Please read the description/textbox first. Circuit where I used only 1 Silicon Diode (be it a 1N4005 or a 1N4007) as a “switch”, by using its/their temperature dependance in a Darlington (= Very high current DC amplifier) circuit.
Of course the FAN keeps working on (!!) on higher temperatures than 60 degrees Celsius (forgotten to tell in the video) !! It will blow constantly...till the diode has cooled down.
Thanks to Barry Blod, he inspired me to make this circuit. 30 July 2022.
IMPORTANT: there are 12 V computer FANS with an electronic chip inside, these fans are not usable in this circuit (!!). Say only the "bare" 12 Volt FANS are usable.
First video is here https://youtu.be/xUZKXFfECNA
Such a Darlington circuit can/will act (with setting via a biasing potentiometer) on extremely tiny changes in their cause/effect where “something” acts at the base of the first transistor in the Darlington.
Be it a “current effect”, a “static charge effect”, an effect “ via an antenna”, or a “voltage effect”.
Everywhere/everywhen a positive charge of whatever kind enters the base of the first transistor in the NPN Darlington (+ charge only refers to NPN, in case of a PNP Darlington, it is effected by negative charges!!!!), this has a kind of waterfall effect (in terms of current) in the collector-emitter lead of the second transistor in the Darlington.
Reason to get a kind of protection: limit the maximum base current to the first transistor via a resistor in the 1 Mega Ohms range. Even 1 K could be enough. In general for 12-18 V circuits 1 K protective resistor is enough.
A somewhat too high base current (be it in the first or the second transistor) will damage the transistors without any warning. The only thing that you will see is that the circuit does not work any longer....When 1 transistor burns out due to a too high base current.
Solution: always use a 1 K resistor to the base of the first or second transistor in the Darlington to limit destructive effects of a too high base current (regards 12 V-18 Volt transistor circuits).
These detection effects (current/charge/voltage) can be used everywhere: detection of voltages, currents, electromagnetic effects, etc. etc.
Using one of these figurations means that you can even (and thus by purpose) drive these circuits (a NPN Darlington or a PNP Darlington) with (+) or (-) charges.
That is in a kind of way showed here, in this video.
Of course: (in general) always use when you want to add AC (read: AC) signals to the base of the first transistor in a Darlington: a separation (non polar) capacitor. Thus when you want send in tiny AC signals into a Darlington, say when you want to amplify AC signals or want to act (say) a relay on the basis of AC signals. Good value: 100 N and say 10 N. Non Polar.
In more or less all these cases: consider each transistor as a current amplification source. In this case and with these transistors (2 x BC 547 b) with a Hfe-Beta of 300 x, it means that the total current amplification is 300 x 300 times, that is say “endless”.
That means that extremely small AC signals can be detected in a Darlington figuration. Or (in this case) not AC voltages, but DC voltages. Better said (in this case): tiny DC voltage/current changes inside a Silicon diode, the 1N 4005 or the 1N 4007.
This is all say old school analog technology.
Why is the 7812 Chip used & protected by a 10 Uf electrolytic capacitor, paralled with a 0.1 uF cap, both paralled with a 220 K resistors?
This is investment in low cost electric components that can make that the 7812 Chip & the whole circuit lasts "forever" (at least 20 years). Only when the elexctrolytics dry out after 20 years.
By the way: the circuit works at its best on 13 Volt – 15 Volt. On 13 Volt you could say: the 7812 will not work as a stabilizer, it needs minimum 2 Volt higher on the 7812 V chip input to make it work properly. Theory.
But and though: it protects everything for higher voltages (13 V-20 Volt), in that case more or less all cases the temperature effect will be the same (say switching the fan on-off, even when the supply voltage gets too high).
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I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 1 August 2022.