Some of my advanced chemistry students would like to try this pharmacokinetic model circuit outlined in an AAPT publication (Cock & Janssen, The Physics Teacher, Volume 51, Issue 8, pp. 482-484 (2013)). It is a simple RC circuit, but I've never built a constant current source and am unsure of what things to watch out for. http://scitation.aip.org/docserver/fulltext/aapt/journal/tpt/51/8/1.4824943.figures.online.f2_thmb.gif The article suggests using a 72 uF cap, 27k/297k/420k/890k resistors, and 172/344/520/700 uA current. Then the procedure is to leave S1 open, and charge the cap for 0.15 s (how can I get timing like that!?). The switches' states are then exchanged and of course the charge discharges with the time constant (over 10 seconds). I've looked at some schematics and I'm confident it can be done with just a few components (like this), but am less sure about how I can vary it over the 4 values stated above. I also don't know how to only allow it to run for 0.15 s...any thoughts?