I am tinkering around with try to simplify a design for a ballistic chronograph that was developed by David Collins in the Nuts and Volts Magazine in June of 2009. Basically, I am using two frames, where each frame consists of (3) IR LEDs (QEE123) and (3) PIN Photodiodes (QSE773). I am trying to develop the entire project using a 5V DC power supply. So, I reviewed the data sheet and noticed that the peak emission for the LEDs is at 20mA and am therefore using (3) parallel branches of a 150 ohm resistor in series with the LED to yield roughly 24mA.
My problem lies in understanding how the the receiver circuit should be set up. In the article, he uses 4 of these photodiodes in parallel, reverse-biased where all the cathodes are run to Vcc and all the anodes are joined and used as the input into a series of op-amps.
The theory behind this (as I understand it - please correct me if I'm wrong) is that under normal circumstances, the emitter is active and will emit IR signal to photodiodes, allowing current to pass through. However, we are looking for when a projectile crosses through causing a small, quick disruption and want to somehow identify and amplify ONLY the voltage drop to produce a TTL signal that will be used to trigger a timer (one frame to start and one to stop). However, with these photodiodes in parallel, doesn't this pose an issue for inconsistent values depending on where the bullet crosses (left to right)?
Any help would be greatly appreciated. I'm attempting to attach Collin's emitter, receiver and amplification circuits in a .jpeg
My problem lies in understanding how the the receiver circuit should be set up. In the article, he uses 4 of these photodiodes in parallel, reverse-biased where all the cathodes are run to Vcc and all the anodes are joined and used as the input into a series of op-amps.
The theory behind this (as I understand it - please correct me if I'm wrong) is that under normal circumstances, the emitter is active and will emit IR signal to photodiodes, allowing current to pass through. However, we are looking for when a projectile crosses through causing a small, quick disruption and want to somehow identify and amplify ONLY the voltage drop to produce a TTL signal that will be used to trigger a timer (one frame to start and one to stop). However, with these photodiodes in parallel, doesn't this pose an issue for inconsistent values depending on where the bullet crosses (left to right)?
Any help would be greatly appreciated. I'm attempting to attach Collin's emitter, receiver and amplification circuits in a .jpeg