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# Axle

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Hi Everyone, Me again. Ive been playing around and it seems that even with an IR filter, range is still limited. It seems the signals are far to small as we move away from the reciever. Ive come across differential amplifiers and they might be what im looking for to boost range. I essentially want to amplify signals of about 1mv to something i can use and i think the attached circuit can do the trick. I believe the attached is a very simple 'op-amp'. Ive included a current mirror active load and grounded one of the inputs to make a high gain amplifier. Just a few questions: 1. Will this amplify the small 1mv signals to something useful? 2. Given no Collector resistor what is the gain of such a configuration? 3. The signals into Q9 is likely very small (few mv only) so should there be a dc bias to ensure the transistor Q9 is always on? Should the bias flow to Q8 as well or should it just remain grounded? Thanks Axle

Thanks AudioGuru. Much appreciated.

Hey again, Still me trying to get things working and to enhance my understanding. So far i think this project has definitely given me the opportunity to learn quite a few things. Ive attached a circuit of a filter and im hoping that someone can verify that my voltage and current calculations are correct. Please look at the attached. Here goes: Situation: No Remote Signal --------------------------- 1. Dark current through the photodiode is 2ua. (This is what i measure with a multimeter) 2. The capacitor looks like a open circuit given there is no signal? 3. Hence the current through R1 is 1ua and the voltage drop across V=IxR=0.4V Situation: Remote Signal ------------------------ 1. Current through the photodiode is now 10ua. (This is what i measure with a multimeter) 2. The signal has a frequency of 38Khz (38000Hz) so the capicitor has a impedance of z=1/(2*pi*f*C) which in this case is about 1/(2*3.1415*38000*(1/1000000))=4ohms. 3. So the signal sees R1 in parralell with (4ohms + R2)? 4. Given the impedance of R1 and the (capicitor + R2) is almost the same the current splits equally between the two branches. As such 5ua goes through R1 with a resultant voltage drop of 1V across it? 5. The drop across the capicitor is small and we can ignore it? 6. The drop across R2 is then the same as R1 which is 1V? I Would really appreciate any ideas from anyone. Pretty Please.... Thanks a million Axle

Been really busy these past 2 days so apologies for not responding sooner. Thanks for the info. Every bit helps me to improve... As per the datasheet you posted, im looking to build the complete circuit in three stages. Stage one is the amplifier which is the darlington configuration. Stage two (as per the datasheet) is then the filter to ensure that only the signals of interest are passed. Hence ive gone for a high pass filter to remove any dc and ambient noise. Given that remotes operate at 36-38Khz, ive gone for the component values below. Stage three converts the pulse to a 0-5V pulse. I thought of using a comparator for this. While all the stages sounds simple in principle im really struggling ??? with the filter aspect of things. From my readings things shouldnt be this complicated! I just need to make sure of the following : For the circuit i posted before this, When calculating the cuttof frequency f3b=1/(2piRC), do i included the emitter resistor of the darlington pair for R in the formulae? I really think my calculations are incorrect for my filter! Ive used the following, R1=10K, R2=90k and C1=0.0001u (capacitor code 101) Thanks

Hi, Me again. Ive made some progress since the last time i posted. Ive contacted one of the local component sellers and they indicated that they may import the Vishay IR module going forward. If they do, and i think they will, then i should be able to get my hands on one of them in the near future. In the meantime i havent given up completely. Im still trying to learn as i go along and i think i have made some progress. Ive sorted out the range issue now. The IR filter was the key. Ive also sorted out my problem of having the same LED brightness no matter how far or how close i am to the reciever. I used a high gain (Ibx 40000!) darlington configuration to ensure that i get about 3v dropped across R1 irrespective of distance. So thanks for the help in getting me this far. I have a few more questions though and would appreciate any ideas on these. 1. I dont have an oscilloscope so im not sure how my input signal looks. I would assume it is a train of pulses with a frequency of 38Khz? 2. My reading on V1 (See attached) is above 3V when a button is pressed. How come i can see this voltage if i cant see the voltage at Vout (after filtering)? Is it that at V1, im seeing the voltage due to the dc bias? 3. After filtering how would the output signal look? Another train of pulses from 0-3V? Anyone with some ideas on these questions? Thanks

Hehe , Modern Tv's and VCR's we thankfully do Have! ;D Im not sure why im struggling but the all component resellers i got in contact with didnt have these modules. It seems they dont import certain components if there isnt a big enough demand for them. Great idea. Perhaps ill call a TV/VCR repairs shop and find out if they can direct me to a shop that might have these modules or if i can purchase an broken TV/VCR. Im not sure where people would dump there old TV's though and rummaging through dump sites looking for components doesnt sound too appealing... It didnt occur to me the problem could be with the multimeter im using. I dont have an oscilloscope and the multimeter im using is a pretty simple one so it most likely the issue. If i dont get one of these modules i think i should put this project on hold for a while. Building one from scratch seems quite complicated and given my lack of experience and limited knowledge its probably a bad starting point! I have been defeated ... (for the time being at least) Just wanted to say thanks again for all the help. Keep well. Axle

Hey, Thanks for the info. Unfortunately these modules arent available here in South Africa. Ive tried quite a few electronics shops and the closest thing they have is the two pin photodiode im using. :( Im stumped at the moment as im not connecting the filter to anything, im just checking the ouput voltage (Vout) relative to ground and i still get a zero reading on my multimeter. If my calculations are fine then surely when i hold the remote close to the photodiode i should get some reading across R2? Keep well

Oops, got a bit excited and forgot the attachment. :) Here it is...

Hi, As an aside (should i start this as a new post ?) i came across high pass filters while doing some research. Sounds interesting and i came up with the attached circuit as a possible filter. I believe that most remote controls use a frequency of about 36khz and based on this ive selected the values of the capacitor and R2 so that it attenuates all frequencies below 30Khz. Are these calculations correct? I dont think so as i dont get any voltage when a remote button is pressed. Vout remains at zero. Any ideas?

Hi Audioguru, Thanks. Will do some research on tuned circuits and rectification. Im still struggling getting my mind around a few things but things are much clearer. Best Regards Axle

Hi Audioguru, Yes its an emitter follower. Im not sure of the polarity as both pins on the diode are of equal length. However i have connected it in the circuit such that if no remote signal is present it provides 1ua of current. If a remote signal is present it provides between 10 and 15ua of current Thanks Axle