In the gantry system the two main items are:
• PIC1: Operates on 2.0 V to 5.5 V DC. It needs only a few tens of milliamps to drive the LEDs.
• Motors: Three of these running on 12 V DC at up to 500 mA. Under heavier load they would need more current but 500 mA is the most they are likely to take.
The best supply for the PIC is a battery of four nickel-metal-hydride rechargeable cells. This produces 4.8 V (a little more when freshly charged). Two such batteries are convenient for running the motors. These give 9.6 V, which slightly under-runs the motors. Or we could use a 12 V battery or a mains plug-top power supply unit. The supply does not have to be stabilised, but the PSU must be capable of providing 500 mA.
The power switching circuit is mounted on the left side panel of the gantry, near to the front. There is a 3-way screw terminal block for connections to the batteries or PSUs: motor 9.6 V or 12 V, common 0 V, and logic 4.8 V. The LEDs and switches are mounted on the side panel with a small circuit board for making the connections.
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