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Question about RL circuits

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Broz

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So I'm working with more IR type circuits and need to detect an IR signal reliably at long distance in strong sunlight. I've tried many things to make the typical 38kHz IR receiver modules work better in strong ambient light. I've had some success, but it's always at a cost, like reduced fields of view. I've read that the best method is to use a photo-diode and filter out the DC (ambient compent) using some type of inductance filter.

So I started hitting the books again to brush up on RL, RC, and RLC type circuits. Then I came across this circuit:

http://www.discovercircuits.com/H-Corner/40khzlight.htm

I'm a little confused. I understand that the first inductor from the left is mainly just sending the DC component of the ambient light to ground leaving the AC component to go through the coupling capacitor to the amplifier. What I'm confused about is the parallel RL circuit between the power supply and the Darlington pair. How is that going to ensure that the 40kHz is amplified? I have formulas for series RL circuits, but this is in parallel. Can a parallel RL circuit act as a bandpass filter? And how do I calculate the center frequency and the Q?

Thanks.
 
The first LR is a high pass filter as you suspect.

The transistor:
At DC the gain is approaching 0 because the coil’s resistance is probably 10 ohms and the emitter resistance is 4.7k. At high frequencies the collector sees 4.7k and the emitter sees nearly 0 ohms. (that’s another high pass filter)

The idea here is to remove low frequencies. Fluorescent lights make noise at 120hz. No it is not 60hz. There are harmonics of 120 up to 1khz that need to be filtered out. Sun has energy at all frequencies. Removing everything below 10khz is a good idea.

You will have a better circuit if it is tuned for 40khz but many circuits are much like what you posted.

Did you install an IR filter? It helps!
 
Thanks for the reply Ron. So you're saying that both filters are high pass filters. If the sun doesn't cause much noise in the higher frequencies then I guess that would work. However if it does, I need to design a better circuit, such as a bandpass filter with high Q to isolate my signal. Is this correct?

I did try IR filters. The material I tried is exposed, developed, photgraphic film. From what I've read the transmission of IR is centered right around the 900-1000nm realm but not tightly centered. I'm sending 940nm IR. However, the range was only increased a few feet. I should note however that this was using an incandescent light bulb to simulate sunlight, which seems to duplicate the problem quite well, as we haven't had any strong sunlight here for the last week. It seems plenty of stray IR is still getting in and flooding the IR module. I can't find a comercially available IR filter that is centered tightly on the 940nm. The only thing I've had real success with is using small tubes as recessed cavities for the IR module. The problem is, I need the full field of view of the IR module for my application. That's why I'm looking into inductance type filters to help. Is it possible that the light bulb I'm using for my simulation is causing more problems than the sun itself? If it does, then maybe I would have better success when the sun comes out and I can do some testing.
 
Ron, thanks for replying again. My next question is not to be taken in a disrepecful manner. But I'm left wondering what the point of your reply is. I take it as one of two possibilities:

(1) You are telling me that the Sharp IR module that you posted a link to is better than the Panasonic 4602's I'm currently using.

Or

(2) You posted that link just to inform me that IR modules in general use a bandpass filter.

If it is (1), I'm willing to give them a try, along with the Vishay's which I'm told are better than the Panasonics.

If it is (2) I have more questions for you, that have many assumptions on my part. The first is, would it be better to stick with stock IR modules or is it better to build my own? The reason I ask is where my assumptions come in to play. I've read that IR receivers that use inductive type filters offer much more ambient light filtering than stock IR modules. I'm assuming this means that stock IR modules use capacitive type filters. I believe this is true just due to the physical size of stock IR modules. There just isn't enough room to put good inductive type filters in IC IR modules. I'm also assuming that capacitive type filters would not work as well as inductive or inductive/capacitive type filters. My guess is that capacitive only type filters have a difficult time dealing with a high noise/signal ratio. I believe this is why some people who want to filter out sunlight from IR communication build their own IR receivers with inductive type filters, such as laser tag guys who claim to acheive several hundred feet in high sunlight conditions.

If (2) was your point, are my assumptions above correct? Is it worthwhile for me to build my own IR receiver using inductive type filters?

Thanks again.
 
Broz,

It looked like you are re-inventing the wheel.
>Reverse engineering- I learn from looking at other people’s work.
>It is good to know if what I am doing is better/worse than what’s out there.

I did not know you knew there are off the shelf options.

The band pass filter in the Sharp unite has no published data. Years ago I designed an IR receiver that had a band pass filter that had a 20 to 50 khz response. A second version had a 120khz filter from the IF of a radio. (very good filter) It is not clear in my mind what happened 25 years ago but I think the filter was better but not much. It also maybe that back then 120khz was too fast for the parts I had.

Sharp sells 36,38,40,56khz versions. It is implied, but not stated, that a 36 and 40khz will not talk to each other. It true then the filter is very good. I would not count on it.

The transistor amplifier you are using has a “band pass” function. The MPSA13 is not fast. High frequencies will be rolled off. Also the capacitance of the transistor and the inductance of 10mH will resonate at some frequency. You can try adding capacitance and tuning for 40khz.

I just looked at the data sheet for a Vishay IR receiver TSOP44. They show the response of the band pass filter! I think it has a IR filter built in. They make “noisy”, “standard”, “fast” all optimized for pulse length coding and “standard for pulse distance coding”. They all have the same band pass filter.

My suggestion is to get the Vishay and/or a Panasonic and build your unite. Then compare. By theory a tight band pass filter is better. (on the other hand) It is hard to build a low noise amplifier using discreet components.

Can you build a better IR receiver? YES Will you? That is the question.
 
ronsimpson said:
Sharp sells 36,38,40,56khz versions. It is implied, but not stated, that a 36 and 40khz will not talk to each other. It true then the filter is very good. I would not count on it.

I've never found any problems using any receiver with any transmitter, perhaps all the ones in the UK are the same frequency?, but it seems unlikely - and certainly when I was writing my IR PIC tutorial I tried various different transmitted frequencies, and it made no noticeable difference.

Can you build a better IR receiver? YES Will you? That is the question.

It's VERY difficult to build a good IR receiver, there have been various efforts over the years, but the cheap modern IC's available seem to outperform them all.
 
Trust me, I never intended to reinvent the wheel. However the standard IR modules just aren't cutting it for me. I'm encouraged by some of the reading I've been doing on the laser tag guys. All of the good sensors that can detect in strong sunlight use inductance type filters. Some are claiming distances of a 1000' in strong sunlight with no type of IR filters covering their sensors and with a good field of view. It's worth a try. I've ordered some inductors and they should be here soon.
 
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