A visible (possibly UV) spectrometer, the design of which (circuits, CAD, software) is open to all.
Those that are available for schools are too bloody expensive, given the level of technology they contain, I'm certain a cheaper alternative can be made that would appeal to schools or DIY scientists.
In my experience schools can have very experienced staff working in their D&T departments. Providing all the schematics will allow a school to (a) fix the apparatus in house should it be damaged or fail, (b) maybe produce more or improve them.
Design will be based on a Czerny-Turner design.
Probably contain an arduino, cos they get you extra blog-cred.
For a basic spectrometer based on above design.
- Cuvette Ebay ~£25 inc P&P
- Concave mirror * 2 (£16 each) ~£47.40 inc P&P
- Diffraction grating ~£10 inc P&P (for 10 1000 lines/mm)
- Sensor (photo-diode) ~£1, have one in electronics kit.
Total = ~£85
Will need servo motors and MDF to build a dark box.