Me: Electronics enthusiast in the 1960s, little chance to practice since then.

Project: Schools in the UK have drawers full of expensive 'Data loggers'. Sophisticated and versatile devices with lots of plug-ins to measure and record mostly physics things. Rarely used because teachers don't use them often enough, crucial bit missing, can't get software to work etc.

A posible solution: Ultra-simple hardware with common software. The software I'm trialing is either Audacity or Acoustica audio editing software. Both are available for free, measure at 10kHz+ (plenty fast enough) and resolve to 16bit (again, plenty).

My current problem: I want to fire a narrow beam of visible LED light (not laser, regulations too complex) at a sensor to produce a signal that can be fed into the mic jack on a PC. The resulting pulse.
These devices often used in pairs to start/stop a timer to measure acceleration etc but a single one could be used by reflection to count the rotation speed of a fan etc.

Photodiodes and Phototransistors are unknown to me and I have found comments on here (Q22414) like:

> Photodiodes may be operated either forward or reverse biased.  Forward
> biased gives most output.  Reverse biased gives most speed. and is
> noisier. Reverse biased mode is most usually used.

I'm thinking that forward bias would be better for me, but where do phototransistors fit in? I suspect that photodarlingtons are a little slow. Help!

My ideal circuit has no IC's, a very low component count and is really cheap. That way any unskilled solderer has a chance of building or repairing it and if it gets lost/broken then it's not a big deal.

Thanks

This gives me a square wave with amplitede varying with temperature. In the audio editing program this looks like a continuous 'histogram' that goes up and down with temperature. Total cost of components about 5£$Euro, one equivalent is here: