# Flashing LED Sign

Discussion in 'The Projects Forum' started by Venus99, Jan 17, 2008.

1. ### Venus99 Thread Starter New Member

Apr 18, 2007
7
0
Hello All,

I have a little experience with electronics but physics is my background. I am trying to design a Flashing LED sign. The sign has 14 parallel paths of strings of series LED's with resistors to account for the fact that some strings have 10LED's some have 2. In total there are 96 LED's and I am aiming to run this off of 25VDC.

Question:
I wanted the sign/array to flash on and off. I don't want it to be sequential just on and off. Is there any way to do that easily? What about 555 IC. Would the 555 be overloaded by the 25VDC?

I have access to equipment to etch boards and breadboard stuff so any suggestions would be greatly appreciated.

Thanks

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
You are correct that the 555 would be overloaded. The 555 would be happier with a 12 volt supply, and could flash your sign if it drove a FET. What is the total current for the sign?

3. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
13
Use a 555 timer to drive a relay that supplies power to the LEDs.

Add a small regulator to knock the +25V down to +12V for the timer and relay.

4. ### Audioguru Expert

Dec 20, 2007
11,231
1,334
A power transistor or power Mosfet is a relay that doesn't wear out.

5. ### Venus99 Thread Starter New Member

Apr 18, 2007
7
0
Thanks for the help so far:

Assuming each of the 14 paths of LED's draws ~20mA... 14x20mA = 280mA So the total current is ~0.3A on the main line when all the paths meet ( unless I am thinking about this wrong).

If I were to set the 555 in monostable ( so it pulses continuosly) setup how do I incorporate the relay to deliver the needed 25V. Does the 555 pulses trigger the relay on and off? If so what type of relay do you suggest. I have a realy that SRD-S-105D(not really sure what that means) rated to 7A and 28V DC 250VAC. With 5 pins on the bottom, would that do the trick?

Also If I need the "timing part" of the circuit at 5-15V (which is what I think 555's are capable of handling) Do you just put the timer part in parallel with the array part and then have a resistor before the timing circuit to dissipate the ~extra voltage? Because the two paths are in parallel?

OR
I could re route all the LED's to make more paths and hence bring down needed supply voltage but up supply current so that it falls into the 555 acceptabe range 5-15V?

Ideally, I want the simplest way to do this possible.
Thanks for the help so far...

6. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295

Before trying the relay, consider a transistor to switch current in the sign. Any NPN transistor rated above 30 volts and capable of 500 ma will do. A darlington type is even better. Put the emitter to circuit ground and the collector will handle the LED's - provided that the LED's are common anode. With the 555 running on 12 volts, use a 2000 ohm resistor between the 555 pin 3 and the transistor base.

7. ### Venus99 Thread Starter New Member

Apr 18, 2007
7
0
Okay,

After thinking for a bit and experimenting with a bread board I got a monostable set up to drive up to 6 LED in series using 12V DC after that they get dim.

From what I understand the pin 3 timer output will drive the FET which acts like a relay allowing current to flow to the LED's frequency depending on the resistances and capacitances in the monostable setup. I am going to change my LED routing so that they run on 12VDC.

My remaining questions are:

1) If I change the LED's to maximum 6 per path will that be enough voltage to run all 16 paths?

2) Which FET do I use? There are so many?

3) How do I set up the FET so it drives the LED array? ie ( where should the (three?) G D S FET leads go?)

4) Attached is a rough diagram of the circuit can anyone expalin what needs to be connected to what?

Thanks.

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8. ### Venus99 Thread Starter New Member

Apr 18, 2007
7
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Wow!
Beenthere...you've answerd some of my questions before I've even posted.

9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Instead of using discrete FETs or transistors or Darlingtons, consider using ULN2803 or ULN2804 IC's. These contain eight Darlington pairs, and each output can sink up to 500mA current at fairly high voltage.

If your timer circuit is CMOS, use the ULN2804; if TTL use the ULN2803.

The ULN2003/ULN2004 IC's are similar, but contain seven Darlington pairs instead.

10. ### Venus99 Thread Starter New Member

Apr 18, 2007
7
0
Thanks everyone!!
I got it to work with an MPS2222A transistor!! I know have the 555IC driving the transistor and then driving 4 paths of 6 LED's and am going to assume it works for 96.

Now I am going design my copper layout and etch.

Thanks everyone!!

11. ### kf6dxx New Member

Jul 23, 2008
8
1
Hello, I've been reading the posts about driving an array of LEDs. I'm getting ready to do something similar: I want to flash an existing array of 72 infrared LEDs that make up an IR illuminator. There are 12 strings of 6 IR leds and each string is powered through a 100 ohm resistor so I assume each LED is driven by about 20mA. The array is powered by 12 Volts. I've built several 555 timer-based astable oscillator circuits and plan to use the output from a similar circuit to trigger a TIP-41A power transistor which will (I hope) flash the array. My question is what size resistor should be used between the 555's output and the TIP-41A's base pin. Any other caveats? Sorry about the long-winded question. Thanks for your time, KF6DXX

12. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Why not use an N-ch power MOSFET like an IRFZ34 instead of a TIP41A? You won't have a voltage drop from the drain to source like you would from the emitter to the collector on the TIP41A, and the drive is pretty simple. Just connect the gate of the IRFZ34 to pin 3 of the 555 using a 60 Ohm resistor to limit the peak current. Once the gate is charged or discharged, gate current will cease. If your frequency is low, the resistor won't dissipate much power. As the frequency goes up, power dissipation in the resistor will increase.

With the TIP41A, you could likely use a 68 Ohm resistor. Your limiting factor is the 200mA maximum current output of the 555 timer. 12V/200mA= 60, but 68 Ohms gives you 176mA max for a bit of a safety margin. You'll need a 5W resistor, since the power dissipation will be 2.4W when the duty cycle is 100%.

13. ### kf6dxx New Member

Jul 23, 2008
8
1
Sgt. Wookie, Thanks for your reply which is greatly appreciated. Maybe I should have first stated that I am not an electronics engineer so any comparison of the IRFZ34 to the TIP-41A, drains and so forth, goes right over my head. I have several of the NPN version of the TIP-41As, left from a previous project attempt which is the only reason I'm trying to use that particular transistor. I hope to flash my IR array at around 30 Hz with roughly a 50% duty cycle. The circuit doesn't have to be particularly elegant or efficient... it's just something I'm going to stick out in the woods and it may get stolen or eaten? Thanks, KF6DXX

14. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
I already suggested a 68 Ohm resistor from the 555 timer's output pin 3 to the base of the TIP41A, which will allow 176mA of base current. This will allow around 3.5-4.5A Ic. Your LEDs draw a total of 240mA, so you could supply the base with less current and still drive them.

Since the TIP41 will be lightly loaded at much less than 1 Ampere, hFE will likely be better than 40. But let's supply the base with a tad more current anyway; 30mA.

12v/.03 = 400 Ohms. Closest standard value is 390 Ohms.
12v/390 = 30.8mA (rounded up)
Even with an hFE of 30, the collector can sink 924mA with 30.8mA base current - more than 3x your current requirement.

Power in the resistor is .372W, but I'd go with 390 Ohms, 1 Watt for reliability.