555 timer IC help

NB_Nesquik

This does not deal with BasicStamp or the BS2. It is just a circuit I need some help with.

I am trying to build led can lights for stage decorations. I found a schematic online and modified it to work how I think it should.

The circuit runs on a 12v 2amp dc power supply. The attached circuit will be replicated 5 times (one for each of the colors: red, blue, green, white, and yellow). The brightness of the LEDs is dimmable by the 10K pot. All of the 5 circuits will run in parallel, sharing the same power supply. The 100k pot. adjusts the spead of the flashing. The switch between the optocoupler and the bank of LEDs flips between getting power from the optocoupler and from the 12V. The optocooupler is mirrored in the diagram so the schematic is more neat. Here are the optocouplers (http://www.taydaelectronics.com/datasheets/A-035.pdf) i am using, and the 555 timer ICs (http://www.taydaelectronics.com/datasheets/A-032.pdf).

Will this circuit work like the way I am planning? If not where are the problems? I want to make sure that the circuit will work before I buy the parts.

erco

I assume you got the 555 flasher part right, as that's easy to find online. But that 10K pot going to your 12V supply isn't the best way to dim the LEDs. It will definitely have some overall effect, but likely neither linear nor independent of the rate adjust pot since you're choking current to the timer circuit in addition to the LEDs. IMO you want an adjustable voltage regulator there (or adjustable power supply), limited to a safe max voltage for your LED/resistor combination. The frequency is fairly independent of the supply voltage. IIRC most 555 circuits will output a total max current of ~200 mA. Also they give you a minimum 50% duty cycle but you can add a diode to change that. See the excellent info on the 555/556 at http://www.kpsec.freeuk.com/555timer.htm

You'll do better using a beefy switching transistor instead of that optocoupler. Do you know how much total current your LEDs draw?

ElectricAye

Your Vdd for IC1 is connected "downstream" of your R4 pot. This means your IC1 will stop working if its voltage sinks below 4.5 volts or whatever. Is that okay for what you are trying to do?

It might help if you better explain what the overall effect of this is supposed to look like, what you are trying to achieve.

kwinn

No, the circuit will not work as posted. For one thing, IC2 will not provide anywhere near the current required by the leds. There are also problems with the 555 timer circuit. It can be made to work by replacing IC2 with a transistor capable of providing enough current to drive the leds and correcting the 555 timer circuit. Also, if you want to blink the leds and control the brightness I would suggest using a 556 (two 555's in one chip). One would control the brightness and the other the blink rate.

Post the part numbers and number of strings of the 3 colors of leds you plan to use along with the blink rate for some more detailed help.

NB_Nesquik

The first picture is a 555 timer PWM dimmer circuit, the second picture is the 555 flasher circuit I first used. They both would work seperately, but how would I connect them to eachother so the LEDs flash and are dimmable?

The LEDs I am using are 3V 20mA blue, white, and green leds, as well as 2V 20mA yellow and red leds. The white, blue, and green strands pull 9V, 20mA per strand and 9V, 320mA for all sixteen strands of each color. The red and yellow strands pull 6V, 20mA per strand and 6V, 320mA for all sixteen strands of each color. All 80 strandss of lights pull 1600mA from the 12V 2A power supply.

@ElectricAye There will be a total of five lights. Each light will have its unique color (either red, yellow, green, blue, or white). The brightness of all five of the lights should be dimmable and each of the lights should be adjustable to flash at different blinking rates. A switch in the circuit will allow the power for the LEDs to come from either the 555 timer circuits so the lights blink, or from the 12V power supply so they stay on and dont flash (I know that if the LEDs blink fast enough, then it will look like they are constantly on, like the PWM circuit. I dont know if or how to use that same idea to flash the lights, and increase the flashing to a point were they look like they are constantly on and then elliminate the need for the switch to change power sources). All five of the lights ahare the same power supply. The flasher circuit runs from a 51% to 67% duty cycle with the 100k pot.

Thanks

kwinn

The output of the "blinker" 555 goes to the reset pin of the dimmer 555. When the blinker signal is low it will hold the dimmer 555 low.

BTW you can also vary the pulse rate and brightness using the control pin. That has been done in the past to make a light organ of sorts.

Forgot to mention that for the 2V leds you can put 4 of them in series and use a 180 ohm current limiting resistor.

Mark_T

Switching lots of current fast - you'll need plenty of decoupling (say 0.1uF + 10uF close to 555, + 220uF for the transistor driving the LEDs).

The opto-coupler idea is a dead duck, they cannot switch large currents - a medium power NPN transistor or an n-channel MOSFET would be the obvious solution.

The standard 555 can output plenty of current to drive a NPN power transistor fortunately (you'll need a base-resistor), or directly drive the gate of a MOSFET. Since you have 16 strings at 20mA each thats 320mA so (not too demanding) - a non-logic-level MOSFET with Rds(on) < 0.5 ohm will be fine, an NPN switching transistor rated at 600mA or more should comfortably handle the load too (make sure its saturated when on).

Choose an operating frequency that's as low as possible without noticeably flickering, higher frequencies lead to higher switching losses. 1kHz or so is a good starting point.