23 July 2021, 10:01 | #1 |
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A500 LEDs
Hi,
Does anyone know what the spec of the power and drive LEDs are on an A500 keyboard? I'm trying to see what else I would need (if anything) on top of the 22ohm resistor thats already in the LED circuit on the keyboard controller to be able to connect them to GPIOs of a raspberry Pi to control them. Currently I'm not worried about mimicking the dimming that a proper Amiga does, just want to connect them as standard LEDs to a pi. Many thanks! Last edited by craggus2000; 23 July 2021 at 15:13. |
23 July 2021, 15:30 | #2 |
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Can't provide anything useful but if i recall correctly this is generic 2 LED's connected in series - so assume double voltage and current bellow 20mA.
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23 July 2021, 16:23 | #3 |
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Yes it is 2 in series. I think i read somewhere that each pair draws 200mA! Which to me sounds too high, but thats just one forum entry I read.
GPIOs are also 3.3v if that makes a difference? |
23 July 2021, 23:56 | #4 | |
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200mA means that they should be very bright but they look same as average green LED from end of 80's (at least to me) yep, 3.3V may be insufficient to make them work (i expect for 2 LED in series somewhere 4.2 .. 4.4V at least to satisfy forward voltage condition - not sure about green LED technology used by LED's and used by Commodore - in past for green LED material used was GaP (bandgap approx 2.24V) but nowadays for so called "pure green" commonly InGaN or GaN is used so bandgap is higher and typical for blue or white LED's i.e. over 3.3V. probably you will need some transistor or logic gate capable to deliver at least 20mA with 3.3V control logic - should be not a big issue. |
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25 July 2021, 10:15 | #5 |
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Could you post the circuit I would use please?
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25 July 2021, 12:59 | #6 |
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This is more about how to power LED (you need at least 5V) - easiest and safest too for RPi is optocoupler - as optocoupler LED is usually IR then forward voltage is bellow 2V - fine from RPi perspective, output transistor can be hooked to way higher voltage than RPi so ++5V is also possible.
This probably most flexible and cheapest way to do safe control from RPi side something powered with more than 3.3V |
27 July 2021, 10:06 | #7 | |
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What about using one of the 5v pins on the Pi itself as the power source? |
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27 July 2021, 16:57 | #8 | |
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27 July 2021, 18:31 | #9 | |
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28 July 2021, 10:19 | #10 | |
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In a 5V-only system, the pnp would turn off when the base is high/open, yes. |
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28 July 2021, 23:20 | #11 | |
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Of course there is many possible solutions, logic level MOSFET for example will work perfectly (condition is to have Vgs significantly bellow 3.3V, for example 1.5V maximum or use P-MOSFET) - any level shifter will work fine too - more in attached AN. But IMHO optocoupler like 4N25 or similar will be cheaper and provide higher safety for RPi. |
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29 July 2021, 23:40 | #12 | |
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Ok, I've been able to make that circuit and it works! However... the after the Pi has shutdown the power led stays on extremely dim. I've tried increasing the resistance between 5v and the transistor, and also between the transistor and the LEDs, but all it does is make the LEDs dimmer when operating. The dimness is still there after shutdown. Any ideas? |
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