Photo transistor help

Thread Starter

shaun577

Joined Nov 15, 2011
4
Hi,

I am having a bit of trouble getting my phototransistor encoder going. I have attached the circuit I have been using. The circuit is going but it's not outputting a low enough voltage when the phototransistor is dark. I need the voltage to swing from below 0.8v to over 1.6v to ensure the schmitt trigger triggers properly. Currently I can get the output to swing from about 1v to the supply voltage which is fine for the high triggering but not low enough for a low trigger.

I am using this Ir led/phototransistor pair to read an encoder wheel.

I have spent the last few hours trying to figure this problem out but with no success. Any suggestions would be much appreciated:)
 

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Adjuster

Joined Dec 26, 2010
2,148
The phototransistor voltage falls low when it is illuminated, not when it is dark.

To get a lower minimum voltage, try reducing R1 so that the LED gets more current. Don't go too far, as yopu don't want to exceed the LED maximum current. Depending on the LED Vf, 330Ω or 270 might get you 10mA, which might be OK.
Best to check the device datasheet though.
 

MrChips

Joined Oct 2, 2009
22,084
Try replacing the inverter with a CMOS gate. Any type of gate might do, eg. CD4069 inverter. With a CMOS gate you can increase R2 to a much larger value. Try 100K or 220K.
 

SgtWookie

Joined Jul 17, 2007
22,220
A CD4093 is a quad Schmitt-trigger NAND gate; you can tie a gates' two inputs together to make an inverter out of it.

The CD40106 is a CMOS equivalent of the 74xx14.

Note that the pin outputs of the 4000 series are not necessarily the same as the 74xx series logic ICs.

We don't know what the typical Vf of your LED is at it's rated current.
If you know what the typical Vf @ current rating is, you can calculate what the value of R1 should be as:
R1 >= (Vsupply - Vf) / DesiredCurrent

Just for example:
You are using 5v for Vsupply
Let's just say that you were using an IR LED that had a Vf of 1.2v @ 20mA.
Substituting:
R1 >= (5v-1.2v) / 20mA
R1 >= 3.8 / 0.02
R1 >= 190 Ohms

Here is a table of standard decade resistor values:
http://www.logwell.com/tech/components/resistor_values.html
Looking in the green E24 columns, you will find that 190 is not a standard value.
180 Ohms and 200 Ohms are the closest. You can go with a single 200 Ohm resistor; 180 Ohms would be too low.
You could also go with a 100 Ohm and a 91 Ohm resistor in series, which would give you 191 Ohms.
You could also use two 360 Ohm resistors in parallel, which would give you 190 Ohms.
 
Last edited:

John P

Joined Oct 14, 2008
1,847
Turn your circuit on its head, with the phototransistor connected to V+ and the resistor to Gnd (but don't reverse the transistor). Now instead of having a swing 1-5V it will be 0-4V. Problem solved.

Oh, obviously the output will be inverted. Just use 2 inverter gates on the output instead of 1, if that's an issue.
 

MrChips

Joined Oct 2, 2009
22,084
Turn your circuit on its head, with the phototransistor connected to V+ and the resistor to Gnd (but don't reverse the transistor). Now instead of having a swing 1-5V it will be 0-4V. Problem solved.

Oh, obviously the output will be inverted. Just use 2 inverter gates on the output instead of 1, if that's an issue.
No. R2 at 20K-ohm is not low enough to pull the input of a TTL gate LOW.
You need to sink 0.4mA to bring Vin to below 0.5 V. R2 would have to be less than 1K-ohm.
 

Thread Starter

shaun577

Joined Nov 15, 2011
4
Thanks for your help. I have tried your suggestions and have found a couple of things.

I calculated the value that R1 should be. R1= (Vcc-vf)/Imax of led
R1=(5-1.3)/60mA
R1=61Ω so I used a 120Ω in parallel with a 130Ω

The output of the schmitt trigger is not meant to be connected to ground, my mistake, it is actually connected to a digital input of my arduino microcontroller.

After trying the suggestion to "turn the circuit on it's head" and experimenting with several different resistor values it is better but still won't go below 1v. If I disconnect the schmitt trigger and just read the ouput voltage with no load on the phototransistor the output swings nicely to just above zero and back up to almost supply:confused: This is what I want to happen but as soon as I connect up the schmitt trigger again it won't go below 1v
 

John P

Joined Oct 14, 2008
1,847
Good grief, TTL. I just glimpsed an inverter there and assumed it was a 74HC14. Throw out that old stuff and get CMOS!

And edited to say, what's the function of the inverter anyway, if you're driving a processor pin? I'm not an Arduinoist, but on PIC processors the digital inputs often have the Schmitt trigger function built in. You might check and see what the Arduino has.
 

Thread Starter

shaun577

Joined Nov 15, 2011
4
I will try a CMOS gate. I don't think the arduino has schmitt triggered inputs. I tried it without using the schmitt trigger chip and the input to the micro jumped up and down and was not stable enough.

I am using the Ir led/receiver to count an encoder wheel. The inverting part of the schmitt trigger doesn't matter. It wouldn't matter if it was inverting or not for my purposes.
 

MrChips

Joined Oct 2, 2009
22,084
You can try this:




I don't know if this will work but it may be worth a try. Any small signal NPN transistor should work. You may have to experiment with the values of R1 and R2.
 
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