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gopalr
01-02-2009, 02:27 PM
i am making a train model with automatic road barriers. i wish to use LDR and a torch to operate the barrier switch but the current output from the LDR is too less to make a relay work..


a circuit to increase the sensitivity of the LDR is required plz help??

akshay66
01-02-2009, 02:31 PM
Which size of LDR are u using?How much output you want?Pls tell in detail.

akshay66
01-02-2009, 08:04 PM
Are you using a torch or laser for source of light I'de preffer laser they are more accurate.

debu
01-02-2009, 10:11 PM
Are you using a torch or laser for source of light I'de preffer laser they are more accurate.
@akshay66: :!: What did gopalr ask?

@gopalr: An LDR is a resistance, so you can't use it directly to switch a relay. How you should use it is, use an Op Amp (LM324 or equivalent IC), as a comparator. Use a potentiometer, and compare the output of the LDR against the adjustable output of the pot. Use the output of the comparator to switch an NPN and sink to switch the relay.

In this way you can adjust the sensitivity of the circuit by adjusting the pot and compensate for any ambient light problems with the LDR.

Hope this helps,

Regards,

Debu :)

gopalr
01-03-2009, 02:12 AM
i am using a torch, and i want around 12 volts output..


Debu,

which NPN transistor should i use i have seen 2 circuits they use BC107-108 but they are not even available in Lajpat Rai Market.

debu
01-03-2009, 09:10 AM
@gopalr: Use a BC547 or BC548. These are very common, and would cost you Rs. 3 each.

Regards,

Debu :)

tekyyu
01-05-2009, 01:36 PM
check out the new topics....
regards
sid

tpsbpl
01-11-2009, 07:40 PM
http://www.technologystudent.com/images5/ldrmtr1.gif
use 47K connected to ground ad ldr use 1k resistance to base of transistor (2N3904) diode 1n4007 , 12 V sugar cube will also work u can use 9-12 v supply

tpsbpl
01-11-2009, 07:40 PM
use 47K connected to ground and ldr use 1k resistance to base of transistor (2N3904) diode 1n4007 , 12 V sugar cube will also work u can use 9-12 v supply
http://www.technologystudent.com/images5/ldrmtr1.gif