http://www.talkingelectronics.com/projects/200TrCcts/101-200TrCcts.html
Capacitor gets charged up via the resistor -> voltage level displayed by the leds.
capacitor discharged via the diode.
ah, so the 555 slowly pulse charges the cap?.. but the diode, unless i'm getting it back to front, it's only allowing a 0/gnd/neg voltage to pass through the diode... or is that purposely done
to allow a little current to pass to the base ?
finally got it 
- the diode threw me off, the cap is also in series with the transistors thanks!
An Extremely Primitive A to D converter. Cute circuit though.
Bob