# How to send voltage when a grounded signal is present?

I'm currently designing an adapter PCB for FAT model PlayStation 2's so that I can use power/eject/LED PCBs and their matching disc drives between any most PS2 FAT model revision.

Technical details:

Early PS2 FAT models have a NPN transistor on the same PCB as the buttons and LEDs connected via a ribbon cable back to the PS2 motherboard. The PS2 sends 3.5 V up to the NPN transistor to turn on the blue tray LED.

Late model PS2 FAT revisions have the NPN transistor on the actual PS2 motherboard. So on late model PS2s, when the LED signal leaves the motherboard it can either be grounded or not connected.

So I need to send 3.5V out from my adapter PCB, so that 3.5 V goes to the early PS2 models power/eject/LED PCB's NPN transistors base pin. I can source 12V at my adapter PCB.

What is the simplest way to make this happen?

NOTE: I've already designed the adapter for early model PS2 FAT motherboards to use late model PS2 power/eject/LED PCBs. All that I did was add a transistor between input and output connectors.

If I understood you right, you need +3.5V at OUT when IN is grounded, at 0V, and 0V OUT otherwise. The simplest implementation of that inverting behaviour, but with only 0V or +3.3V at OUT is:

simulate this circuit – Schematic created using CircuitLab

It might also be possible to use a single logic inverter, like the 74HC1G04, requiring only a single component, but this will only work if you can ensure that the potential at IN complies with 74HC input thresholds and slew rate.

simulate this circuit

Both of the above produce +3.3V outputs, given a +3.3V supply, not +3.5V. If you require exactly +3.5V out, then you'll have to derive +3.5V some other way, from 12V:

simulate this circuit

That will get you a +3.5V source of up to 100mA or so, which you can use to power any of the previous circuits, instead of +3.3V. Then they will all produce 0V or 3.5V outputs. Note the "L" in the part number LM317L for the regulator IC U1. This is a physically small device, but you could use the more common, and much larger LM317 (no "L") too, if you wish.

Lastly (from me at least), you can also obtain a 0V or +3.5V output from an NPN BJT inverter powered from +12V, with a resistor divider to constrain the output:

simulate this circuit

Whether these solutions will work depends entirely on the current requirements of whatever is connected to OUT. In reality, you might not even even need +3.5V out, especially if you are driving a BJT base, in which case all these measures to get exactly +3.5V are a waste of time.

• Thank you for your reply. I'm pretty sure 3.3v out will be fine. The simpler the better. May 22, 2023 at 9:52
• Could you suggest a SOT-323 PNP transistor and the resistors to use with it. As I am extremely limited on PCB space and the 2N3906 in SOT-23 package suggested is really too large to fit. Jun 3, 2023 at 9:38