Joystick port supply voltage only 3.5V?

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Don't Panic. Please wash hands.
  • I've had a small electronics project to add autofire to my 3D-printed joystick. I am using a 555 timer for this, whose supply voltage range is 4.5V-16V. Knowing that the C64 outputs 5V on its joystick ports I figured I could drive the 555 with that.


    However, I just measured the Keyrah db9 output voltage, and it's only 3.5V? Is this normal, i.e. do I need to add a step-ups regulator to my circuit to go from 3.5V to 5V?

  • Keyrah does not supply any voltage to the joystick ports at all - see documentation. This is to comply with the USB spec: The power consumption of an additionally-connected device is not known, but the USB device must report it's power consumption in the USB descriptors. These would be wrong if you connect anything that consumes power.


    The GND line is actually an IO line of the controller, so adding VCC to the other pin may destroy the unit.

  • Hi Jens,


    thanks for the reply. FWIW I will still try the circuit, but this time with a version of the 555 timer that can run on the 3.5V. It's a "micropower" version of the timer, i.e. it draws mere microamps, so I'm hoping to fly under the radar of of USB spec.

  • FWIW I will still try the circuit,

    Please don't - it won't be successful, as the GND line is not connected to GND, but to an MCU pin. It toggles on a regular basis and is therefore no good for supplying any kind of component. Further, even a "micro power" version of the NE555 won't really consume "micro power", as the amount of power drawn is set by the external circuit. The energy to charge/discharge a capacitor must come from somewhere!