help with a custom dimmer

[dacflyer]

i just got finished with my GPS clock for the most part, still need to get a case made for it now.
anyway i want to make a custom dimmer for it, using a foto transistor or a cds cell ( fotocell )

it will need to operate off of 12v.
here is the features i would want.
adjustable daytime dimming... ( how bright i want it during the day )
adjustable night dimming , and an adjustment for when it should trigger from day to night dimming.

i have tried to think how to go about this, and any tips would be helpful.

this can use more than 1 sensor if needed, but if possible to use just 1 sensor..
i'd like to try to do this with hardware if possible rather than software.

[haklesup]

The CDS should be easier to design around since it acts more like a variable resistor it can be used to control bias voltage to a VCO that drives the PWM circuit. Use it as one leg of a voltage divider

A photo transistor might be more critical for bias currents, I haven't really used one before, I think they work by substituting Light for the base current but otherwise need the same design practices as any transistor.

https://www.google.com/search?q=light+s ... iAod4V0M4A

[sghioto]

Here's one of those 555 circuits modified using a CDS. This circuit is continuously adjusting the brightness of the LED depending on the ambient light level. The pots will allow you to set the initial brightness. I have not tried this circuit but looks plausible.

Steve G

[dacflyer]

i needed something with a lil bit of hysteresis.
so here is what i did so far, needs some tweaking.

i am using a 741 OP amp and a cds cell. this part is working fine for a straight on/off function of a led.
the output is feeding a small transistor then to the led and a relay..
the relay is a SPDT on the contacts side the common is fed to the adjust. side of a LM317T.
and the NC and NO contacts are connected to 2 trimmer pots ( these will adjust the day brightness and night brightness. ) and they are connected to GND.

so when the cds cell gets light, the relay is closed and my led display will be bright
(adjustable brightness ) and when it gets dark, the display will dim ( also adjustable )
the switch over sensitivity is also adjustable. and the hysteresis will keep it from flipping back and forth
with small light changes.
but what i need to do now is increase the current handling of the LM317T.
i have seen some power supply schematics where i can add in a bypass transistor to handle more current.. if i get this done, then i'll be set.

i had to try and draw out all i wanted to do 1st, then breadboard it section by section..
sure it could have been built into the clock programming, but i am not good with that, and i wanted to keep it simple as well..

i'll post my results later on....
any thoughts or tips ?

[sghioto]

The LM317T is rated at 1.5 amps. How much current do you need to drive the display?

Steve G

[dacflyer]

i need at least 2 - 2.5 amps.. the 1.5 itself is loading the heat sink too much.
i used to have a larger version of the lm317, but i can't find it anywhere.. i think it was rated at 3 or 5 amps. can't remember.

[dyarker]

Then you're going to need a heat sink with at three times the surface area of the current one as well.

[dacflyer]

i could use a 2N3055, but that would be over kill.. was looking for something in a T-220 package like the LM317T

[CeaSaR]

TIP31, max Ic = 3A.
TIP41, max Ic = 6a.

TO220 package.

Use these for drivers.

CeaSaR

[CeaSaR]

From your description, I see 2 things in my mind:
RC pwm control method and NPN/PNP operation.

Lemme 'splain my convoluted thoughts.

The way RC control works for a servo is that a certain pwm frequency corresponds to center (our reference 0) while above turns the servo in one direction proportionally while below turns the servo proportionally the other way. Since you want to be all hardware, this would be something you could sum out with a capacitor to get an analog level for the pwm. This could possibly run into an integrator to get it to a + - level that you could send to the second circuit.

This above would be the Day/Night section with proportional level of light/dark.

Second circuit would need to be fed with a positive /negative center ground supply (capacitor split?) to feed the NPN/PNP control. NPN would control the high side while the PNP would control the low side. Here you would have your individual controls for max bright etc. And the sweet thing is that the single CDS cell would do most of the main control.

I don't have any schematics, but hopefully this will inspire someone with more acumen than I to help out. Either that or it will inspire someone to tell me I'm full of it.

Hopefully I helped get the gears spinning.

CeaSaR

***Edit

I don't mean that you should use an R/C controller to decide between light/dark, rather a 555 type timer where the CDS cell is part of a voltage divider sent to the frequency or duty cycle pin and the light level controls the output accordingly.

Hope I didn't cause any confusion.

[dyarker]

i could use a 2N3055, but that would be over kill.. was looking for something in a T-220 package like the LM317T

An LM317 is voltage regulator integrated circuit, while the 2N3055 is an NPN power transistor. Not directly swappable for each other.

In either case the power that needs to be disapated by the heatsink is the voltage dropped across the device by the current through the device.

FOR EXAMPLE the LM317 is set to output 5V. The supply is 12V. That leaves 7V ACROSS THE DEVICE.

You said the heatsink was hot with 1.5A through the LM317. That is 7V times 1.5A equals 10.5Watts. That must be too much power for the size of heatsink you are using. And that is why I said 3 times the surface area for 2.5A load. To handle the additional power and be a little cooler.

Good luck with this.

[sghioto]

He means using the 2N3055 as a pass xistor to handle the additional current needed. Another option is to use a resistor between the 12 volt supply and the input of the regulator to carry some of the power. A 2 ohm resistor at 2.5 amps will drop 5 volts leaving only 2 volts across the regulator at 5 watts. A couple of 1 ohm 10 watt resistors in series might be sufficient if you have the space and ventilation.

Steve G

[dacflyer]

yes i was planning to use the 2N3055 as a pass transistor.. have not had time to play with it this weekend, i was on call all weekend and had too much work to do.. 14hrs OT this weekend


Ceasar >> neat idea, but i already have most of this circuit bread boarded, and i am not willing to start on a whole new circuit as of yet.. but i know what your talking about
had someone tell me i could not use a regulator for what i wanna do,, but he does not think like i do..
he likes to make everything a microprocessor circuit,, i like off the shelf stuff. or out of the junk box

[CeaSaR]

Dac,

I saw you had it most of the way done, but I had to throw that out there, JIC. Besides, you never know when something like that could come in handy.

Anyway, you said you wanted a TO-220 package and as I previously mentioned, a TIP41 should work wonders.

Hope we can see a photo or movie once it is fully finished.

CeaSaR

[dacflyer]

i'll see what i can do.. might be still on a bread board til i get it mounted more permanently.
i just got a gopro camera, but i am having a hard time to play MP4 files,, tried to view it online and with VLC player, windows player..i can't get it to play with out buffering or worse... usually VLC player plays anything..
will try it again with another video,, possibly my video was corrupt ?