Electronics > Beginners
LED control circuit, would appreciate feedback and suggestions.
Zero999:
--- Quote from: doublec4 on June 14, 2019, 05:10:01 pm ---
--- Quote from: Zero999 on June 14, 2019, 02:19:25 pm ---The smaller transistors are probably used as a MOSFET driver.
PWMing the entire circuit will not work because it the MCU will have to restart every cycle. The LEDs will most likely be configured common anode. If you can disconnect the common anode connection for the LEDs and PWM that, it will probably work.
--- End quote ---
That was my thinking but... the brand of lights is Whelen if you guys didn't see the little label in the picture I posted. They have a programmable flashing module that apparently we just so happen to have here at my work... I was just playing around with it and it can dim the lights with only the power and gnd connected... I started prodding the power/GND connections with a Fluke 177 true RMS meter.
My power supply is putting out 12VDC to the programming module... The readings I got at the output for the LED module for each brightness level are as follows:
100% brightness --> 12VDC --> N/A VAC --> 200Hz AC --> 0Hz DC
70% brightness --> 9.6VDC --> 3.8VAC --> 200Hz AC --> 0HZ DC
50% brightness --> 7.5VDC --> 4.75VAC --> 200Hz AC --> 0HZ DC
20% brightness --> 4.12VDC --> 4.33VAC --> 200Hz AC --> 0HZ DC
0% brightness --> 2.7VDC --> 3.45VAC --> 182Hz AC --> 0HZ DC
I set the meter to both AC and DC to see how the readings would differ. I'm wasn't sure if the AC setting would respond different to a PWM signal or not. I'm not experienced in taking these sorts of measurements but I did read that using the AC setting to check DC voltage would not damage the meter.
Now where I am confused is that if I remove the module from the programmer and just wire it directly to my power supply and adjust the DC voltage to lets say 4.12VDC as was read for the 20% brightness... the LEDs do not come on. They turn off around 7VDC direct from the power supply... So when I took the above measurements, am I reading some kind of average 'ON' time from a PWM signal perhaps?
And since I have the programmable module, before anyone suggests just using that... its quite large. Too large to fit into a portable device (it's meant for flashing/programming vehicle lights). I'll see if someone in my building has a scope to see if the programmer is outputting a PWM signal then?
--- End quote ---
That's what you should expect to happen. 20% brightness will be 12V pulses with a duty cycle of 20%. The string of LEDs probably have a turn on voltage threshold of around 7V, so will not turn on, until the voltage exceeds that threshold.
The meter is not measuring the peak voltage of the signal, but the DC component, i.e. the voltage you'd get if you averaged the signal.
doublec4:
Got it on the scope ... see attached. So yes, with 12V supply and the programmer set to 40% brightness, there is a 200Hz frequency, 5ms period... 40% on time
So now I have to revisit the 555 timer circuit to do this
My last little bit of curiosity is this... when I first power on the LED module (no PWM), my power supply reads a higher current draw and then after about 5 seconds of being on, the LED modules settle at about 1/2 the peak current of when it was first turned on... Now when I PWM them at a lower brightness (ex: 40%), the current draw is still slightly higher than the "settled" current when I just directly connect the 12V supply to the module. I thought PWM should reduce current consumption since there is a considerable "off time?"
doublec4:
For those still following...
I ended up with an acceptable dimmed state by using this calculator/circuit:
https://houseofjeff.com/555-timer-oscillator-frequency-calculator/
Put in my required 200Hz frequency... ended up with the following values because I had them on hand to breadboard
C1:0.01 µF R1:56 KΩ R2:330 KΩ Period:0.005 sec. f:201.5363 Hz. Duty Cycle: 53.9 %
Was still kind of bright... found this info here:
https://electronicsclub.info/555astable.htm
Put a diode in parallel with R2 and the dim state looks great. Looks like the frequency is somewhere around 300Hz + (have to scope it again) but it works...
Thanks to all who helped!
Zero999:
--- Quote from: Jwillis on June 14, 2019, 08:33:14 pm ---
--- Quote from: Audioguru on June 05, 2019, 07:25:09 pm ---Which circuit is correct?
Do these circuits both work?
--- End quote ---
Both those circuits will burn out the LED . A limiter resistor is still required for the LED to keep it below its maximum current tolerance.
--- End quote ---
Well no, because the LED module mentioned in the original post, already has a built-in current limiting resistor.
With bare LEDs an additional series resistor would be required, but I hoped the original poster would have been aware of that, even though this is the beginners section.
--- Quote from: doublec4 on June 05, 2019, 05:52:31 pm ---
--- Quote from: Zero999 on May 23, 2019, 10:20:01 am ---A simple 555 timer circuit will do the job. You could use a potentiometer with a switch in series with the whole circuit to make sure it turns fully off, drawing no power.
--- End quote ---
Also, I tried this circuit for fun with some regular 5mm LEDs that I had on hand... I could only get them to very faintly light and then ever so slightly dim. It seemed as if the adjustment range of the pot was extremely narrow.
--- End quote ---
I've built that circuit before and it works perfectly. There are two things which I can think of that will cause this: the polarity of one of the diodes in incorrect or the potentiometer is bad.
The frequency is fixed and depends on the RC time constant.
Assuming the potentiometer has a much higher resistor compared to the 1k connected to pin 7.
F = 1.44/(RC)
--- Quote from: doublec4 on June 14, 2019, 08:12:50 pm ---For those still following...
I ended up with an acceptable dimmed state by using this calculator/circuit:
https://houseofjeff.com/555-timer-oscillator-frequency-calculator/
Put in my required 200Hz frequency... ended up with the following values because I had them on hand to breadboard
C1:0.01 µF R1:56 KΩ R2:330 KΩ Period:0.005 sec. f:201.5363 Hz. Duty Cycle: 53.9 %
Was still kind of bright... found this info here:
https://electronicsclub.info/555astable.htm
Put a diode in parallel with R2 and the dim state looks great. Looks like the frequency is somewhere around 300Hz + (have to scope it again) but it works...
Thanks to all who helped!
--- End quote ---
That circuit is very similar to the one I posted, except it has a fixed duty cycle and if that's acceptable, then good.
doublec4:
Thanks Zero999
I was going to use your circuit to find the "sweet spot" with the pot and then just measure the resistance once I had a suitable duty cycle.
I think maybe the module is somewhat sensitive to the frequency. I'm not sure how to calculate the frequency in your schematic , but perhaps it was too high for the module.
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