Is this a pre-amp circuit?

Hi,

Just a quick question. Im looking at a circuit diagram that supposedly uses an IR beam break system. I was under the impression that such systems didn't need a preamp and could be wired straight into the pic, but this apprears to use a preamp circuit.

**broken link removed**

What is the function of the circuit outlined in red, and would it be nessesary for a beam break system?

Thanks!

This is a pre-amp with a voltage gain of 101. It is AC coupled. Whether it is needed is dependent on the type of sensor you are using. Right now, it is a light dependent resistor.

The circuit in red amplifies the signal by an AC gain of 101 (DC gain is 0). Whether it's needed depends upon the strength of the IR beam, the sensitivity of the detector, and the distance from the IR source to the sensor.

Hobe said:

What is the function of the circuit outlined in red, and would it be nessesary for a beam break system?

Thanks!

Click to expand...

It appears the function of the amp is to detect the conduction transitions of the IR diode to produce a pulse to IC3, perhaps to count jelly bean bags on a conveyer or to serve some other process control purpose. Myself, if it were necessary to interface along these lines, I would use a comparator instead of overdriving an op amp with slow rise time like the LM358 and to eliminate the 1/3 Vcc DC at the output.

Hi,

Thank you for all the information. In my design i was going to use something like this:

https://media.digikey.com/photos/Fairchild Semi Photos/QVB11134.jpg

Would a IR rx and tx in such close proximity need a pre amp circuit like this?

Could you also tell me how you got the gain figure of 101K?

Hobe said:

Hi,

Thank you for all the information. In my design i was going to use something like this:

https://media.digikey.com/photos/Fairchild Semi Photos/QVB11134.jpg

Would a IR rx and tx in such close proximity need a pre amp circuit like this?

Could you also tell me how you got the gain figure of 101K?

Click to expand...

First and as I have already mentioned, the circuit you posted is not a proper interface, IF required for many applications.
Second, Fairchild's data sheet is very short on electrical info, so one can only say that it probably would be fine for a flag interupt device UNDER MOST CONDITIONS; conditions you have not expanded upon.
Third, the gain in your schematic is calculated as Av=Rf/Rs +1 where Rf is the feedback resistor R5 and Rs is the source resistor R4 for the non-inverting amp.

The AC gain is 101, not 101k.
101 is one hundred and one. 101k is one hundred and one thousand.