Back to basics with this photodiode 2-transistor LED light circuit activated by darkness. How does it work? Can it be improved? Are there any problems with the circuit design?
By Julian
Youtuber, shed dweller, solar charge controller aficionado
Not showing the resistor in series with the LED on the circuit diagram is also pretty naughty. Any poor sap trying to learn from this kit is going to get the idea that you can connect an LED straight across a battery and everything will be fine…
"Would you have large current through the NPN's C-E path?" My assumption would be no because of the PNP's E-B limiting it. That (E-B) junction wouldn't be saturated. Thoughts? Would be nice to see an ammeter as others have suggested 🙂
You are a child. I do not buy this kind of "electronic lessons" even to my son! We do it together, so he understands better, how to design. And..actually.. do you know, how to create a photo-resistor from a transistor?
Julian, your explanation of how the current is limited to a safe value is slightly off. The NPN transistor is not saturated, it is fully active and has a high gain. The gain will typically not be greater than say 100 and its base current is a maximum of 60uA (not 600uA) and so its collector current will not be more than 6mA. So the circuit is inherently safe if built as shown. But you're quite right it would be good design practice to add a base resistor to limit the current especially in this situation where the user may change the 100k. The saturated gain of a transistor refers to the condition where the collector voltage has fallen near to zero because all the supply voltage has been dropped across the collector resistor. In order to get a very low collector voltage it's necessary to provide more base current than would be predicted by dividing the collector current by the gain. This is called forced gain and a typical value of 10 would be used. In this circuit the transistor will never saturate because there is no collector resistor and there's always about 5.5V across it.
that was great ! please sir may have more?
Does it have to be a NPN and PNP or can it be done with two NPN's or two PNP's???
Base of the NPN transistor will see 54 microamps max (if the supply is 6v), multiplying that by the gain of general purpose transistor, optimistically 200 you will have less than 11milliamps flowing through the base of the PNP.
Nice, slowly explained for dummies. I guess this is quite educational. Slowly explaining is really your strength.
i had so much fun with these kits as a kid in the 70's & 80's. wish I'd stuck with the profession.
Am I the only one who noticed the operation of the cct inverted, without explanation, during the messing around?
Would have been interesting to add an ammeter to the circuit to see what current is flowing. Could also work out the transistor gains.
I like the back to basics videos. Filling in the gaps in my electronics knowledge. Makes me wish I still had my Tandy 200 in 1 electronics set from when I was a child!
As an electronics design engineer, I find myself getting impatient and seeing everything you get wrong Julian before you realise it. But now I see the real value in what you do here: your line of reasoning is clear and verbalised throughout, and as you fix your oversimplifications, and come to the right answer, we see the process of reasoning these issues through. That way, we get to learn how to be better at investigating and solving problems, which are at the very heart of engineering. I have long underestimated you in my arrogance, and have missed the real treasures you bring to your audience. And your methodical and relentless approach is a lesson the most seasoned of us old hacks. Well done, and keep up the most excellent work!
I've got the exact same version of the book. The math is way above me but it teaches a lot