Bogdan - someone mentioned "digital pot" here a month or so ago and I found a design guide by Maxim (Dallas Semiconductor) on the web. I am left with the impression that a digital pot behaves as if it were the traditional pot except that the adjustment is done electronically and the resistance changes in discrete steps (though the increment is very small). The inputs on one type appear to be such that your up/down can be accomplished as you describe - in fact I think these were developed with that in mind for consumer electronics.
The power handling and range might be such that you could adjust a PWM circuit built from a 555 or similar common IC. Send me a message if you can't find the site and I'll send it to you.
An alternative - on the current version of the LM317 applications datasheet is a circuit that describes a "programmable" voltage regulator where multiple transistors are used to select a resistor that when selected or turned on, gives the desired voltage output. The more steps the finer the control of the light. You'd have to come up with the circuitry to select the right level. This is a bit more crude but if you've got the parts laying around it can be cheap. You'd have to wrap a power transistor around the LM317 in order to handle the power.
There is probably a slick way to do this with a PIC since some have PWM outputs but I haven't enough experience with them yet to offer much help.
light dimmer
- 19th August 2003 10:46 AM #1bogdanfirst
light dimmer
i need some special type of light dimmer.
i want to control some 12V lights, max. 4A(all lights). the dimmer will be controlled by 2 buttons. one will increase the light intensity, and one will descrease. also, it must have the ability to be controlled by some digital signal, 2 bits. one for increasing and one for discreasing the light intensity.
i thought of a 4029 counted and a PWM with a 555. but i don't quite know how to connect them together....
anybody can help me?
- 19th August 2003 11:31 AM #2stevez 0
- 19th August 2003 04:32 PM #3bogdanfirst
i thought of using the pot.......but i loose a lot of power on it....low efficency. same on the LM317.
i want to use a PWM to control the light because it is more efficient. i can find a MOSFET wich can handle 4 amps as i need with no problem. using the LM317 with another transistor would mean quite a power loss.
the thing is that i need a 555 to be controlled by a voltage level.
sort of get a specific output from the timer, and the duty cicle modifies acording to that from something like 0-90% or something like that.0
- 19th August 2003 05:21 PM #4stevez
Good point on the power loss. I don't think the digital pot consumes much power - it appears they don't handle much power either but enough to control things in many applications.
I ran across the datasheet for a MIC502 which was designed for thermal management. Someone mentioned it here a while ago. The output is a PWM signal - from zero to 100% duty cycle. Input 1 is a thermistor but Input 2 can be a voltage input (therefore the digital pot, I think). Either input can be used to control the PWM output. The output is taken directly to the base of an appropriate power transistor. This might work for your lighting situation however the frequency that I noted is 30 Hz that might be noticed as a flicker. The digital pot might be expensive - or it might not.
I think your approach of putting a 555 to work in a simple configuration makes good sense - I just don't know how to offer you specific help. I've seen hundreds of 555 schematics - many that will do just what you describe. I just don't have one handy.0
- 19th August 2003 07:20 PM #5bogdanfirst
ok
so i will stick to the idea with the 555
the circuit is powered from 12V
so i need to create a 555 cirrcuit as a PWM and control the duty cicle with a voltage. vary it from 0% to more than 90%(i know i can't get 100% from 555)
if you can find one of those schematics that you say you have...0
- 20th August 2003 04:46 PM #6tdb
The 555 isn't the best IC to use if you want that range of duty cycles. There are many circuits that use op-amps. The typical method is to create a triangle wave and then use a comparator to determine where on the slope of the triangle the cut-off voltage is. Check this article out:
http://www.homepower.com/files/pwmhp75.pdf
I can think of two ways to automatically control the voltage into the comparator: using a digital pot or using an ADC. If you go with a digital pot, check out the DS1804 from Maxim (formerly Dallas Semi). It's an incrementing/decrementing digital pot. If you go with an ADC, use the counter you talked about and go with an ADC that has a parallel interface (i.e. 8-bit). You can get either of these components fairly cheaply.0
- 21st August 2003 04:07 PM #7rookie
Re: light dimmer
try this http://www.kitsrus.com/kits.html#k67 just change the motor with a bulb or light.try add some mosfet if you are using high load.
Originally Posted by bogdanfirst 0
- 21st August 2003 06:44 PM #8bogdanfirst
thanks dtb!!!!
it is just what i need. i have all the parts for it exept the MOSFET. just one thing.....
do i need a regulated 12V suply? just don;t want to use a 78l12. because i don't have it and it is preety hard to find it.
maybe a non stabilized power suply is good for what i need.0
- 22nd August 2003 04:44 PM #9tdb
You don't need a regulated 12V supply. The 78L12 regulator is used if you have a 24V supply. In your case, you can omit it from the circuit. Incidentally, a 78L12 is a very common part so I'm surprised that it's not as easily accessible, even in Romania. Have you ever tried going through Digikey for parts (www.digikey.com)? I'm not sure how they handle international orders, but it may be worth some investigation. Also, most power switching MOSFETs will work in this application, as long as it can handle at least 4A (or 6A to be safe).
And I may be wrong, as Rookie pointed out, about a 555 not being able get the 0-90% duty cycle range you wanted. One thing I'm not sure about is whether you can get 0% out of the circuit shown. The lowest the CV pin will get is 1.56V (based on 12V supply) when the pot is turned all the way down. I'm not sure, just asking.0
- 22nd August 2003 06:21 PM #10bogdanfirst
thanks!
this is why i thought the 12V reg is there for. the idea is that i want to use the outputs from a 4029 and with some voltage dividers to get the duty cicle i want.
about the 78L12.....well it is not the problem that it is not avgailable. the idea is that i am living in a quite small town, so the shops for electronic components are not so rich here. so i have to order the components over the internet. they usually arive in 3-5 days. it is a common component as you say. but i can't order just it as a single, though i could oreder a few more to have spare. but still...shipping will cost more than the component itself. anyway, i have a lot of 7812 so i will use one of these till i need to order more components and order the reg too.
now, the 555....yep, you are right. it cant get 0-90%
actually i needed 0-100%, but i know you cant get that. the widest range of pmw i have seen was from 20 to 90%. i need from 0.
now all i have to do is make a voltage divider and get some resistors from the 4029 to get the appropriate modulation.0
| Tags |
