Hi All, Attached pics show my working prototype for a garage light which turns on when a Human approaches.
Powered by an old car battery charged by a solar panel so I can find my torch when the mains power fails.
The mmWave Sensor is powered by a 1117b 33 smd voltage reg, it’s OT2 pin is connected to the Gate of a K2962 mosfet which switches the negative rail to a buck / boost dc-dc converter (LTC3780) which converts the battery power to 3.6V and limits the current to 0.9A AND IT WORKS !!
The 80 led light is recycled and has no specs. It has 80 leds in parallel, all with resistors. It seems to run well on 3.6V and 0.9A.
Someone might have suggestions to improve the circuit however my only wish is to eliminate the buck / boost module and use a simpler method to provide CV and CC to the light.
Success after lots of experimenting.
Regards, Stephen
Hi Stephen
That circuit looks OK. There is only one thing to be wary of. Using a low side switch in this manner means your LED neg or ground is not the same as Battery ground. This is not going to matter when you say connected as shown. Your problem will arise if you graduate to using NeoPixels or other addressable LEDs. You will have to rethink in that case as the LED ground WILL matter. But that is another scenario.
The only way to do this will be a resistor, a big one. At the moment you say you are drawing about 0.9A @ 3.3V. That would mean your 12V would be providing about 0.3A. If you use a resistor you would be dropping about 9V @ 0.9A = 10Ω 10W (approx. That means you would be burning up 10W as heat in the resistor and so wasted power.
So if you can tolerate the power waste a resistor will be OK, just remember that it will get HOT. Also insert the resistor in the positive 12V line so the Mosfet still switches to ground.
Cheers Bob
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Hi Bob,
With my first iteration of the circuit I used a mechanical relay via a 2N7000 mosfet and a BIG resistor, 5W was the biggest I had, and it was cooking! And I don’t like wasting power like that.
So then I replaced the resistor with the buck-boost module and the relay with the mosfet.
I also had some excitement with the second circuit iteration.
I unintentionally reversed the breadboard meaning the + & - rails were on the other side and the 12V feed was reversed.
When I turned on the power, the first electrolytic cap immediately started to distort FAST. Before I had time to react and turn off the power the cap exploded, louder than a cracker, fluff and shrapnel everywhere.
Luckily I always wear safety glasses in the workshop
The voltage reg was the only other casualty.
Stephen.
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Hi Stephen
Probably light up like a toaster. It pays sometimes to do the arithmetic first.
Cheers Bob
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Hi Bob,
I did some calcs and knew the resistor had to be a hefty one but 5W was the biggest I had.
Also, not having an electrical background, I’m not real hot with the arithmetic so I usually seek advice with more critical circuits.
Thanks for your input,
Stephen
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Hi Stephen
Full marks for trying anyway. You look like you learned a couple of lessons here, the hard way. Just as well learn here instead of later where the results could well be more dramatic.
Cheers Bob
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