Photodiode gate circuit noise

Question

I'm building a chronometer for high speed projectile speed measurement. Currently i'm in the process of designing the projectile detection gate. It needs to be able to detect obstructions in light for a duration of just under 1 μs.

The design is supposed to work as follows:

• The reverse biased photodiode will be lit by a stable light source (battery powered led)
• If the path of light is blocked from the light source the voltage on MP1 will rapidly fall.
• The voltage on MP2 is slightly negative biased to ensure it is below MP1 under normal conditions
• Since MP2 has a RC circuit it's voltage will change slower than MP1
• If the voltage on MP1 gets lower rapidly the gate will output 1.

So far so good, it seems to work with simple tests of breaking the path of light.

The problem i currently run into is noise, for some reason the signal at MP1 (measurement point 1) is very noisy, it seems irregular (for as far as my equipment can tell) and it measures over 20mV peak to peak, which renders the sensor unusable.

The following is measured with an ADC of an arduino (atmega328p) @ 200Hz +/- 1mv per unit.

Blue line is MP1, Red line is MP2, Output is not drawn cause it will offset the scale too much and is jumping up and down like crazy as you can imagine.

The gradual fluctuations are caused by changes in light (probably by my hand) and are intended. However the irregularity in the blue line is causing the problem. As can be seen the noise is higher than 20mv and also higher than the offset voltage and object detection limits. This causes the output to constantly bounce up and down.

• Is this amount of noise normal for an Photodiode (SFH213)? and thus should i just deal with it?
• Is there anything obvious that i'm missing that could cause this noise?
• Could i filter the noise in a way that would still enable me to see dips in the light of slightly under 1 μs?

Answer 1

First, use a proper transimpedance amplifier immediately after the output of the diode. Second, make sure the emitter is as strong as it can be.

You do want to use the photodiode in reverse leakage mode, but you want to hold its voltage constant. Converting the current signal to a voltage signal right at the diode is a bad idea.

Trying to modify this circuit is pointless. Start with a real photodiode detector circuit, then see what you have.

Answer 2

I'll take a guess here - you need to monitor the output as well. I suspect your output is randomly triggering and the transient feeding back to the input. You don't see the effect on the - input due to the low-pass filtering of C1.

As Olin has stated, you desperately need more light. Get yourself a (for instance) 3W LED on eBay - they are cheap.

Do the math, and you'll see that you're only getting about 1/2 microamp from your PD.

Now, about circuit selection. Start with your simple circuit. What is its intrinsic response speed? Let's say you have 20 pF equivalent across the resistor. Then, since the PD is acting as a current source and has (effectively) infinite impedance, the response at the amp will be a single-pole low-pass filter with a time constant RC. In this case, (20 x 10^-12) x (36 x 10^3) gives .72 x 10^-6, or about 0.7 usec. This is a decent (but not great) response if you're looking for usec accuracy. Thing is, your signal is very low, and your comparator is not really up to the task. Note that a 393 has an input offset voltage of as much as +/- 9 mV and is looking at a 4 mV difference.

So, how can you get more signal? Well, in this case you might try increasing R1/R2. Problem is, when you do that you increase the input time constant. Increasing by a factor of 10 will give you 200 mV with a 40 mV input separation (yay!) with a time constant of 7 usec (boo!). What to do?

The classic solution is the TIA. Here you can get much better response time for the same signal level. In this case, though, dealing with changes in ambient level can be a problem. As long as ambient changes slowly, and is comparable to the LED level, you can do OK.

An issue often overlooked when considering a TIA is that, when using the PD in photoconductive mode, you need a second, opposite polarity bias voltage, while the direct circuit only requires a single supply. Well, OK, you can make a little DC-DC converter to give you a - supply, but I don't recommend it in view of the noise issues it will introduce. Maybe when you have more experience.

But notice that the complexity of the TIA vs the direct detection approach hinges on the assumption that the PD current is low. That is true in this case, but does it have to be? With more current you can use a smaller load resistor and get faster response. Like I say, you can get high power 3W IR LEDs on eBay for little money. Use one of those puppies and you'll get a much higher PD current and higher input voltage (with greater voltage differential in the resting state). Adding a cheap IR filter in front of the PD is ALWAYS a good idea, too. Granted, a much greater LED drive current is needed, and this will affect your battery selection, but that's a tradeoff you'll need to make.

As I started off saying, I suspect that, with the resting voltage difference so much less than the guaranteed input offset, the comparator is switching randomly, and the transients are feeding back into the input and showing up. Just out of curiosity, do you have a decoupling capacitor on the comparator? Like, 0.1 uF ceramic placed as close as humanly possible to the power/ground pins? Do you have a ground plane? Both of those may well be an issue.

TL:DR - Get a beefier LED. And put an IR filter on the photodiode. And pay attention to your construction techniques.

Answer 3

Thanks all for the comments and answers, all in all they helped a lot. The mystery of the noise is solved. This evening i powered the system on again and got a blue line displaying a waveform in the 50Hz range which i could easily track back to the Led lamps lighting the room. I turned them off and got a nice line on the zero (dark and no artificial lighting).

When i introduced an LED the levels rose an a tiny fluctuation could be seen on the blue and read line as seen in this graph:

A minuscule excitation of the gate (rushing my finger past the diode) gave me the following graph:

The initial offset voltage might need some tweaking, as i understood from @WhatRoughBeast the LM393 needs an input difference of 9mV to produce a stable output. This is manageble.

So the question remains what caused the noise: To answer that i had to think back to the situation i was in. I sat in my room near the window. No artificial light but plain daylight. There was no direct sunlight in the room. However there is a tree next to the window and the sun was shining on it's leaves shimmering in the sunlight... I suppose this has been the cause of my distortions, the frequency of all the leaves on the tree moving in the wind and changing the light in my room. Wow that is a sensitive sensor!

So to answer the questions i asked in my post: 1). No those are not normal noise levels. 2). Maybe a tree ;-) 3). The circuit is fine.

Special thanks to @WhatRoughBeast and @PDuarte for your helpful answers and remarks, even if though the problems solved themselves i learned a bit from those comments and the long read of @WhatRoughBeast (thanks for the great effort and your willingness to teach a fellow technician)!

Answer 4

Just in case someone stumbles across this in the future, a laser diode harvested from an old laser pointer can make a cheap high-intensity visible light source for a photodetector application.

Answer 5

Maybe the noise comes from your power supply. I certainly wouldn't trust any small signal such as this, when biased directly from the supply.

Either use a zener diode or precision reference (like the TL431) to obtain a stable potential slightly less than 4.5V (say, 3.9V), and use that to derive all biasing potentials:

^{simulate this circuit – Schematic created using CircuitLab}