K8118 ultrasonic bat detector

Hi all!
I have just finished my k8118, and it seems to be ok, but in my opinnion gain is soooo high even at lower volume that i think that something must be wrong.
is there any way to know if it is ok this way? Too much noise and i think is because of really high gain.
Maybe sending a recording? maybe changing volume pot with another value?

thank you in advance!

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The noise you hear may be ok.
The noise is a “side effect” of the mixer.

Yes, you can do the following tests:

  1. Set the volume control to about middle position.
  2. Turn the frequency control to the lowest position.
  3. Now rub two fingers together near the microphones or slide a fingertip over a sheet of paper held near to the microphones. A scraping sound should then be heard in the headphones. Also jingling bunch of keys near the microphones can be used as a test.

If all the tests are ok, I think the unit is working properly.


Thank you!
As i said, i can hear ultrasonic now, but the noise is SO SO strong even at lowest volume that i cant believe i cant do nothing to modify it! :frowning:

what about the idea to change potentiometer value? would it work?


Can you hear some noise even if the potentiometer is in its minimum position?
Should be muted.


It is not muted at all :frowning: You can hear the effect of the volume pot and the effect of the freq. pot, but, in order to record something with my external recorder, i must set input gain to 0.1 !

If this is the right way, i wouldnt recomend this product :frowning: But maybe with some helo i can fix this ‘issue’.


This is not normal. Should be muted when the volume potentiometer is turned fully counterclockwise.
The trace between the potentiometer RV2 and C22 may be broken.
Please make sure C22 is properly soldered and the points +V/2, marked red in this picture, are connected together:


Hi, and thanks for the detailed potential solution.
I have two questions, ilustrated with the attached files.

Is this the right way to number the pins of the Volumen pot?
I suposse REV2A and B are the stereo audio channels, and numbers 1,2,3,4,5,6 are the pins.

If so, you say that 1 and 4 must be connected to the positive pin of the C22? May i do that with an aereal wire to be sure?

(sorry about my english, i’m doing my best…)

thank you very much for your patience and help!



Yes, if you have no multimeter to check the connection, you can add a separate wire between these points to verify the connection.

Please be very careful when soldering to the pin of C22 to avoid damaging the traces on the board!


Hello again!
I have tried your solution and I think it helps, but not as good as I expected. So I would like to know if it is now your best noise quality or if I am missing something else.

Could it be possible that the fact of having used some cables to put some components in a box (pot’s, switch, mics …) has led to this increase in base noise?

Is there a possibility to optimize this cable design to improve this noise a bit?

I would like to share later a couple of images of the box adaptation that I have been able to do.

thanks once again!


There is always some amount of noise due to the mixer.
Here you can listen a sample:


Hi vel255,

I bought this kit some time ago for my nephew and have enjoyed building it with him. Recently as an additional side project I decided to breadboard it with him but my understanding on the circuit and electronics is limited. I have a few questions which in part relate to the above +v/2 points and the noise of the mixer as well as an additional question regarding frequency control.

  1. any reason why the circuit cant be powered by 9v battery and what relation this has to the voltage division +v/2 points at the opamp and potentiometer stages.

  2. the potentiometers on my kit are marked B which i understand as being audio taper does it matter if frequency control is made with a linear pot, is it not suitable to control frequency in a linear manner?

  3. related to noise of the mixer section would substituting any other type of ic help?

Any thoughts appreciated!


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9V is too high voltage for the microcircuit 74HC14 (IC4).
The maximum recommended supply voltage for 74HC14 is 6V.

Both of the potentiometers RV1 and RV2 are linear pots.

I think the noise is related to the mixing process, not related to the components used.


I’m afraid that the noise level is simply TOO much to enjoy listening to bats :frowning:
I have tried in quiet places with bats flying quite close to the window, but… the K8118 lives now in a box… waiting for any improvement that the comunity could share.


Hi Vel255,

Thanks for your responses.

I had looked at the voltages of the IC’s but it slipped my mind that the 74HC14 required 6 volts.

Regarding the pots thanks for clarifying I guess it makes sense that the pot feeding voltage to the last op-amp stage is linear.

When you say mixing process are you referring to the op-amp stages?


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I’m referring to the whole mixer section containing two op-amp stages (IC1B and IC1D) and two analog switches (IC3A and IC3B).



Thank you for confirming seems complex then from that side :thinking:

Im starting to gather what i have around to attempt a breadboard of the project although the only thing i have handy are 10k linear pots.

Would there be a critical difference using a smaller valued pots to the rest of the circuit or would it just limit the resolution/trajectory of both frequency and amplitude?


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For amplitude this should be OK.
Using 10k pot for frequency control will limit the frequency range to the high frequencies (about 60kHz - 90kHz) only.
Using 50k pot the frequency range is about 20kHz to 90kHz.


Thanks, glad i asked, will try to get another pot.

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Hi Vel255,

Hope all is well and your having some time for Summer.

I have another question regarding my bread board endeavor + original question of noise which i understand is not only the opamp.

Any opinion on a TL974IN which says its has very low noise/distortion audio applications.

I’m not sure of the inherent difference of to the TL274IN the latter having a 3mhz bandwidth and TL974IN have a 12mhz bandwidth, voltages seem to be in same range for application.

Any advantage to using it perhaps?


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I think there is no problem to replace the TL274IN with TL974IN.
Indeed, you can get lower noise too…