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Light Meter Circuit

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muashr

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Hi,

Has anyone got experience with this circuit? or can help me out with this?
**broken link removed**
The LEDs do not light up as the intenstiy of light increases. The voltage tried ranges from (10- 15)V. My questions are:
What type of phototransistor should be used (two legs or three legs and wavelength)?
What type of LEDs should be used (green or red or yellow etc.)?
Why the potentiometer is needed between pins 1 & 5?
What potentiometer's specification (power) work?
I tried to make this circuit work by applying voltages ranging from (10-15)V and adjusting the potentiometer but the it only results in heating up the IC and potentiometer.
Any ideas
Thanks
 
Yes - do not use this circuit. It will not work as intended for several reasons. Even if a 741 had enough output voltage swing and output current capability to drive the LEDs, the LEDs will not come on sequentially like a bar graph. The output circuit is all wrong, indicating that whoever posted this circuit never built it. LEDs and resistors do not function the way this circuit intends. If you want a bar graph display, better to start with something based on the LM3914 or 3915.

ak
 
"heating up the IC" That does not sound good.

The pot should not get hot. It is a offset adjustment. Long story.
(the LM741 is not perfect. If Q1 was not there the (+) input and the (-) input would be looking at ground or "0" volts, so the output pin-6 should also be at o volts. Because of "not perfect" the LM741 might see zero as small small voltage. With the pot you can force the LM741 to see zero as + or - 0.015 volts. R1 and R3 sets the voltage gain to 101. 0.015 volts X 101 = 1.5volts. That will light (almost) one LED. Probably you will set the pot so one LED is almost on at no light. This way a very small amount of light will lite the first LED.

Any type of LED. All the same! Probably designed for red. Different types of LEDs turn on at different voltages.

You need to have a transistor where light can get to the silicon! Photo transistors have a clear plastic case.

If the LM741 is working right. Lick two fingers on the same hand. (make slightly wet) touch "+9" and LM741 pin-3. The LEDs should light. The current through your fingers will pull pin-3 up in voltage very slightly. The amplifier will make that voltage 101 times bigger (up to 7 volts max) and light some LEDs.


**broken link removed**
 
Who comes up with this garbage? This circuit is wrong in so many ways. Do people not care about the content on their website?
 
Yes - do not use this circuit. It will not work as intended for several reasons. Even if a 741 had enough output voltage swing and output current capability to drive the LEDs, the LEDs will not come on sequentially like a bar graph. The output circuit is all wrong, indicating that whoever posted this circuit never built it. LEDs and resistors do not function the way this circuit intends. If you want a bar graph display, better to start with something based on the LM3914 or 3915.

ak
Who comes up with this garbage? This circuit is wrong in so many ways. Do people not care about the content on their website?
This circuit is also in one of the Forrest Mims' books. Here is the link.
https://books.google.de/books?id=a_...age&q=bar graph light meter using 741&f=false
 
"heating up the IC" That does not sound good.

The pot should not get hot. It is a offset adjustment. Long story.
(the LM741 is not perfect. If Q1 was not there the (+) input and the (-) input would be looking at ground or "0" volts, so the output pin-6 should also be at o volts. Because of "not perfect" the LM741 might see zero as small small voltage. With the pot you can force the LM741 to see zero as + or - 0.015 volts. R1 and R3 sets the voltage gain to 101. 0.015 volts X 101 = 1.5volts. That will light (almost) one LED. Probably you will set the pot so one LED is almost on at no light. This way a very small amount of light will lite the first LED.

Any type of LED. All the same! Probably designed for red. Different types of LEDs turn on at different voltages.

You need to have a transistor where light can get to the silicon! Photo transistors have a clear plastic case.

If the LM741 is working right. Lick two fingers on the same hand. (make slightly wet) touch "+9" and LM741 pin-3. The LEDs should light. The current through your fingers will pull pin-3 up in voltage very slightly. The amplifier will make that voltage 101 times bigger (up to 7 volts max) and light some LEDs.


**broken link removed**
I have 3 questions.
1. When selecting LEDs should I select LEDs with smaller current (2mA) or higher (20mA)?
2. If I had to use Phototransistor with two legs. Should I then connect Emitter to GND and Collector to the point where the base is connected (see figure above)? Please advise how a 2 lead phototransistor should be used in this circuit.
3. What should be the power rating of resistances and potentiometer in such a circuit?
 
If you want to make a light meter (if the light meter is the priority not the circuit you posted), you can use easily LM3914 and a LDR.
LM 3914 LM.jpg
 
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