Constant Current Source

Well without any accuracy figure quoted I would just use a couple of diodes a transistor and two resistors.

2 diodes in series to bias the transistor base and a resistor in the emitter of about 620 ohms, with 0.6 approx volts across it will give a collector current of approx 1 mA.

This would be a current source or sink depending on whether you use a NPN or PNP transistor....

John

Thanks so much sir, I shall try it.

How accurate do you need?

A 1N5297 is a 1mA CCD (constant current diode). +/- 10% I think. You can get CCDs in lots of values and they only need about 1 Volt of overhead.

Another option is to build your own from a JFET and a resistor ( that is what a CCD realy is).

Third option is to use an LM137 Voltage regulator configured as a current source. It requires the LM137 and one resistor.

Same method as John's, there's a picture here: http://cr4.globalspec.com/comment/26532 , but you'll have to change the zener to two diodes and reduce the value of the resistor, again, as John said.