# Op amps and Photo interruptors - Questions on them

Discussion in 'General Electronics Chat' started by Cemre Tas, Jul 14, 2015.

1. ### Cemre Tas Thread Starter New Member

Jul 14, 2015
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0
[Embed] (Starts at 1:10 for mobile).
Basically the idea is that the photo interrupter shoots a beam of IR light from an emitter into a collector: [Embed]
When the beam is intact the output current is based on VCC being supplied and the resistor value the photo interrupter has, when the beam is broken the output is pulled from ground so no current is applied.
Now this leads into an current to voltage amplifier configuration: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html#c2
Basically this converts the current back into a voltage, the voltage is fed into the panel meter. When the panel reader reads a higher voltage it starts to creep up, when it reads a lower voltage it starts to creep down. The idea is that based on the output current by the photo diode the beam can be controlled.
My question is this: In the video he describes that when the beam is broken the output [voltage] of the amplifier is increased. So when the weight is put on it, which brings the beam down, the beam is broken which ends up increasing voltage output from the amplifier which when fed into the beam pulls it up. How does this work, since the current to voltage amplifier is based on the current wouldn't breaking the beam, which makes the current output 0, mean that there would be no current to amplify and thus no voltage so the beam should fall?
Likewise when the beam is not being blocked at all I'd imagine that the amplifier would reduce the voltage and bring the beam back down, but I'm just confused why this seems reversed logically. When the unblocked beam should give a higher voltage and a blocked beam should give none.

Also anyone know the difference between a typical 0-5v panel meter and one that's in the mv range? Is it just a smaller resistance in the coil? Since I see one that measures from 0-5v or 0-30v costing \$5 while one that measures millivolts costs \$40+.

2. ### wayneh Expert

Sep 9, 2010
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You'll get more feedback if you 1) supply a schematic and 2) don't make a potential helper go watch a video.

3. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,297
A millivolt meter is harder to construct because you need a lot of turns of very fine wire compared to what is in a meter for a larger voltage. All meters actually measure current, not voltage. For DC meters, the resistance of the coil is designed to limit the current per Ohm's Law such that when 5 V (for example) is across the coil, the meter deflects to the 5V marking on the scale.

The current-to-voltage amplifier (called a transconductance amplifier) is at its heart an inverting DC amplifier. Notice that the + input on the National Semiconductor schematic is connected to Vbias. This is an offset voltage, probably 1/2 of whatever the power supply for the circuit is. The bottom end of the photo diode is connected to GND, so the photodiode has several volts across it (notice that it is reverse biased). When light hits it, it starts to conduct reverse current, pulling the - input of the opamp below the + input reference, and this causes the output to increase because that's the definition of the - input -> when it goes one way, the output goes the other way.

Overall it is a very cute system, but he didn't say how hard it was to calibrate it.

ak

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4. ### Cemre Tas Thread Starter New Member

Jul 14, 2015
10
0
I'm a bit confused still. The meter deflects upwards when a higher voltage is applied to it and downwards if a lower one is applied. If the beam is forced up this allows the photo interrupter to conduct the IR light which generates a reverse current as you said which leads to a smaller - input to the opamp and a higher voltage output to be fed into the beam. Wouldn't this simply make the needle/beam want to keep going up in this case?

5. ### AnalogKid Distinguished Member

Aug 1, 2013
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Reverse the leads to the meter.

ak

6. ### Sensacell Well-Known Member

Jun 19, 2012
1,183
276
One very important detail to note is that the photo interrupter used to make the scale does NOT include a schmitt trigger circuit.

The video below details a logic level output opto-sensor, this will NOT work in the proposed system.
The scale works around the closed loop servo idea, a minute variation in the position creates a change in output voltage, this gets amplified and applied back to the meter coil to act against the force. The opto sensor is actually putting out a linear signal in this mode, not an on-off logic signal.