Phototransistor switching freqeuncy

Thread Starter

Alasttt

Joined May 13, 2015
68
Hi,

I have an infrared emitting LED that shines onto a phototransistor. I am inputting a square wave into the LED and looking at the output of the phototransistor. At a low frequency the output is correct. However my data is 4khz, and at this frequency the output of the phototransistor is nothing like a square wave. See the pic attached for input data and output of the phototransistor.
I also noticed that the frequency of the output wave is only 2khz, so I think its something to do with the phototransistor not being able to switch its output state quick enough.



Any thoughts ?
 

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GopherT

Joined Nov 23, 2012
8,012
Hi,

I have an infrared emitting LED that shines onto a phototransistor. I am inputting a square wave into the LED and looking at the output of the phototransistor. At a low frequency the output is correct. However my data is 4khz, and at this frequency the output of the phototransistor is nothing like a square wave. See the pic attached for input data and output of the phototransistor.
I also noticed that the frequency of the output wave is only 2khz, so I think its something to do with the phototransistor not being able to switch its output state quick enough.



Any thoughts ?

What type (brand and model) of scope are you using? What probes are you using?
This matters because some cheap USB oscopes are really low frequency and you get an effect called "alias-ing". Essentially, destructive interference of input signals and sampling frequency of your scope.

Also, post a schematic. I know it seems like it shouldn't matter but it can if your resistor values are too high.

A photo transistor can easily handle 40 kHz.
 

Papabravo

Joined Feb 24, 2006
13,963
Looks like your output circuit has one driven state and one floating state. This suggests a missing resistor. A schematic would be most helpful. It is the lingua franca of electronics.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
I have used two oscilloscopes, a picoscope 3205 and a tektronix tds 210. I should also add that I am using an infrared emitter and receiver, not a normal LED with visible light. The circuit works at low frequency, I have attached the schematic and the datasheet for the receiver. It is an SY-32PT

Its a simple circuit, apologies for the phototransistor image as the schematic software I used did not have this component so I had to use an image.

Hope this helps
 

Attachments

Thread Starter

Alasttt

Joined May 13, 2015
68
Then you have it connected backwards.
Hi,

thank you, reversing the transistor and still taking the output from the emitter gives more or less the correct output you can make out the highs and lows. But it is horrendously noisy. See attached, might a capacitor sort this ?zoom.PNG
 

GopherT

Joined Nov 23, 2012
8,012
Yes. The flat edge pin goes to 5v via a 5.6k resistor. and the other pin goes directly to 0v.
Yes, as DL324 pointed out, swap the collector and emitter. On the other hand, your drawing is best corrected by moving the ground symbol to the emitter and the supply + resistor to the top (collector pin in your drawing). With those changes, you now have a more "standard" format for a schematic. That is, more positive to the top of the drawing.
 

GopherT

Joined Nov 23, 2012
8,012
Hi,

thank you, reversing the transistor and still taking the output from the emitter gives more or less the correct output you can make out the highs and lows. But it is horrendously noisy. See attached, might a capacitor sort this ?View attachment 98294
No, no. Now that the transistor is flipped, you need to move the resistor between + supply and collector. Take the signal from the collector of the transistor. I hope that helps.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
No, no. Now that the transistor is flipped, you need to move the resistor between + supply and collector. Take the signal from the collector of the transistor. I hope that helps.
I have connected collector to + via resistor. and the emitter directly to ground. Taking output from the collector just gives a straight 5v line. Output from the emitter gives something like the expected output. but its very noisy
 

GopherT

Joined Nov 23, 2012
8,012
I have connected collector to + via resistor. and the emitter directly to ground. Taking output from the collector just gives a straight 5v line. Output from the emitter gives something like the expected output. but its very noisy
Post your new schematic. And double check that your new schematic matches your breadboard.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
No, no. Now that the transistor is flipped, you need to move the resistor between + supply and collector. Take the signal from the collector of the transistor. I hope that helps.
Data.PNG
Thats the input to the infrared emitter, and what I would like to see at the output of the phototransistor.
line.PNG
This is the output at the collector.
em.PNG
This is output taken from the emitter
 

GopherT

Joined Nov 23, 2012
8,012
First, change R1 on the photo-transistor from 5.6k at collector to 47k or 100k. Make sure your probe is at the collector and the clip on the probe is connected to the emitter of the phototransistor (which is connected to ground).
 

GopherT

Joined Nov 23, 2012
8,012
@Alasttt

See post 15 above and make sure your IR source is well aligned with the photo transistor. Some have only a 10 degree spread of the light. Likewise, the phototransistors have a specific focal angle too. It is easier if you separate them by 3 or 4 inches and inspect for co-linearity.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
First, change R1 on the photo-transistor from 5.6k at collector to 47k or 100k. Make sure your probe is at the collector and the clip on the probe is connected to the emitter of the phototransistor (which is connected to ground).
I have changed it to 910k, the closest I had. The probe has a + and - clip. - clip is connected to 0, +clip is connected between the resistor and the collector. the output is still a straight 5v line.
 

GopherT

Joined Nov 23, 2012
8,012
I have changed it to 910k, the closest I had. The probe has a + and - clip. - clip is connected to 0, +clip is connected between the resistor and the collector. the output is still a straight 5v line.
910k is too much and signal impendence is so high everything falls to noise.
 
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