First Project help, please.

[pebe]

Hi Jib,
1) An FET is a Field Effect Transistor. The type I have used is an IRF510. You can see the datasheet here:

http://www.datasheetcatalog.com/datashe ... F510.shtml

The Fairchild sheet will give the connections.

I would buy them at BitsBox here, but any stockist would be able to supply them.

http://www.bitsbox.co.uk/transistors.html

2) Failure of one LED would only affect the other two in the string. But they are extremely reliable and normally would only fail if you fed them with excessive current. Their datasheet should show the normal and absolute maximum currents. Use them with normal current.

3) The LEDs have to be fed in strings of three with individual resistors as I have shown. If you put the LEDs in parallel, the ones with the lowest forward voltage would take most of the current and the others would appear dimmer. By turning the boost voltage up to 30V you could have 6 LEDs in a string, if it would help. But the 555s would have to be fed direct from the 12V battery because their absolute maximum supply is 18V.

A forward voltage of 3.2V to 3.8V is quite a large spread, so you should check the voltage of each one – especially if they came from China! The easiest way is to make a simple test rig. Take a couple of pins and insert them into a piece of wood a few mm apart. Then connect the negative side of the 12V battery to one, and connect the positive side of the battery via a 470ohm resistor to the other. That will allow approx 20mA current flow through the LEDs. Clip the leads of a multimeter set to volts across the pins to measure the voltage. Connect the LEDs, in turn, to the two pins and ensure they light up. Then note the voltage and sort into three piles according to voltage. Ensure each string has similar voltage LEDs and adjust R* in the circuit accordingly.