A500 LED spec

[craggus2000]

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!

[pandy71]

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.

[craggus2000]

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?

[pandy71]

Originally Posted by craggus2000

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?

doubt on 200mA - this looks like ordinary 20mA LED also 200mA LED's in 80/90 was usually unavailable for visible light devices (this kind of current was nothing unusual for GaAs IR LED's)
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.

[craggus2000]

Could you post the circuit I would use please?

[pandy71]

Originally Posted by craggus2000

Could you post the circuit I would use please?

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

[craggus2000]

Originally Posted by pandy71

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

What about using one of the 5v pins on the Pi itself as the power source?

[hooverphonique]

Originally Posted by craggus2000

What about using one of the 5v pins on the Pi itself as the power source?

You can power them using 5V from the Pi, but you still need a transistor (or other switching element) to drive them. Something like the schematic below - the stuff in the box is the existing keyboard circuit, so you just connect its GND pin to ground and PWR or DRIVE to the circuit shown to light a LED. Unfortunately, the LEDs are high-side switched, so you need two transistors to control them from 3.3V (unless you want to hack the keyboard).

[pandy71]

Originally Posted by hooverphonique

You can power them using 5V from the Pi, but you still need a transistor (or other switching element) to drive them. Something like the schematic below - the stuff in the box is the existing keyboard circuit, so you just connect its GND pin to ground and PWR or DRIVE to the circuit shown to light a LED. Unfortunately, the LEDs are high-side switched, so you need two transistors to control them from 3.3V (unless you want to hack the keyboard).

Single PNP transistor can be used as in orginal Amiga schematic but negative logic (i.e. 1 disable LED and 0 enable LED) and pullup resistor required but still IMHO best way (RPi safe) is to use cheap optocoupler.

[hooverphonique]

Originally Posted by pandy71

Single PNP transistor can be used as in orginal Amiga schematic but negative logic (i.e. 1 disable LED and 0 enable LED) and pullup resistor required but still IMHO best way (RPi safe) is to use cheap optocoupler.

I think not - when you apply 3.3V to the base of the pnp (through a resistor), it will be on because 3.3V < 5V-0.6V, and even if you switch the gpio to input (high impedance), the current will probably travel through the internal gpio tvs diode and still turn the pnp on (because the Pi runs at 3.3V).


In a 5V-only system, the pnp would turn off when the base is high/open, yes.

[pandy71]

Originally Posted by hooverphonique

I think not - when you apply 3.3V to the base of the pnp (through a resistor), it will be on because 3.3V < 5V-0.6V, and even if you switch the gpio to input (high impedance), the current will probably travel through the internal gpio tvs diode and still turn the pnp on (because the Pi runs at 3.3V).


In a 5V-only system, the pnp would turn off when the base is high/open, yes.

True but you can connect pullup to 5V - question is about RPI GPIO capability to sink current - that's why i prefer to use optocoupler - substantially simplify circuit, remove issue with different Vcc and finally prevent GPIO in RPi to be damaged.

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.

[craggus2000]

Originally Posted by hooverphonique

You can power them using 5V from the Pi, but you still need a transistor (or other switching element) to drive them. Something like the schematic below - the stuff in the box is the existing keyboard circuit, so you just connect its GND pin to ground and PWR or DRIVE to the circuit shown to light a LED. Unfortunately, the LEDs are high-side switched, so you need two transistors to control them from 3.3V (unless you want to hack the keyboard).

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?