not so bright...

Our red LEDs use about 2V and about 20mA so use about 0.04W (we should measure this really) and for a red led the brightness is about 70 lm/W so a total luminous flux of about 3 lm if it is 100% efficient. A typical efficiency for Red LEDs . It emits most of its light forwards so illuminates, say 1m2 at a distance of 1m (to make the numbers easy!) this corresponds to about 3 cd or 3 lx (they are the same value at a measurement distance of 1m!)

These are very rough numbers, but in a dark room, Luxmeter app on the iphone measured the LED brightness as 1 lx and a candle as 2 lx too so our LED seems to be similar to a candle (although I'm not convinced how dark the room was or the accuracy of Luxmeter at these light levels) 

We would need to measure things more precisely to get absolute light intensity, but we can measure our light source brightness relative to our standard LED for now. 

How can we make it dimmer? One easy way is to switch it off for half the time, and then it will emit only half the light! If we switch it on and off really quickly, quicker than the persistence of vision it will be perceived as just a dimmer source. In practice, this means more than about 60 times per second (60Hz) in bright light or about 10 times per second in dim light (10Hz) due to the difference in integration time between cones cells (10-15ms) and rods (100ms). 

We can do this by programming a microchip PIC 12F675 chip to control the LED. We can use JAL a simple programming language to write a program and then compile it and load it onto the chip, so that when we supply power to the chip it flashes the LED. 

Here is an example program that we can modify to make our LEDs flash!

Actually, the only bits we need to modify are the delay statements (giving delays in microseconds) and the instructions to set the LED pin high (LED on ) or low (LED off) - easy! ...and here's the hardware circuit!

(much easier circuit than our flashy LEDs because we did all the work programming the chip and don't need lots of different components!)

To start with, we will just make them flash slowly to make sure everything works, and to flash our name in morse code (because we can!)