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## Control Signals
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To control the valve operation, a set of relays is employed. The initial hardware is an 8-position relay, though only 4 are implemented in software. The relays are opto-isolated from the control side of the board for safety, though the 24vdc passed to the valves is still considered low voltage (under the 50vdc limit). A micro-controller (described in the next section) controls the relays with a 5vdc signal. The relay board uses an active-low configuration, so an NPN transistor is utilized to further isolate the microcontroller from the relay board. The microcontroller interacts with the base of the transistor, enabling the control pin to go low, enabling the relay.
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Use of the [Sainsmart 8-channel relay hardware was selected](https://www.sainsmart.com/products/8-channel-5v-relay-module) based on reliability and personal experience. It is rated for 30vdc operation under continuous load. Given we are drawing ~200mA @24vdc, we are well within specifications.
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# Micro-controller
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Leveraging a [Adafruit QT-PY ESP32-S3 board](https://www.adafruit.com/product/5426) as the new control unit. Pin usage as follows, see image below:
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- A0 - NPN 1 controlling valve 1
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