Whats a Sky Quality Meter ?
Essentially, it's just an extra-sensitive lightmeter that is used for measuring the darkness, or, to be more exact, the brightness (or 'skyglow') of the night sky. Astrophotographers will use the measurements to set their exposure times = the brighter the background sky, the shorted the exposure time you can get before the skyglow will 'wash out' the image.
Astronomers measure 'how dark it is' by calculating the 'limiting magnitude' = the dimmest star that can be distinguished against the sky by a visual observer.
How they work
An optical sensor is used to measure the light level in uW/cm^2 (microWatts per sq. cm) or lx (Lux), and some clever mathmatics converts that into a 'magnitude per square arcsecond' (aka 'limiting magnitude') sky quality value (typically from 14 to 20).
Almost any light sensor will do, and this means it's even possible to use the camera in your smart phone ! The only app. that seems to exist is the 'Dark Sky Meter' for the iPhone 3/4. It won't work on iPad or current iPhone. For Android, the well publicised 'Loss of the night' app. is NOT a meter (it's a 'wizard' that guides the user to count visible stars from which it 'infers' the limiting magnitude) The 'problem' in developing an app. seems to be one of calibration. There are just too many different camera's in use with too many smart devices to 'preset' values, however (plainly) the smart device itself knows all about it's own camera response to anbient light levels (since it has to take well balanced photos) so something 'should' be possible If you do find an app., remember that many 'free' (and even some 'charged for') apps will pester you to make 'in app purchases' and (like Google voice search) many 'work' by exchanging data with some 'server' (which means a phone call, which could cost you £thousands in roaming charges)
A hardware unit contains a simple photo-sensor that generates a series of pulses depending on the lighgt level. Counting the pulses leads to the sky brightness
A typical commercial Sky Quality meter is the Unihedron SQM / SQM-L (the standard model has a FOV of about +/-60 degrees, whilst -L model has a lens than reduces the FOV to about +/-20 degreees and is more suitable for city use (where streetlighs would otherwise 'swamp' the result)). The Unihedron SQM has as 'start' button', a 'beeper' (to indicate when the measurement is complete - it beeps at 1 per second during the measurement - since it can take up to a minute in really dark skies) and a 4 digit display (nn.xx). It operates from a 9v battery.
The 'problem' (other than cost) is the time needed to get an accurate reading. This is due to the small size of the sensor (small sensor == very little light)
Whilst the obvious solution is to gather more light (fit a lens, or use a bigger sensor), there are other approaches :- Photometer (e.g.Zacharov,1962) DSLR = see, for example, dark skies awareness (the method is explained here) CCD1 Camera (e.g.Cinzano etal.,2000) Imaging with fisheye lenses(e.g.Cristaldi etal.,2000) Solarcell = perhaps the 'ultimate' sensor is the 3.5" sq. amorphous Si solar cell used in the IYA (International Year of Astronomy) lightmeter, which is capable of generating sub-second readings (actually, up to 2k Hz !). This unit operates from 500 uLux to 200 kLux (and has a noise floor of only 20 uLux (at 1 mLux)). Sinsitivity, approx 350nm - 650nm, is essentialy in the visible light region (unlike many photo-sensors that require iR / UV filters)
Building your own SQM
A typical DIY SQM (built around an Arduino Nano) can be found on sourceforge projects
My version uses a PIC chip, and runs from 5v (so I can use a £1 PowerBank (rechargable LiPO battery pack)
The SQM 'sensor'
The 'easy' way is to use a simple light-to-frequency converter that allows you to 'accumerlate' by counting pulses until some fixed level (measured total ammount of light) is reached. The time taken to reach that level can then be expressed in 'magnitudes per square arcsecond'. Depending on the data sheet, device sensitivity may be specified in Lux or uW/cm^2. To convert, 1 Lux = 0.1464 uW/cm^2 (1 uW/cm^2 = 6.83 Lux) or use this on-line units converter.
The sensor used in the Unihedron SQM is the AMS TSL237, which is hard to find (one UK supplier is 'Mouser electronics' however whilst the chip is only double what you might expect (at £2.49) they will rip you off £12 for delivery. Possible suitable alternatives are the TSL235R, the TSL230R (Hi sens mode), TSL237S (not T, which is reported to be 50% less sensitive) or TSL238. Anything that operates down to about 0.001 uW/cm^2 (0.007 lux) should be suitable since you will be typically be measuring from a typical moon-lit sky of about 0.3-0.2 Lux (0.03 uW/cm^2) down to the typical 0.002 Lux for a really dark sky. Most of the 'light sensor' HAT's offered at outrageous prices by the 'project part' sellers (such as the TSL2561) typically use the TSL260R (which is limited to 0.1 uW/cm^2, the TSL261R (0.25uW/cm^2 limited) or TSL262R (1 uW/cm^2 limited), all of which should be avoided.
The main issue is how to measure very low light levels - whilst the TSL235R generates 80Hz at 0.2 Lux (0.03 uW/cm^2) and above, the bottom end (.001 uW/cm^2) you may only get about 10Hz.
You will also need some means of calibration (as the TSL235R typically generates 10 Hz at '0 lux') Once calibrated, you can submit your results to the Globe at night project