Hello,
I was wondering if any one could help me on how to make an IR phototransistor act as a switch? I have a IR emitter that is in line with the IR phototransistor but it doesn't do anything when i block them. (http://www.radioshack.com/search/index.jsp?kwCatId=&kw=ir diode&origkw=IR Diode&sr=1) When i test the volts coming out of the phototransistor it changes from 5.2 volts to 5.0 volts and i thought it would actually change to about 1 or 0. Any help would be great thanks!

Tyler

 

Can you post a copy of the schematic showing how you connected up your phototransistor?

hgmjr

 

Yes here is a picture of the schematics.

http://www.geocities.com/Yosemite/Trails/7471/pwdtrack.htm

Also im kind of confused with the resistors R7-R10, it shows it is connected to the positive and then another resistor on the negative would you happen to know the logic behind that?

 

The circuit should work if the parts work properly.
RadioShack might sell manufacturer's rejects at a very high price.
The range might be very short, especially in daylight.

R7 to R10 bias the input of the comparator to half the supply voltage so that when the photo-transistor conducts then the comparator can sense it.

 

The voltage at the collector of the photo-transistors should be less than 1 volt when it is not blocked and near the 5V supply when blocked.

What voltages are you measuring when the sensor is unblocked and when it is blocked?

hgmjr

 

Thanks Hgmjr and Audioguru for your posts so far i appreciate your time taken to help me out with this.

When it is unblocked it reads 5.13 volts and when it is blocked it reads 5.17 volts. i also tried these schematics with my IR and the led light never turned on, it appeared there was never anything going through the IR.

 

Thanks Hgmjr and Audioguru for your posts so far i appreciate your time taken to help me out with this.

When it is unblocked it reads 5.13 volts and when it is blocked it reads 5.17 volts. i also tried these schematics with my IR and the led light never turned on, it appeared there was never anything going through the IR.

You say that the IR led did not come on. Can I assume that you used a digital camera in a cell-phone to confirm this fact?

hgmjr

 

What i had connected to it was a normal LED that would light up when the IR phototransistor was inline with the IR emmiter, like a remote control or something. The LED never did light up though, so could this mean my IR phototransistor is bad?

i used these shcematics for the LED light:

http://3.bp.blogspot.com/_940DBYqYeYo/RuoRY7sPf2I/AAAAAAAAAaw/j5YrhE8ALsQ/s320/IRD_circuit.png

 

The circuit should work if the parts work properly.
RadioShack might sell manufacturer's rejects at a very high price.
The range might be very short, especially in daylight.

I was wondering if these phototransistors from Jameco would work better?

http://www.jameco.com/webapp/wcs/st...reId=10001&catalogId=10001&productId=1069958&

 

I found out the problem, i wired the IR Phototransistors backwards so it is working now. I was wondering if there is a way to speed up the phototransistors from reading the Ir emmitter? if i run my hand through them really fast it doesn't notice it, which isn't very effective for a pinewood derby finish line.

 

Your schematic has an IR photo-diode feeding a transistor.
If you used a photo-transistor then please show us how you connected it.

A photo-transistor has a fairly slow response time.
A photo-transistor used as an emitter-follower is slower than if it is common-emitter.
A photo-diode driving an opamp is much faster.

 

I just built my own circuit using the radioshack Infrared Phototransistor, and it turns the circuit on / off pretty fast with a laser pointer. You can get a laser from Wal-Mart for $3 in their "As Seen on TV" display, but if that's not there when you go, you can go to the pet section and buy a laser there for $4. Just look for cat toys.