ESP8266 Community Forum

- Wed Nov 11, 2015 12:58 pm #33711

Measure the resistance of the photocell in the dark.
Multiply that value by 2.2 and round it off to the nearest resistor value you can find.
Connect the resistor to +3.3V and the other end to tout.
Connect the photocell to tout and ground.

That should cover a big part of the range of the ADC assuming that the resistance difference between dark
and light is big enough

tnx i have managed to wire up everything and it works now. I get data range from 0 to 1023. But still i have few questions.

1) Why multiply it with 2.2 (is this because of 3.3V -1V)?
My photocell has around 18K ohm x 2.2 = rounded 40k ohm. See point 2)

2) 3.3V is not fixed. 3xAA are in range from 2.7V to 4.1V(slowly dropping ...)
So should i use i dont know .. low dropout regulator LM1117 (1.8V) and than i would have fixed 1.8V? Is this ok plan?

If point 1) is correct and if i use LM1117(1.8V) than i would multiply it with (1.8V - 1V) = 0.8V ... so photocell 18K ohm x 0.8V = 14.4k ohm.

Is this correct path or not?

- Wed Nov 11, 2015 2:10 pm #33714 1) I calculated the value with the following method:
(Available_voltage - Desired_voltage) / Desired_voltage
(3.30 - 1.00) / 1.00 = 2.30
So it's actually 2.30 times the photocell value; 18kOhm * 2.3 = 41.4kOhm but the 40k is close enough (it only makes a difference of 20 millivolts..)

2) It's a good idea to use a voltage reference so that a value always maps to the same light intensity regardless of battery level. A LM1117 can indeed serve as voltage reference, in that case just use the formula above to calculate the required resistor (14.4k is correct yes)