Hi, i want to detect the change in the resistance of LDR while seeds like corn passes between light source and ldr. Resistance changes from 1500 ohm to almost 1700 ohm when seed passes. means 200 ohm resistance changes. i want to monitor this change on Arduino serial monitor in range of 0 to 5 Volts (0 volt at 1500 ohm and 5 Volts at 1700 ohm.
thanks in advance.
could you measure the current flowing?
Normally you would put the LDR in series with say a 10k resistor and put this combination across the 5 volt power rails. You measure the voltage (with an Analog pin) at the point where the LDR and resistor are connected together.
The voltage is also dependent on the strength of the light source.
We could throw math at this, but I prefer a simple visual to show the issues at hand:
Using a 10K load for the LDR, your voltage swing is very small:
LDR are not particularly good for your use, a phototransistor is far superior.
Anyway, you can play with the simulation, just click the switch to go between the two LDR states. Adjust the Pot to change the voltage divider resistance.
Circuit Simulator Applet (falstad.com)
$ 1 0.000005 10.20027730826997 50 5 50 5e-11
S 160 128 272 128 0 1 false 0 2
r 272 112 400 112 0 1500
r 272 144 400 144 0 1700
w 400 112 400 144 0
w 400 112 464 112 0
R 464 112 464 64 0 0 40 5 0 0 0.5
w 160 128 160 208 0
174 160 208 160 288 1 10000 0.005 Resistance
g 160 304 160 352 0 0
w 160 208 224 208 0
w 176 256 224 256 0
w 224 208 224 256 0
p 160 128 96 128 1 0 0
o 12 64 0 4098 5 0.1 0 1
The greatest output deviation occurs when the load resistor is almost equal to the LDR resistance.
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