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### Author Topic: Light / Dark Activated Relay circuit help needed  (Read 7786 times)

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#### johnplagis

• Newbie
• Posts: 6
##### Light / Dark Activated Relay circuit help needed
« on: September 22, 2012, 20:18:59 PM »
Reference :  Light / Dark activated relay

Hello I made the third circuit described in the page above, with some slight modifications in parts as follows:

I used the following parts:
UA741CN

My problem is that when I provide +12V the relay is activated, but then nothing change as I'm moving my hand in front of the ldr.

p.s the ldr I'm using in normal circumstances counts 37~40 k (when not in the circuit) is that the problem? Do I need a ldr with more resistance (around 100k)?

#### kam

• Hero Member
• Posts: 1849
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #1 on: September 23, 2012, 03:22:32 AM »
Hello John,

i use 5-500K so the 40K is much different. You can therefore compensate this difference by changing the resistor above the LDR (i use 100K resistor). You need a resistor that is about 1/5th of the LDR's resistance at maximum resistance. So, if the maximum resistance for you is 50K (measure both in light and dark to find the max resistance) then use a resistor 10K instead of 100

#### johnplagis

• Newbie
• Posts: 6
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #2 on: September 23, 2012, 10:11:08 AM »
Do you have the product number of the ldr you are using? Well i meausered it in dark situation and it get me a value of 2Mohm!?!

#### kam

• Hero Member
• Posts: 1849
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #3 on: September 23, 2012, 12:35:04 PM »
2MOhm !!!!!! Wow! Try a 220K / 330K / 470K resistor, one should work.
I dont have a part number of the ldr, i cannot even recall from where i bought it. It was long time ago.

#### johnplagis

• Newbie
• Posts: 6
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #4 on: September 23, 2012, 18:33:28 PM »
Ok I'll give it a try in the upcoming week and I'll report back here the results!!! Thanks for your help.

#### johnplagis

• Newbie
• Posts: 6
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #5 on: September 25, 2012, 16:41:26 PM »
hello again, unfortunately all my tries were unsuccessful. I wasn't able to found a 5-500 k ldr from the local store. So I have four different ldrs which measure the following values when are not connected to the circuit.
ldr 1: 8k when uncovered and 46k when I move my hand in front of it
ldr 2: 4.2k when uncovered and 58k when I move my hand in front of it
ldr 3: 3.2k - 13.4k
ldr 4: 6.9k - 45k

how to count the maximum value?

I also tried with the resistors you told me but with no luck.
Are there any other measurements which I could make into the circuit to check if everything works ok?

#### kam

• Hero Member
• Posts: 1849
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #6 on: September 25, 2012, 22:46:41 PM »
The max value is when the LDR is in darkness.

You can have 2 multimeters and measure simoultanousely the voltage on the 2 inputs of the comparator. The idea is that when the positive input (set by the 10K pot) is higher than the negative input (from the ldr), the output goes high and so the relay arms.

The multimeter on the negative input must show different values when you move your hand above the ldr
The multimeter on the positive must change when you rotate the 10k pot

#### johnplagis

• Newbie
• Posts: 6
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #7 on: October 06, 2012, 10:29:05 AM »
on the ldr I'm counting -16k to -43k (based on how close my hand is) while on the potensiometer I count 11.4k when is fully rotated, if i try to rotate it then I can't measure it (always giving 1, no matter what scale I use on the multimeter).
Also I noticed different values when I'm measuring the ldr switching negative and positive cables of multimeter , it then gives me a positive value (30,2k)

#### kam

• Hero Member
• Posts: 1849
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #8 on: October 07, 2012, 01:53:14 AM »
normally the ldr has no polarity. so this is very strange. Do you measure the ldr on the circuit connected? if yes, then this is wrong. you must disconnect it from the circuit.

Also, i need the voltages on the + and - inputs on the comparator for different states of the potentiometer (low high) and light (no light - full bright)

#### johnplagis

• Newbie
• Posts: 6
##### Re: Light / Dark Activated Relay circuit help needed
« Reply #9 on: October 08, 2012, 15:13:38 PM »
I thought that you wanted to meausure it while online.
6k to 12 k when it's not connected to the circuit (6k in my room with no light open and 12 k when I have my hand in front)

the volts counted on V+ and V- pins of ic, result at 9.32 v when the potensiometer measures 10k and 9.37 when measures 0k  no matter where my hand is! (wired?)

Also I found this on ebay, is this compatible with your circuit?