Initial Design Notes

[TerryJC]

All,

As discussed a week or so ago, the main design decision that we have to make for the electronic design of this project is the type of white LEDs that we need for the trackside lighting. The 74HC4050 hex inverters cannot do the job even if we put an LED on each device, most white LEDs of the 5 mm type consume about 20 mA each and the inverters have an absolute maximum rating of 20 mA. This is a bit close for comfort.

I therefore think that the easiest way to do this is to utilise a IRLZ44NPbF MOSFET. This is the same type as was used for the lighting project, so we already have some and we won't need to keep lots of different types of spares.

I have selected a white LED from RS Online and attached it datasheet below. Please check that this is suitable, especially from the mechanical standpoint, eg will it fit into your lamp fittings?

These are 20p each, but the Minimum Order Quantity is 20. so the total cost will be £4.

[peter]

this seems good to me
I let Geoff know and he thinks as long as we can adjust brightness if necessary then OK and he looks forward to receiving them so he can build the lights
thanks Terry

[TerryJC]

Peter,

What does Geoff mean when he says 'adjust brightness if necessary'? If he means at will, then I would have to drive the lighting with PWM. It isn't a problem, but I would need to know before I started.

If, on the other hand, he simply wants to set the general level of illumination during Setting to Work, then I'll be able to reduce the brightness using a larger series resistor.

[TerryJC]

I have now ordered the proposed LEDs.

[Geoff]

Hi,
with respect to the brightness of the LEDs it would be just to reduce the level of brightness to suit the location so a larger series resistor is all that is required.
Regards
Geoff

[TerryJC]

Thanks Geoff. I assumed that's what you meant, but thought it as well to be sure.