thanks for the reply so the time constant for one side would be r*c = 470*10uf = 4.7 ms ? The components affecting the current would be the led, capacitor and resistor? if this was flowing through c1 and q2 was turned on then would that be the case as if q1 was off then would you need to consider that in the path? r1 is connected to the pos side of c1 and current would flow and charge c1, what affect would r3 have on the neg side c1? would this affect the time constant/current flowing through c1? and how do you determine the saturation current for a transistor eg bc108? is it the case that across an led that is not on there is a voltage drop of approx .7v then when it is turned on approx 1.4v drop, for a standard led? how would you solve for the impedance of this circuit or would that not apply to this? would this be classed as an ac circuit with the current changing direction? so many questions I know but once I get my head around how this works properly then I dont think I will have many probs understanding similar circuits. many thanks graham