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Nerfo meter - measuring the speed of Nerf darts - final setup

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HendriXML:
In the final week of the summer holidays I would like to do some scientific experimenting and constructing together with my son (9).

The goal is to measure the speed of Nerf darts. The setup will contain 2 laser beam light ports. The frame will be 3d printed in one piece.

On the right side 2 pcb's (green rectangles) will be vertically mounted with the photodiodes (blue circles) on the backside.

At first the change of exposure is measured using an oscilloscoop. With that it should be possible to do very precise time determinations. Maybe in the future a dedicated timer and Arduino with some user interface will do the timing. So the signalling part of the schematic can be implemented later on. But it is better to already save some space for it.

About the schema:
The component values are chosen mostly by guessing. They will be calculated later on.
The laserdiode emission is automatically regulated, so it should be possible to react fast on only a 10% drop of exposure. (The photo diode will be in a "tunnel"). Another reason is to not get the U1B op amp in positive saturation.
The transition from exposed to non exposed should be optimal in speed. Going from non exposed to exposed is less important.

Feel free to comment!

Kleinstein:
For fast reaction the photo-diode should be operated with some bias voltage (e.g. 3-5 V) in reverse direction. This reduces the diode capacitance - usually by something like a factor of 3 to 10 and makes the sensor correspondingly faster.
The feedback path in the transimpedance-amplifier should not use a pot. The exact amplification should not matter that much. Pots tend to have more parasitic capacitance to ground and this can lead to instability. In addition they can pick up more stray signals / hum.
For high speed the TIA gain should not be too large, as this slow down the TIA. Some 20 K is a common values for a fast TIA.

The adjustment should be better done at the trigger level of the comparator. For use with a DSO, there is no need for the comparator - a series resistor at the OPs output would be a good idea to isolate it from capacitive load (cable).  µCs tend to already include a comparator (the AVR internal one seems to be quite good), but often only one.
For the simple TIA shown the good old LM393 should be fast enough.

It is odd to use current regulation for the Laser(s): the modules normally just need a constant voltage, and bare laser diodes may need actual power regulation and extra optics.  One might get away with bright LEDs (red or IR), if there is little background light.

HendriXML:
Thanks for the reply!


--- Quote from: Kleinstein on August 25, 2019, 07:53:23 am ---For fast reaction the photo-diode should be operated with some bias voltage (e.g. 3-5 V) in reverse direction. This reduces the diode capacitance - usually by something like a factor of 3 to 10 and makes the sensor correspondingly faster.

--- End quote ---
I should have placed the photo diode differently. The bias voltage is only 500 mV, I would like to keep it battery powered so I don't have a negative rail.


--- Quote from: Kleinstein on August 25, 2019, 07:53:23 am ---The feedback path in the transimpedance-amplifier should not use a pot. The exact amplification should not matter that much. Pots tend to have more parasitic capacitance to ground and this can lead to instability.

--- End quote ---
The pot will essentially determine the optical power of the laser diode. I will experiment with it to get 25% of the max current, and then replace it with a normal resistor. This capacitance to ground is also an issue with those 10 turn trim-pots?

The comparator will collapse at 90% exposure relative to the (longer term) regulated one.

Regulating the laser diode is done for the above reason, but also to counter attack longterm aging of the diode and the drop in brightness while in operation.

The setup will have delays and inaccuracy's in it. I hope however to make them as consistent as possible between the two gates  :-+.

Kleinstein:
500 mV of reverse voltage for the PD is already better than zero bias. With battery operation the available voltage is limited, especially as the TLC072 is not rail to rail.

There should be no need to use a relatively power hungry zener diode to stabilize the 3.3 V level. Normally just a low power 5 V LDO like MCP1703 should be good enough. Even just the battery voltage can be stable enough. R6 also draws quite some power.

I now understand the laser regulation,  using the same photo-diode as the detector.  I would still adjust SensorVoltRef. and keep the TIA fixed. One may have to choose a suitable resistor though (e.g. 5 K, 10 K, 22K maybe 47 K). How critical a pot is depends on the resistance - at lower resistance (e.g. 10 K) a pot is probably OK, especially a relatively small form factor.
The Laser control still looks odd, as Q2 has emitter and collector swapped.
Worst case the 2N7002 will need more gate voltage to get some 30 mA out.

wilfred:
Circuits designed to measure the shutter speed of cameras would be a source of ideas for a sensor. These typically shone a light through to  a sensor mounted at the film plane. They would set a counter running when the sensor received light. Your application would probably invert that and use the dart to block the light. With the time the light was blocked as the dart passed the sensor and the length of the dart you have what you need.

I once had a device like this that used a 2N5777 http://www.njsemi.com/datasheets/2N5777%20-%202N5780.pdf
https://archive.org/stream/ETIA197/ETI%201977-10%20October#page/n45/mode/2up

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