Low voltage indicator working in the range of 13 Volt up to 28 Volt. It can also indicate a tiny current change in a DC circuit with a certain DC load. Please read the textbox, especially the final lines (UPDATE 20 december 2020).
You can set the precise voltage where the indicator works (red or green LED flips “on/off”) in a 0,3 Volt precise range.
Important: in this setup the indicator circuit needs a “series resistor” in series with a “load” (could be an electronic circuit or whatever device that needs a certain current and voltage and has to “watched over”).
Because the indicator is supplied out of the voltage drop that “falls” parallel to the first series resistor (in this demo 100 Ohm) it also means that the maximum current with which you want to supply a circuit is limited by that first resistor in the row + the fact that such a first resistor can get warm or hot. That means energy loss (….) but for circuits up to (say) 200 mA the circuit is in my opinion usable.
For people that like to do experiments: you can also hook up the indicator nót with a resistor in series to a “load” (electronic circuit) but parallel to a voltage source (watch schematic 1 in the video). Please note that you can need a protective series resistor (that also will create a voltage drop…) to prevent that the indicator circuit burns out. Start e.g. with a 3K3 resistor on voltages of maximum 28 Volt, slowly test and try series resistors of smaller values (they may not get hot) till the indicator circuit works properly. Watch UPDATE final lines of this textbox.
Advice: for serious applications use a 10 K cermet potentiometer to align the switch-voltage where it has to work. Also important: zener diodes are temperature sensitive, so in cold situations you need (perhaps) another alignment.
And with other zener diode values you will find somewhat other results. The zener diode values that I used give more or less the maximum voltage bandwidth in the showed application (13-28 V), where the circuit is supplied out of the voltage drop of the first resistor in the row. In a parallel application of the indicator circuit there are perhaps more possibilities with other zener diode values, perhaps you can get it to work on 7-8 Volt or so.
Next video (watchover a 24 V battery schematic) is here https://youtu.be/YUWUbX5LgdQ
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Regarding all my video’s: I constantly keep them actual, so the original video’s with the most recent information are always on YouTube. That is the source, and search there. When my video’s are reproduced or re-edited on other websites/channels you can not (!) be sure about the original content (=really working electronics with real properties for a purpose) and important adaptations to the circuits.
Be aware of that, I saw on the internet my circuits reproduced in a poor or not proper way.
I also found that people probably republish my circuits under phantasy names and/or with phantasy properties, attributing electronic properties to them that they were never made for, so phantasy properties. Sometimes they want to find gold with them. I take distance from all these fake claims; I cannot help that it happens, sorry. Upload 20 december 2020.
UPDATE: when you want to use the circuit to watch over a 12 V battery or a 24 V battery: connect a 1 K resistor to the + (and solder the "100 Ohm" parallel resistor out!). Connect the indicator with that 1 K series resistor connected between + and - of the battery. For 12 V: solder the 3,9 V zener in, for 24 V: solder the 6.2V zener in. Set the exact indicator voltage ("low") with the 10 K trimmer. It works now between battery voltages of 10 V and 28 V within a 0,3 V band.