A simple transistor and opamp circuit which doesnot work

Hi,

I have been working to make a very simple circuit work but have not been successful. I tried to simulate it using multisim and also tried it on a breadboard. I am wondering if anyone could help me with this. The circuit consist of an opamp, a phototransistor and some LEDs. Please have a look at the schematic from Multisim. The LEDs are supposed to be on or off one by one depending on light.



Please see also the external source links for this circuit.

**broken link removed**

Link 2

In both of the links one can easily notice that the only base and collector of the transistor have been used. That means one needs to have a phototransistor with three legs whereas I am using a phototransistor which has only two legs i.e. emitter and collector. This leads me to two questions.

Question 1: Can such a circuit be realized with a transistor which has only two legs i.e. collector and emitter? If yes what adjustments have to be made in order to make it function properly?
Question 2: Since the circuits in link 1 and link 2 both are employing only base and collector of the transistor isn't the transistor being used as a photodiode instead of phototransistor? If the transistor is being used as a photodiode why the symbol of photodiode is not mentioned? Can a photodiode be used instead?

In addition I have an additional question.

Question 3: The circuit in link 1 shows that both collector and base (almost) are grounded. How that works?

Thanks

I do not know MUltiSim.
How to you get light from U3 to U4? U3=LED??

I think U4 collector should not go to ground but to +9 volts. (supply)

ronsimpson said:

I do not know MUltiSim.
How to you get light from U3 to U4? U3=LED??

I think U4 collector should not go to ground but to +9 volts. (supply)

Click to expand...

In the description of Multisim it is written to use both U3 and U4 in combination during simulation. U3 acts as a source of light for phototransistor. And I have checked it it works. If you don't know Multisim it doesnot matter. However, have you seen the link 1 and link2? Could you comment on these and on my questions?

Q1: Yes. None.
Q2: Yes. Perhaps the author didn't have one. Yes.
Q3: The base is not grounded. N.B. R2.

The 741 is not a rail-to-rail output opamp so can only pull up to ~7V with a 9V supply. That is not enough to drive all the LEDs unless they have a very low forward voltage rating.

I agree with Ron, the phototransistor collector should go to +9V.

All three circuits have a +9V and -9V power supply which is plenty for a lousy old 741 opamp to drive 10V worth of red LEDs.
The base-collector of a photo-transistor is a photo-diode. Since it has no voltage across it (it is between the inputs of the opamp) then it is used as a tiny solar cell that generates a small voltage. A photo-transistor NEEDS to have a voltage across its collector and emitter to work as a photo-transistor.

audioguru said:

All three circuits have a +9V and -9V power supply which is plenty for a lousy old 741 opamp to drive 10V worth of red LEDs.

Click to expand...

.... provided the bottom LED cathode is taken to -9V instead of to 0V as shown.

Photo detectors can be used in either of two modes:

One mode treats them like a tiny solar panel. They produce a small voltage when illuminated, but the load resistance has to be very, very high (>megΩ). Ordinary LEDs can be used as photo-detectors this way.

The other mode treats them like a leaky reversed-biased diode where the reverse leakage current increases with increasing illumination, You apply a reverse bias voltage (~-5V), and then you detect the leakage current as a voltage developed across a load resistor.

The posted circuits in the two links correctly apply the photodetector in the first Mode, above. The MultiSim circuit will not work because the opamp is not biased properly .