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Trying to get this circuit to work on breadboard

Discussion in 'General Electronics Discussion' started by Wiginometry, Dec 6, 2017.

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  1. Wiginometry


    Nov 29, 2017
    I found this online. It seemed fairly straightforward however when I put it together on a bread board it just constantly Beeps and the led stays lit up.
    Thought it was just me having problems with this but I guess tapendra mandal has a YouTube video on his drawling his works but it seems others are having the same issue any ideas?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    To use an LED as photodiode (receiver) you need to reverse the polarity. Connect the receiver Anode to pin 3 of the LM358, cathode to +9V.
    As shown, the LED will not work as receiver but as emitter, too.

    Turn the potentiometer all the way up to +9V, the output should go low and the buzzer/LED be off. Then turn the potentimeter down until the output goes high, buzzer starts to beep, then turn the potentiometer back a bit so the output is low again. The circuit is now trimmed.
    Note that the transmitting LEd should light the receiving LEd as directly as possible to generate a good signal. A better choice than using two LEDs is a fork coupler which not only has the correct alignment of tgransmiter and receiver but also a much more sensitive phototransistor as receiver.
  3. Wiginometry


    Nov 29, 2017
    Thanks that actually helps alot. I actually popped an emitter bulb not paying attention when I first set it up on my bread board
  4. Audioguru


    Sep 24, 2016
    Even if the polarity of the receiving LED is reversed, the maximum allowed reverse voltage of most LEDs is only 5V but this circuit gives it 9V.
  5. Alec_t


    Jul 7, 2015
    With only 100Ω series resistance your transmitter LED will be passing about 70mA if the battery is fresh. Is the LED rated for that much current? If not, it won't last long.
  6. Wiginometry


    Nov 29, 2017
    Lol it didn't after I got it situated on the board I literally blew the bulb into two pieces.

    It reminded me of when I was on a mission in Iraq they had a substation in a back alley village. It would be an electricians worst nightmare a transformer actually exploded behind us that's what that bulb popping reminded me of.

    Lesson learned? Always check the math
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