of the relays are selected), and then Control Output 0 is set low, causing Relay-1 to switch 5V relay coil supply to Relay-8. The app can now control the Relay-8 board as needed, and the board LEDs will show which relays are active.
Relay-1 could be replaced with an SCS (Silicon Controlled Switch) device.
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