Trying to make a large marquee with a 3 channel LED chase. It needs to have two modes of operation; 1st is dimmed slightly with no chase effect. 2nd is full bright with chase.

Each channel powers 128 LEDs, 32 x 4. It was calculated that it would take .8 amps per channel. I'll be using a 13.8v regulated power supply.

I reviewed the LED blog and at this point would like an expert to review for errors. Please see attached jpg.

Your insight and replies are greatly appreciated. Should make for a great school prop if/when it works.

Thank you.

 

I don't like that dimming arrangement at all. With the switch in the dim position, the supply rail for the 555 and 4017 is routed through the dim pot and another parallel path through the timing resistor and pot of the 555.

Instead, how about having a switch that just sets the clock to a high frequency. The chase would run normally, but the LEDs would be at 33.3% duty cycle and appear to all be on. Keep in mind that LED on times less than 35ms will appear dimmed as well due to the oddities of human vision.

 

- If you want 25mA you will be closer with 39Ohm instead of 43.

- Use bypass capacitors for the 555 and 4017.

- Supply the 555 and 4017 directly from the main power supply , not from after the 50Ohm resistor.

- If you notice erratic behaviour of the 555 or 4017 put an electrolytic capacitor directly from the LED resistors (power supply side) to the sources of the Mosfets.

- In the circuit position A and B is not indicated.

- With the switch in the upper position the 555 stops working. Is that on purpose?

 

Please see Rev B attached.

Made changes to the resistors, marked the sw. positions and removed the ill conceived connection back to the 555.

I put in three diodes to supply the 3 IRF510's directly when in the sw. A position.

Not sure about changing or adding capacitors. Please elaborate.

Thank you.

 

Forgot to add power when in sw. pos. B.
See attached Rev. C.
Thanks!

 

With the switch at A, you'll be driving the pins of the CD4017 while its power is off. This may be OK, but I suggest checking that it doesn't cause problems.

 

When you calculate the capacitor/resistor size for the frequency out of the 555 that drives the sequencer, that is the capacitor to change for dim.

e.g. Instead of a 5Hz clock, switch in a MUCH smaller capacitor to make it a 10kHz clock. That way, the LEDs will appear to be on continually, but each one only lit 33% of the time. e.g. "dim"

A SPDT switch could be wired to choose which capacitor is in circuit, giving either dim or chase at full brightness (it's the same thing operation, dim is just chasing faster than your eye can see it, look up "PWM LED Dimming").

That would save a lot of initial layout for a power potentiometer, as well as be far more efficient due to power not being dissipated as heat in the dimming potentiometer. Variable resistors aren't the most reliable method of dimming LEDs due to the very narrow range needed to go from dim to full brightness. (This is an expanded version of the same information in post #2 by KJ6EAD)

 

If you want to use the diodes D1 D2 and D3 they would have to be directly connected to the gates and you will need to increase the gate resistors to at least 1k. This may be to high at the desired frequency.

If SW is in position B and the outputs of the 4017 are low then D4 will conduct through D1 D2 or D3 and the potentiometer. D4 needs to be in series with the potentiometer (pointing towards the LED resistors.

Connect the 555 and 4017 directly to 13.8V. Switch only the power part.

However, a better and more elegant solution is what thatoneguy suggested. Power pots aren't exactly cheap.

 

Too much to absorb at once from Post#2.

I reviewed wikipedia irt astable mode. In that section it shows how to calculate the frequency and duration. What is Ln(2) in the computation?

http://en.wikipedia.org/wiki/555_timer_IC

Looking for 1.8 hz on the chase, 100%
Then say anything above 60 hz at 33% for all on dimmed?

Then a resistor and cap needs to be swapped out?
C1 and VR1?

 

Okay on to Rev E. this swaps out to another resistor and smaller cap.
Your comments are very much appreciated. Winging it on the cap value. Could use some advice there.

 

\(ln(2)\ =\ natural\ logarithm\ of\ 2\ =\ 0.693\)

A Wikipedia article is no substitute for a component data sheet. There's a nomograph on page 8 that can help you determine correct timing component values. For the chase pattern, a 1μF or 2.2μF ceramic capacitor would be my guess.

For the fast (dim) mode, a 60Hz clock would result in each set of LEDs flashing at 20Hz so the frequency needs to be at least 180Hz. There's no need to have both a switched timing resistor and a switched timing capacitor, just the latter. There are also numerous online 555 calculators but I recommend working the math yourself; it's somehow more satisfying, and educational.

http://www.ti.com/lit/ds/symlink/ne555.pdf

 

Thank you! I am getting closer?

At 180hz, if 99% duty cycle then each channel would be on for 33% of the time at 60 cycle and then appear 1/3 as bright?

OR does the the duty cycle need to be lowered as well?

 

LEDs on 33% of the time will not appear 33% as bright. You can't easily adjust the duty cycle with your current circuit design. You can change from 33.3% to 16.7% or 11.1%, but I don't think that would be desirable. Breadboard your circuit and observe the effects of frequency changes. Three or four LEDs on each channel should be enough to visualize it.

 

Okay, it is hooked up and is working! But I need some tweaking help.

In the slow chase mode, the Variable Resistor does not effect the pulse rate.
It is stuck at about 1 or 1.2, hz - and it needs to be closer to 2 hz. Maybe re-check wiring.

In the fast chase(all on) it is too bright and after about 10-15 seconds, it stops chasing - though I when touch the 555 or the heat sink of the 510's it starts up again.

Someone mentioned putting a capacitor in, please let me know where and what size/type.

All advice very much appreciated.

 

Checked the VR's and needed to jump the center to the side. It now has the correct speed on both.

Just need to take care of the erratic behavior. Need help with that.

 

1. Do you have bypass caps?

2. Can you try using 100k instead of 1M, 200k instead of 2M, 220pF instead of 2.2nF and 22nF instead of 220nF?

Maybe you have some leakage current on your board?

 

I found that pin 4 of the 555 (reset) didn't have +v to it. It works great now. Very stable.

Only thing left it is to dim it a little when all are on. Any easy way to get the duty cycle changed to 10% or so?

What are bypass cap? Where would they go?

 

You can change the duty cycle to 16.7% by moving the LED channels on the 4017 outputs to Q0, Q2 and Q4 and moving the reset line to Q6.

You can change the duty cycle to 11.1% by moving the LED channels on the 4017 outputs to Q0, Q3 and Q6 and moving the reset line to Q9.

Unfortunately, doing it this way also affects the low speed chase.

 

What are bypass cap? Where would they go?

http://forum.allaboutcircuits.com/showthread.php?t=45583

If it works, leave it alone.