RP2040 need to drive a 4V LED

[SezaroSteve]

Hello,

The HDD LED in my Amiga 3000D operates on 4V. I did not know this at the time, so I hooked up my RP2040 (soldered two pins on the LED header) just like I have done in my other Amiga's and.... nothing. No HDD Activity LED.
As a reference, I measured the power LED and it works on 4V. As the HDD LED is of the same type I became suspicious that the Zulu's 3V is to low to make the LED work at all.

I attached a "lab power-supply" to the LED. The PSU can supply 0 - 15V up to 3 AMP very granularly and with this, I confirm my own suspicion. These original "Commodore" LED's need a starter Voltage of 3.65 V.
At 3.65 V, the LED starts to shine very faintly. At 4.0 V the HDD LED reaches normal brightness.

So the HDD LED is good (pfew..) but I need to feed it 4.0 V to work and the ZuluSCSI only provides 3.0 V.
I know how to reduce a voltage by placing a resistor inline on the + wire. But I have no idea how to safely increase the voltage from 3 to 4 V without damaging the Zulu.

With my limited knowledge of electronics, I found these small Adjustable Step Up DC power converters that can do this, but are they safe, as most of these units, take the + and - from the source, step it up and output a higher Voltage. They only have 4 pins: 2x "in" and 2x "out". But I guess this puts a strain on the source (higher Amps? I dunno).

Examples:
https://www.techtonics.in/dc-dc-3v-35v-to-4v-40v-adjustable-step-up-power-module-high-power-boost-converter

https://www.ebay.ch/itm/174959053725?hash=item28bc5f4b9d:g:gqUAAOSwhgpia46a&amdata=enc%3AAQAIAAAAwMbMfc9bc4VVBqrtSA5pkOTTya5hcfk4RBfmtPDpxqBeJ7tSEjU2F8s3pov3s11QFcenpY0UIjIPHTYipOGjWG819OfoXetr6gk%2BbfLoYnR5ukBS9CcwGGXsZz8P6mGHFbqzYKIZcEwo2pVbtvtN98xZucVU%2BHsxz621qDPPqJwAzQm7nh772d2ckP6gZfm6UvN0ScAEgwOKfNXVBjn%2FUk0ElHRaGoH4bjglTHQRE6JQpUBMjcsitM6O7tBY9yn%2FLA%3D%3D%7Ctkp%3ABk9SR5TBpfmVYg

[PetteriAimonen]

If you have +5V available in the system, easiest way is to use a transistor to control the LED.
The transistor can be any small NPN transistor.
Common types such as 2N2222, 2N3904 or BC547 are suitable.

This tutorial explains more of the details.
The basic circuit is:

R1 is from the ZuluSCSI LED output to the transistor base and can be any value between 1 kilo-ohm to 10 kilo-ohms. The Amiga LED probably has the LED resistor R2 already integrated.

In your case VSS would be the +5V supply from the computer, while ZuluSCSI would supply 3V to the transistor (0.7 V is enough to trigger NPN transistor).

[SezaroSteve]

You mean the +5V on Pin 1 of the motherboard header? (otherwise I don't know what you mean with "driver", sorry for being such a noob)

[PetteriAimonen]

The 7407 driver IC which is used to control the LED originally can take input from ZuluSCSI.

[SezaroSteve]

The 7407 driver IC which is used to control the LED originally can take input from ZuluSCSI.

I understand now. But i'm not going to touch the motherboard. I'm leaving everything there alone. The Motherboard was rebuilt/restored at great financial- and labor-expense and i'm not going to solder a bodge-wire onto some chip's leg. The entire solution must not touch the orginal Amiga at all.
I'll gladly steal +5V from a Molex or Floppy connecter to feed the base-pin of the transistor, that's fine. Everything else is purely Zulu -> LED.

I'll be building a test-rig for this first. I will not touch the original HDD LED before I am 100% sure that it will not get damaged.
The LED's don't exist anymore (they are 10mm wide in one-piece) and if damage that LED, i'm in big poopoo as replacements are impossible to get (or insanely expensive).
(there are several after-market LED "3000D LED-boards" with several flat LED's besides each other to mimic the 10mm wide original which look terrible imho).

I'm now looking for a LED that can take the place of the original 4V HDD-LED's (has a forward voltage of 3.6V) on the test-rig. I can't find LED's that are specced for 4V and also have a current of about 5mA.
They are all 3.3V or 5V and 10 or 20 mA nowadays.
In my test-rig, I could take a modern, round, 5V 20mA LED and use a 50 Ohm resistor to reduce 5V to 4V correct? (and assuming it has a forward-voltage of a little less than 4V)

(I sound more knowledgeable than I am. I learned about Ohm Law and what a transistor is, only today).

[PetteriAimonen]

Ok, this is probably more than what can be resolved remotely. I thought that schematic was for a "LED board" not motherboard.

Maybe you can seek out a friend or someone else local who has more electronics experience and can guide you through this?

[SezaroSteve]

That schematic IS for the A3000 LED Board. The 4707 Chip is on the motherboard. The LED-Board has no IC's on it.

This is what the A3000D LED-Board looks like. There are two generations (red and green PCB) but they are the same otherwise.

[SezaroSteve]

Thanks for putting up with me this long. I don't have anyone that can help me so i'll have to figure it out. The test-rig should allow me to blow up as many LED's as needed until I get it right.

[SezaroSteve]

One last question if I may: where does the ground-wire from the Zulu goto? It's +3V will go to R1, but it's ground?

[PetteriAimonen]

All grounds should be connected together. They probably are already connected together through the power supply anyway.

[aperezbios]

@SezaroSteve the circular-looking plated through hole labeled 'C' (for cathode), is the right-most pin on the LED pin header, J304. The square-looking plated through-hole pin to the left, closest to the "LED" silk-screen, is the positive pin.

[SezaroSteve]

Hi yes I know. That's not what I was asking :-)
The + from the Zulu goes to R1. (normally this ends up at the + pin of the LED)
The - from the Zulu goes where? (on the other side, not the Zulu Side). (normally this ends up at the - pin of the LED)
Petteri answered the question thnx.

[SezaroSteve]

Succes !

I ordered a 2N3904 NPN Resistor as per your advice and hooked everything up according to the schematic. And it works perfectly.

I used a modern-day 5V 20mA yellow 5mm round LED at first. When this worked, I lowered the 5V on my "lab powersupply" (Vss) and created traffic by running a disk benchmark. I then slooowly upped the Vss Voltage until the HDD LED on the LED-Board started to work.

As the exact, precise specs of the original yellow HDD LED are unknown, I was very careful and increased the "R2 resistor" bit by bit while also increasing the VSS voltage in turn, until the desired brightness was achieved.

I literally used the LED's brightness as the means to determine the correct resistor. The HDD LED lights up to almost the same brightness as the Power LED at normal brightness, which is the result I was after.

Long story short: I ended up with a 50k Ohm resistor connected to the base-pin of the transistor (R1 in the schematic).
Between the powersupply and the LED, I ended up using a 460 Ohm resistor on R2 with a Vss Voltage of 5.04 Volt as this is the voltage that the 5V Rail in my A3000D delivers.
The brightness is the result of the Ohm value of the resistor.

The brightness is a bit hard to compare as the HDD-LED flickers (I copied a large file between two (virtualized) SCSI HDD's to make the LED light up). But you get the idea.

Next thing to do is to package it nicely in a clean solution that can simply be plugged in between a Zulu's LED Header and the 3000D HDD LED. I'l report on that when I've built that too.

Time for Piccas: in one you see the HDD LED being lit up. In the second I drew up the final schematic.

[PetteriAimonen]

Nice to hear you got it solved and thanks for sharing the information!

[SezaroSteve]

And here is the finished product. I call it the "A3000 Desktop ZuluSCSI LED Adapter thingy".

I use a 3-Pin Case-fan style connector to connect/disconnect the adapter from the HDD LED ( I only use two of the 3 ping though).
It works perfectly :D