# Light Meter Problem

#### muashr

Feb 11, 2014
5
Hi,

Has anyone got experience with this circuit? or can help me out with this?
http://circuitswiring.com/bar-graph-light-meter-by-ic-741/
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

#### Martaine2005

May 12, 2015
4,776
Hello,
Are you using a dual power supply?
The pot is a 10K.
LEDs can be any colour.

Martin

#### GPG

Sep 18, 2015
452
So many things wrong its hard to know where to start The pot wiper should be connected to -9 for starters

#### Colin Mitchell

Aug 31, 2014
1,416
Have you connected a 9v battery from the 0v rail to the +9v rail and another battery from the 0v rail to the -9v rail?

Now remove the transistor and put a 1k then 10k then 47k then 100k then 220k then 330k in place of the transistor.
If the LEDs don't change, the circuit is faulty.

#### GPG

Sep 18, 2015
452
If the LEDs don't change, the circuit is faulty.
The fact that the pot and IC got hot............

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Talk about horrible design! That thing takes the cake. It's also EXTREMELY power hungry! That poor little 9V battery is screaming "You're Killing Me"!

Chris

#### GPG

Sep 18, 2015
452
The question is: do you need a light meter ?
If so there are better ways.

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
The question is: do you need a light meter ?
If so there are better ways.
Ha, I spiced that circuit for the hell of it. I think you were being too kind when stating there are "better" ways. Any way is better than that way.

Cheers,
Chris

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
If you want to stay with an LED type display I would recommend something like the LM3914(Dot-Bar) Display Driver. You may be able to directly control it with the proper LDR.

Another option and a whole lot of fun to learn is Picaxe

Cheers,
Chris

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