radiogareth
Senior Member
A quick search did not yield any posts, so I thought I'd offer this.
One of my pupils is making a cricket light monitor for which he will use an LDR, not least because its response reasonably matches that of the human eye. He then wondered about monitoring UV levels (sunburn etc) and spent a while searching for UV photo-detectors. Lots of ££sss!
Then I remembered a post somewhere (probably on this forum) about LEDs acting as PV devices that only respond to the wavelength of light they were designed to produce. A quick test with a 10Meg input impedance DMM, a UV LED and another powered UV LED (It was a cloudy day) showed a good 1.9 Volts when strongly illuminated. Next try was on a sunny day and equally useful readings resulted. Time to connect it to a PICAXE ADC input, bearing in mind that the 'input impedance' is nominally 47K. It reacted as I hoped (so the DC input resistance must be HUGE) but the string of LEDs did not decay when the sunlight was blocked. I wondered if the LED junction capacitance was storing the voltage, so I put a 10M resistor across the LED. Sorted, it responds very well to sunlight, yielding ever higher values the more intense the sun gets (nearer noon).
On this basis he can progress a very useful project
They (UV LEDs) would make good sensors for when the sun is actually out (as opposed to cloudy) for solar panel trackers, especially the amorphous types that only really yield in blue sky conditions.
One of my pupils is making a cricket light monitor for which he will use an LDR, not least because its response reasonably matches that of the human eye. He then wondered about monitoring UV levels (sunburn etc) and spent a while searching for UV photo-detectors. Lots of ££sss!
Then I remembered a post somewhere (probably on this forum) about LEDs acting as PV devices that only respond to the wavelength of light they were designed to produce. A quick test with a 10Meg input impedance DMM, a UV LED and another powered UV LED (It was a cloudy day) showed a good 1.9 Volts when strongly illuminated. Next try was on a sunny day and equally useful readings resulted. Time to connect it to a PICAXE ADC input, bearing in mind that the 'input impedance' is nominally 47K. It reacted as I hoped (so the DC input resistance must be HUGE) but the string of LEDs did not decay when the sunlight was blocked. I wondered if the LED junction capacitance was storing the voltage, so I put a 10M resistor across the LED. Sorted, it responds very well to sunlight, yielding ever higher values the more intense the sun gets (nearer noon).
On this basis he can progress a very useful project
They (UV LEDs) would make good sensors for when the sun is actually out (as opposed to cloudy) for solar panel trackers, especially the amorphous types that only really yield in blue sky conditions.