circiut that converts light pulses to continous current

a circuit that converts non-continuous light on an optical-transistor into a continuous current that can be measured

Samuel,

A few questions:

What do you want the measured current to represent? Magnitude of the pulses? Frequency of the pulses? Some sort of time or frequency average?

Here's how I would do it in the general sense:

1) Use a simple envelope detector circuit to get a voltage from your optical transistor. The relation between the time constant of the RC portion of this circuit and the frequency of your optical pulse train will determine what the final current signal represents.

2) For the next stage of your circuit, convert the voltage signal to a current signal. Op amps provide a convenient way to achieve this. Linked below is a rundown of the theory

http://www.allaboutcircuits.com/vol_3/chpt_8/7.html

One thing to note: The lower the input impedance of the circuit you are using to measure the current, the better. Using a rail-to-rail op amp means that you need a voltage supply V+ >= (20mA) * (Rload) to be able to measure the full scale.

Another thought:

Consider a voltage regulator or a large cap to filter the signal after the peak detector circuit. You may observe some noise at the frequency of your pulse train, again depending on your choice of time constant. It's up to the specification of the problem as to how steady the final signal has to be.