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Creating a simple S-Meter Sensor for uBITX (LM358 Version)

Creating a simple S-Meter Sensor for uBITX (LM358 Version)

If you are already using S-Meter, you do not need to view this document. This document is for those who do not have an S-Meter Sensor or want to use some of the features using S-Meter on the Nextion LCD. There are various ways to attach S.Meter to uBITX. You can find things on the Internet that are far superior to what I have created.

uBITX Firmware CEC version is based on unmodified uBITX. However, to see the strength of the received signal, you have to attach the sensor. Of course, there is no problem in using CEC firmware without a signal meter sensor. Most people I know have not installed an S-meter yet.

I was suggested to make an S-Meter Sensor using the LM358 which is a bit cheaper than the LM386. This circuit is a modification of the basic form of the OP-amp, which is very common. This model set the offset voltage to 0Volt. (The previous version using LM386 was 2.5~3Volt),  So one more part was added and five were used.

1. Parts list
  LM358 * 1
  1n4148 * 1
  Resistor 1K * 1
  Resistor 10K * 1
  Capacitor 0.3nF ~ 40nF

  (I recommended 0.3nF if you use Nextion (or TJC) LCD, 0.3nF => marked 301 on capacitor )  

The LM358 has been rotated 90 degrees. attention to the Pin number.

3.Wire UP
The figure below is 'uBITX Wire UP' at I added a number to connect to it. (Red color number)


I made it again with IC Socket.

Connect 1N4148 to pin 1
Connect 10k to Pin2 and Pin1
The pin numbers of the LM358 are shown in red.

Connect 1k to Pin2 and GnD
Connect C1 to GND and 1N4148

I have made it possible to replace the capacitors as shown below to do some experiments.

It is the final finished.

Maybe if you use C1 at 1.5nF, the S-Meter may not be accurate.
In this case, if you replace C1 a little bigger, it will probably work stably.
However, if you are using Nextion LCD or TJC LCD, we recommend 1.5nF. In the next beta version, I will introduce some fun features using this S-Meter.

My plan is to introduce one or two different versions of S-Meter in the future.
It costs maybe $ 3 to $ 9, but it is not expected to be so difficult to make.


  1. Looking forward to newer versions of S-Meter! I just got into uBitx and I have been researching all you have done!! I have already updated my firmware for my 2 line factory display with version 1.08. I have a uBitx version 3 that I just put together and waiting on my uBitx version 4 from India. I just ordered and received the Nextion 2.8 enhanced display and plan on installing it. I need to get some components that I do not have like the pins to add to the cable to plug into the uBitx board. This is all new to me!! Please forgive if I don't use the right terminology.

  2. Can this S-Meter be made using the LM386N??? I have a couple of these that I bought when I was building a Pixie kit. Is the wiring the same??? Thanks for any response.

    1. Juddie

      Thanks for your interresting
      Please refer to Link below for LM386 wiring.
      But if you plan to use Nextion LCD, I recommend the LM385 version.

      The Signal Meter will also display the same version of LM386.
      But I have added some fun features that we can use to study with Signal Meter.
      I made it based on the LM385 version.

      The LM386 version is planned to be tested in the future, but until now we have only tested with the LM385 version.

      Ian KD8CEC

    2. Thank you sir. I am going to build the circuit you have here using the LM358. I am going to order the chip on Friday. I have resistors and capacitors and well as diodes to do the build. Again, and I know I have said it before, I really appreciate all this work you have been doing!! It is like having an elmer right here to learn from and ask questions. So again, THANK YOU! 73 have a good 4th of July from Juddie WD8WV

  3. Made the S-meter ctt and it works fine... Simple but effective!!

    1. Joe
      I am going to make one more improvement in relation to S-Meter in the future. I think the price is less than $ 8.

    2. I built this and it trips my TX on and off! Will try the NANO version soon.

  4. interesting that you have a lm386 version. 386 is a 1 watt amp. 358 is basically a voltage amp, low power. I expect that the Nextion is basically an A to D voltage sensing system so the 358 or any low power amp should be fine. A voltage amp, no "wattage" really required. But, one thing that bothers me is that when you incorporate an AGC similar to the one that ND6T uses it basically clamps the audio so all audio has the same level. So, when using an AGC like that one (here } your leveling the audio, strong signals have the same volume as weak signals. He is doing that by sampling the audio level at the high side of the audio volume control and sending a representation of that audio level to a RF attenuator, thus, keeping most all audio at the same level regardless. So, the levels read by an S meter would be very much affected by that circuit {which samples at the same high side volume control point) , yet, the uBITX very much needs an AGC.

    1. I have not found the Reply button for a while :)

      The reason I used LM386 is simple, because it's easy to get around. I used the LM386 by subtracting it from the Pixie qrp rig, I used the LM358 to lower the offset voltage.
      I have not used ND6T's nice AGC yet, I will use it sooner or later, I know KIT has been released recently and I think it will be easy to use.

      Ian KD8CEC

  5. Another question Ian. I have everything now to build the s-meter. I was reading your document and discovered I don't have the right capacitors. But, what puzzled me is that in red text you say to use the 301 capacitor if using a Nextion display, but further down in conclusion you say if you are using the Nextion display to use the 152 capacitor. I have to order the capacitor and just want to verify actually which one I need.

  6. Good evening Ian. I just wanted to let you know that I built the s-meter circuit, but I have not been able to get it to work. I used the exact components you have listed including the .3nf cap. I had 5 volts going to pin 8 of the LM358 but for whatever reason running uBitx Manager and doing your steps all I would get by reading the min value from uBitx was 0 - 1 and when I did the max value from uBitx I got the same reading. Not sure what is going on. I have tried 3 times in the past 3 days to get it to work. So I don't know what else to do. I removed the circuit (which was hooked up temporarily) and turned the s-meter off. I run the ADC and get the following readings:

    A2 (F Switch) 1020
    A3 (PTT) 1020
    A6 (CW-Key) 1023
    A7 (S-Meter) 766

    When I start the ADC the A7 starts around 668 and goes up to 766 and floats there between 766 and 765. I am just wondering if I have screwed something up.

    Hope you may have a suggestion. Wish your circuit pictures showed more of the hook up of the components, and then where you hooked it onto the uBitx.

    Juddie WD8WV


    2. I hooked the 4148 diode's cathode to VIOLET wire of Raduino connector which actually goes to A7 pin of Arduino board.
      Similar symptoms here, too. Wandering numbers, no matter how big signal I am feeding to radio.

      The violet wire is working, Using ADC monitor function you can see its leveln on the screen.
      Perhaps we just should use a single fet or transistor to amplify the VOL HI signal, rectify it to a cap and forget all op-amp bells and whistles.

    3. Well, this is embarrassing...I had Vol HI and M reversed. Had to replace a faulty potentiometer and messed the yellow and orange wire.
      After reversing them all works.

      Thank you, Ian.

  7. In your version with the IC holder, do the pins on the cables plug directly into the raduino board or do they require connection to the wires as per the diagram. I am not sure if the sockets left after the LCD removal for the nextion are used for connecting this/

  8. For the ubitx v6. On the raduino has 12 volt. Your project showing 5 volt. Well this be ok?

    1. There is a 5 vdc regulator on the back of my raduino board on my ubitx v6. I think that it should be 5v somewhere there on the board.

  9. Hi Ian...I am on a v5 board running 1.2 and a Nextion 2.8” display. It’s working well. I built this circuit and added it, but I see no difference in S-meter action. Is there anything I need to do in software/configuration to make it aware that this has been added? Or is it more likely a problem with my build?
    -Tim WT1IM

    1. I Have the same problem with v6 board, I check with the oscillo, I have the signal on A7 than change with the reception level, but nothing append on the S-meter display. Any idea where is the error ?

  10. I am an enthusiastic in ham radio and very interested in bitx and ubitx. Thanks for providing such wonderful projects and software for ubitx.These are very helpful to us


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