Photo-transistor circuit design... again

[BushSnake]

Hi all

I've got a little private project going that requires an IR beam for a trigger, and will be used as a portable device in an outdoor environment. It has to be sensitive to be triggered by the a piece of wire being waved through the beam at relatively high speeds, and this section of the system has to be done on analogue components only. This sounds fairly simple, but I'm having a lot of trouble getting the system to work on variable distances, and especially in direct sunlight. I've worked on a couple of designs but it seems like I'm doing something wrong as this kind of thing must have been done in the past and it can't be that difficult.

My problem is as follows: From what I've worked out I need a variable resistor to determine the biasing of the photo-transistor. Based on some initial trials, the value of this resistor needs to be between 1k and 10M, depending on the distance between the sending and transmitting units of the IR system (i.e. the amount of light entereing the photo-transistor). But I'm battling to design a voltage controlled resistor circuit that will do this for me. I've attached the circuit diagram of my design... so please comment on that?

I've also attached the complete circuit necessary to return a 0 or 5V from an IR beam break. Please feel free to give other design ideas, or to comment on the design? I can't claim to be the world's best circuit designer... which is why I'm asking some gurus on this forum! I've looked at a number of designs on the web, and elsewhere on this forum, but I still can't find a design that I'm happy with.

Thanks in advance

Andre

 

[mneary]

To find something small and fast, you need to look for edges. So use two phototransistors that are illuminated by the same beam. While they are in balance there should be little or no output, but when there is movement or interruption the output changes.

 

[Damo666]

In addition to what's been said above, the use of ir lenses would also be a good idea, as they'd make the beam narrower and therefore easier to break, align, etc.

 

[MikeMl]

I needed an outdoor IR beam-break detector to detect a runner on a path in the forest. I ended up using an IR LED pulsed at 40kHz and a TV remote control receiver module that you can buy surplus for ~$1. Works up to about 3m in sunlight. The detection rate is fast enough to detect a break of a few ms. I posted about this on these forums about a year ago.

 

[BushSnake]

Thanks. I wasn't aware that you can actually buy the existing component, and I'll definitely need to have a look at that. Does anyone have a circuit design for one of these modules?

I'm a bit worried about the sensitivity, as well as the time delay. For a TV remote a few ms delay is no train smash, but this system need a response time of less than 5ms, and preferably as fast as possible. I could get the desired sensitivity with the above mentioned design, but you'd have to manually set the biasing resistor depending on the distance between the transmitter and receiver. This is not a problem, but I basically need a circuit / component that can set the resistance of 16 resistors to equal levels with one dial. This is why I was looking at the voltage controlled resistor. Any further ideas?

I'll definitely be using IR lenses with black tubes around the receiving photodiodes to minimise the environmental noise, but thanks for the reminder

 

[MikeMl]

Since for TV remote use, the modulation rate is < 5ms, the IR module (like the Sharp GP1U58Y Series) will follow the short break.