Newbie help - converting quick LED pulse to audio

Discussion in 'The Projects Forum' started by newbiefail, Dec 7, 2013.

  1. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi all

    New to electronics and need some help! Ive done a lot of research and need some help with a one-off project.
    I purchased an RFID reader from eastern Australia for a mountain bike race. The unit works fine, but the rider has no indication of a successful RFID read apart from a small onboard LED that would be in a unit anyway.
    So I want to convert this only indication of a successful RFID read to another external brighter LED and audio response, to assist the rider when a successful RFID read has occured.
    So I soldered a little white cable off the onboard LED to try and get a voltage. The multimeter gives me some erratic reads, but I think its around 50mv when the LED lights up.

    My first attempt was a simple NPN transistor circuit with a LDR for the switch. I had varied results, but didnt really work out (I think) due to the low voltage and speed of the LED pulse.

    My next several attempts over several days involved a 555 circuit, to try and extend the output time using a cap and transistor to get the desired 1secound output. Some involved using the LDR, some the cable off the onboard LED but nothin seems to work.
    Currently (as shown in the attached images), but setup sort of works. Again, much like the NPN setup, the speaker outputs such a short pulse that the speaker is almost inaudible.

    So if anyone has any suggests, please fire through! Getting to the end of my tether :(

    I think i firstly need some help in terms of getting a consistent read from the onboard LED. The LDR had trouble detecting such a quick pulse. If anyone has any other suggestions in getting another signal from the purchased board, that could also greatly help.

    *sorry about the hair on the breadboard! haha*

    Kind Regards
    Steve
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How about YOU trace out the schematic of the stuff on your breadboard and post it...

    What is the voltage between the red and blk wire?

    What is the resting voltage on the wht wire measured with respect to the blk wire?

    Which direction does your DMM kick when the on-card LED lights? Higher than the resting voltage, or lower than resting voltage, above"
     
  3. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi Mike

    Thanks for your post - hopefully the attached is of assistance?

    When i had the multimeter connected to 0v and the white cable, it is 4.9v. When I scan an RFID, the voltage doesnt change (or is very small).

    But when I had the multimeter on each side of the onboard LED, it rested at 5.1mV and when I scanned, it jumped to anywhere between 20mV and 40mV.

    What is DMM sorry?
     
  4. MikeML

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    DMM=digital multi meter

    With power disconnected, measure the resistance between the wht wire and red wire, with both possible meter lead polarities...

    You didn't tell me the voltage from red to blk?
     
  5. Alec_t

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    Sep 17, 2013
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    This should give a ~0.4sec burst of ~600Hz tone for each positive-going input pulse from the voltage across the LED :-
     
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  6. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi Alec
    Thank you so much!!!
    Looks like I need to head out and get some PNP's

    Mike
    Ill get those other answers to you in a few hours, just at work at the moment - want to get home to keep working on this project though! :)

    Thank you both for your help. Mountain Biking in Western Australia is also very grateful :)
     
  7. Alec_t

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    My circuit was designed around the bits I thought you already had. If you don't have a suitable (32 ohm) speaker then you could use a piezo element instead. Or a simpler circuit could use a cheapo buzzer and (probably!) avoid the need for the NPN and PNP
     
    Last edited: Dec 9, 2013
  8. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi Mike

    Please find attached some pictures that may help explain the resistance across the wires. I took photos just to make sure Im giving you the correct values and measuring it the correct way.

    Im using 4.8v as the power source

    Thanks!
    Steve
     
  9. Alec_t

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    Sep 17, 2013
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    Having re-read post #3, I realise my suggested circuit would be fine provided the 4.8V supply to my circuit doesn't have a common ground with the RFID reader.
    Were you planning to use a single supply to power both the reader and the pulse-to-audio circuit? If so, my circuit would need modifying.
     
    Last edited: Dec 9, 2013
  10. MikeML

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    Ok, you posted two pictures each of:
    Red Ohmmeter to Red wire, Blk Ohmmeter to Wht wire (Open or OL)
    Red Ohmmeter to Blk wire, Blk Ohmmeter to Wht wire(16.8K)

    I also want the other two combinations:

    Red Ohmmeter to Wht wire, Blk Ohmmeter to Red wire?
    and Red Ohmmeter to Wht wire, Blk Ohmmeter to Blk wire?

    Just post the Ohmmeter readings.
     
  11. Alec_t

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    My interpretation of the voltage measurements in post #3, plus the Ohms values in post #10, is that the white wire is presently connected to V+ and to the LED anode. The LED is switched via its cathode.
    For triggering the audio generator it would be helpful to have a wire connected to the LED cathode rather than the anode.
     
  12. MikeML

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    Well, by asking all these leading questions, that is exactly what I was trying to figure out. However, I'm still not convinced. Let the OP make the additional measurements.
     
  13. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi Alec

    It would be very beneficial for me to use the same ground source, as the units will be mobile and one power source would be a lot easier!

    Note: I did setup your circuit and the buzzer simply stayed on. No matter what I did, I couldnt turn it off. The next statement is a reflection of my avatar. I did some research on what I thought was a NPN transistor, and infact, I was using a 5v regulator (embrarrased!) After changing this however, the circuit failed to work - perhaps due to the common earth.
     
  14. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    I would like to trigger both an audio signal (0.5-1sec beep) as well as a bright LED that I can attach to the outside of the unit. I can solder the white wire to the other side of the onboard LED if thats prefereable
     
  15. newbiefail

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    Dec 7, 2013
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    I hope this helps! :)
     
  16. MikeML

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    Since those readings where in mA (not Ω), you must have taken them with the meter in the DCmA mode, presumably with the device powered?

    I wanted you to take Ohmeter readings with the device not powered.
    You previously measured two of four possible combinations of red-black wires on the meter to the white-black pair and white-red pair on the device.
     
  17. newbiefail

    Thread Starter New Member

    Dec 7, 2013
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    Hi Mike
    Im sorry, ill get there eventually!

    Red Ohmmeter to Red wire, Blk Ohmmeter to Wht wire OL (Open or OL)
    Red Ohmmeter to Blk wire, Blk Ohmmeter to Wht wire 17.9k ohms (16.8K)
    Red Ohmmeter to Wht wire, Blk Ohmmeter to Red wire? OL
    and Red Ohmmeter to Wht wire, Blk Ohmmeter to Blk wire? 18.1k ohms
     
  18. radiohead

    Active Member

    May 28, 2009
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    You can have the weak signal fed to the gate of an SCR that will turn on a buzzer or other alerting device. You can have a normally closed momentary switch so the operator can cut voltage to the SCR to reset the circuit.
     
  19. alfacliff

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    Dec 13, 2013
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    led's generate voltage as well as light up. connect another led of the same color to your audio amp, it should make noise when the other one lights, if it is modulated. this also works for testing infra red leds on tv remotes, hook an infrared let to your scope, and watch the pulse train from the remote when you push the button.
    cliff
     
  20. Alec_t

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    Do that. Then, with the reader powered up, measure the voltage between the white and black wires with and without the tag recognition.
    Can you confirm the black wire goes to battery negative (0V) ?
     
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