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need urgent help with LM393 IC

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ary

New Member
I want to build lght based sensor circuit using LM393 ic. The purpose of the circuit is to turn on or off the relay depending upon the intensity of light. I have built this circuit using transistors , which worked well. but now i want to build this circuit using LM393 ic. I am new to this IC . I have tried to build the circuit using only one comparator of this ic, but circuit is not working.LDR is used for light sensing. On pin +3 of this ic i have connected resistive divider of 10k and 0n -2 pin of ic i have connected resistive divider of LDR and variable resistor.ouput is conncted to transistor for relay driving, i dont know whether to pull down or pull up output. plz someone tell.
 

ericgibbs

Well-Known Member
Most Helpful Member
I want to build lght based sensor circuit using LM393 ic. The purpose of the circuit is to turn on or off the relay depending upon the intensity of light. I have built this circuit using transistors , which worked well. but now i want to build this circuit using LM393 ic. I am new to this IC . I have tried to build the circuit using only one comparator of this ic, but circuit is not working.LDR is used for light sensing. On pin +3 of this ic i have connected resistive divider of 10k and 0n -2 pin of ic i have connected resistive divider of LDR and variable resistor.ouput is conncted to transistor for relay driving, i dont know whether to pull down or pull up output. plz someone tell.
hi,
The LM393 comparator has an 'open' output, it requires a pull up resistor to +Vsupply, choose the resistor to give about 5mA thru the resistor.
 

Arumugam

New Member
Y should use the pull up value depends upon the which transistor are you going to use.Generally It will be around 1k to 10K.
 

Hero999

Banned
If you just need one comparator, you could use the LM311 which has the advantage of being able to drive a load of up to 50mA and can power small relays directly.
 

jmb4370

New Member
Using the LDR into the LM311 comparator, as described above, it seems the output does not go high or low immediately when the light is displayed upon the LDR. I think the overlap in ON and OFF points is the hysterisis. How can this be eliminated, so the threshold point is the same for when light is shining on the LDR, or when the light is not shining on the LDR?
 

Hero999

Banned
Please post the schematic.

LDRs are very slow and can take a second or so before the resistance changes enough.

Some hysteresis is required because if you don't have it, the circuit can oscillate before the output settles to a steady state.
 

jmb4370

New Member
I searched the web further, and was able to solve things by using a bypass cap (0.01 uF) and a 10K resistor both placed in parallel between pins 2 and 7 of the LM311.

The circuit works just as I wanted, although it needed some modification to add the 555 to give a timed pulse for the output.

I will try to post the finished circuit, or at least a link to where I found the bypass to minimize potential oscillations.
 

Hero999

Banned
The 10k resistor you've added actually adds hysteresis not taking it away.

Your problem was a lack of hysteresis. The 10k resistor added some positive feedback making it more stable.

If you use a dual comparator such as the LM393 then you could get rid of the 555 but you'll probably need to add a transistor to drive the relay so it depends on what's more convenient for you.
 
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