This is the schematic diagram of my solar tracker based on an automatic satellite dish system. The discussion is on the Led sensor circuit in the right upper corner. The sensors are standard green color leds. The transistors are the c1740. Under normal temperatures the sensor readings are ok. But around about 18 degrees Celcius or lower (also under circumstances of no light), three out of the 4 transitors suddenly start conducting and the Mega reads values over 1000 (=full sunlight) on the analogue inputs. This is really weird. When I heat the sensors a bit up (e.g. with my warm breath), the circuits turn to normal operation again. I'm really lost Any idea's?
A led as light sensor can be used in this way:
Some leds (preferably a red led) can be charged like a capacitor. The charge is removed by the voltaic effect of the led. So the more light, the faster the charge is removed. But one led can do this hundred times better than another led.
But using a led as capacitor, is very sensitive for electric noise. The led must be close to the microcontroller.
So please get rid of the green leds. Use LDR's or photodiodes, or phototransistors.
Thanks, well this is one way of using the light sensitive property of standard leds and it works like a charm except for the weird temp issue. Btw, leds may produce a voltage up to 1.7v when exposed to (sun) light. I'm aware of the other approach you mention and its limitations. If I can't solve the temp issue I'll switch to one of the other sensor types. For now, it keeps me busy and I like it
Possibly unwanted RF oscillations - the detector circuit is very high impedance in low light so susceptible to
capacitive feedback. Remove the capacitors from emitter to ground, then add smaller caps from base to
ground of each transistor - should suppress oscilation and noise at source, prevents the Vcc rail injecting
AC into the transistors' bases via the LED capacitances.
Why two LEDs per leg - one ought to work just as well (effectively current sources in this mode).
Thanks, sounds plausible. Will try this. The capacitor from emittor to ground was meant to suppress EM noise picked up by the long wires between the sensors and the Arduino; they are mounted near the Arduino for that reason.
I am using two leds per leg, one as forward looking tracking led and the other mounted in an angle to increase the sideways sensitivity. I'm still testing the system so design changes are likely.
Charm,
Check out this link Electronic Projects, about halfway down the page, lots of designs using led's as light sensors. I have built most of them and they work extremely well. Haven't tried the ones with the outputs for microcontrollers yet, can't see tying up a Arduino when an FET will do the job.
Guess I could use an Attiny45 and an RTC to return to east after dark. Dang, another project. Naw, I'd be better off adding a panel and forgetting all the tracking mechanism.
Good luck
TomJ
Hi TomJ,
thanks and yes I've seen the site and still wanted to setup an Arduino. The reason was simple: I needed a project to get acquainted with Arduino and C++ programming and decided to build a mobile solar tracking system.
Any idea's?
