TLC555 2 Led Flasher issue

Thread Starter

ShockBoy

Joined Oct 27, 2009
186
Wookie, Checked the LED's and swithcing them around gives the same outcome, only different colors. When L1 is the only LED in, it blinks just fine. But when L2 is added, L1 stays on and when it is its turn to blink, it indeed grows in intensity but only a bit. L2 blinks just fine. I think it might have something to do with this: (Whenever the output is low, a low-impedance path is provided between the discharge terminal (DISCH) and GND.) Thanks for the assistance!
 

SgtWookie

Joined Jul 17, 2007
22,230
OK, then it's because the TLC555 output source current is too low to raise the voltage at the junction of R3/R4 high enough to turn off LED1.

You could increase R3 and R4 to about 2.5k Ohms, and the problem should clear up - but the LEDs would be very dim, as only about 4mA current would be flowing through them when on.

Going to my version in reply #12 should allow full brightness of both LEDs.
 

ELECTRONERD

Joined May 26, 2009
1,147
Shockboy,

Instead of having a voltage divider in addition to the LEDs, I would lower the resistor values to 235Ω (half) and have them going in series with both LEDs. The LEDs should then be in parallel.

Austin
 

SgtWookie

Joined Jul 17, 2007
22,230
Shockboy,

Instead of having a voltage divider in addition to the LEDs, I would lower the resistor values to 235Ω (half) and have them going in series with both LEDs. The LEDs should then be in parallel.
Austin,
You're not understanding how the circuit is supposed to work.

The output (pin 3) of the 555 is supposed to be either high or low.
When the output is high, LED1 should be turned off and LED2 should be turned on.
When the output is low, LED1 should be turned on, and LED2 should be turned off.
The two 470 Ohm resistors are necessary to limit their respective LED's current. They are not "voltage dividers" per se; however since the TLC555 is not able to source enough current to LED2, it's acting like one.

Decreasing the resistor values to 1/2 would result in even MORE load on the TLC555, which would make the situation much worse. Besides, when the TLC555's output went low, LED1 would be subjected to too much current, and would have a very short life.

Just a quick calculation; 12.7v battery through a 7812 regulator, output of the regulator is 10.7v (minimum 2V drop across the regulator)
Let's say LED1 had a Vf of 2v @ 20mA
10.7v - 2v = 8.7v remaining. 8.7v/235 Ohms = 37mA (rounded down), which is likely 12mA over the maximum current specification for the LED. It would not last long.
 

SgtWookie

Joined Jul 17, 2007
22,230
This schematic was designed around an NE555 IC.... http://wild-bohemian.com/electronics/flasher.html << here is the explanation for the circuit and why it works with an NE555.....
I'm afraid that the originator did a rather poor job of explaining how the timing components R1, R2 and C1 work. He mentioned using a low value of R1 to get close to 50% duty cycle, but made no warning about going too low. Many newbies are not aware that the discharge pin (7) shorts the junction of R1/R2 to ground in order to discharge C1.

It would have been much better to start with a lower value for C1; say 1uF, and use larger values of resistors; say 10k for R1 and 100k for R2. If R1 is less than 100 Ohms per volt of Vcc, the 555 timer may become damaged, and power draw on the supply will become excessive.
 

ELECTRONERD

Joined May 26, 2009
1,147
Austin,
You're not understanding how the circuit is supposed to work.

The output (pin 3) of the 555 is supposed to be either high or low.
When the output is high, LED1 should be turned off and LED2 should be turned on.
When the output is low, LED1 should be turned on, and LED2 should be turned off.
The two 470 Ohm resistors are necessary to limit their respective LED's current. They are not "voltage dividers" per se; however since the TLC555 is not able to source enough current to LED2, it's acting like one.

Decreasing the resistor values to 1/2 would result in even MORE load on the TLC555, which would make the situation much worse. Besides, when the TLC555's output went low, LED1 would be subjected to too much current, and would have a very short life.

Just a quick calculation; 12.7v battery through a 7812 regulator, output of the regulator is 10.7v (minimum 2V drop across the regulator)
Let's say LED1 had a Vf of 2v @ 20mA
10.7v - 2v = 8.7v remaining. 8.7v/235 Ohms = 37mA (rounded down), which is likely 12mA over the maximum current specification for the LED. It would not last long.
Yes, sorry about that Sgt. I rushed over things.

Austin
 

Thread Starter

ShockBoy

Joined Oct 27, 2009
186
Well, there's probably something wrong with the chip. I tried Bill's circuit and no go. LED2 just blinked 2 times then out for the count LED1 was still on. Tried changing out the resistors R1 and R2, Just varied the flash rate. I'll see if I can pick up another chip and test it. Thanks All. Just a thought; Since the LED1 stays on with a spike of current where it's supposed to flash, is there a way to use that to make it work the way it's supposed to? Kinda like a workaround.
 

SgtWookie

Joined Jul 17, 2007
22,230
Well, there's probably something wrong with the chip. I tried Bill's circuit and no go. LED2 just blinked 2 times then out for the count LED1 was still on.
Bill's circuit won't work with a TLC555 because the output pin 3 will never be able to source enough current to charge C1 through R1; the lower LED & resistor just is too much of a current draw.

Tried changing out the resistors R1 and R2, Just varied the flash rate. I'll see if I can pick up another chip and test it.
Did you try the circuit I posted with the transistor and base resistor?
Thanks All. Just a thought; Since the LED1 stays on with a spike of current where it's supposed to flash, is there a way to use that to make it work the way it's supposed to? Kinda like a workaround.
That's just what the circuit I posted was; a workaround.

Did you try it?

If you have tried it, and it wasn't quite working for you, you may have to increase R5 somewhat.
 

SgtWookie

Joined Jul 17, 2007
22,230
No Sgt. I have yet to try your circuit. But will! C1815 still good for the transistor?
Did you not see my reply #17 on page 2 of this thread?

And just in case you missed a later comment, you may need to increase R5 in my schematic from 2.2k to 4.7k. Try it with 2.2k first though; it should work just fine like that.
 

Wendy

Joined Mar 24, 2008
23,415
The funny thing is, you don't need a base resistor with a CMOS timer. If you tried that with a conventional 555 the magic smoke would escape.

CMOS timers are very different in many ways to a regular model.
 

SgtWookie

Joined Jul 17, 2007
22,230
The funny thing is, you don't need a base resistor with a CMOS timer. If you tried that with a conventional 555 the magic smoke would escape.
Well, there's more than just the limitation of the TLC555's source current capabilities that I'm considering.

The pin 3 output has to go high enough to stop the current flow through LED1.

Don't forget that the BE junction of a transistor "looks" like a forward-biased diode. If R5's value is too low, LED1 will never turn off; its' current will be sunk through the be junction of the transistor.

As it is, I'm asking the TLC555's output to source 4.5mA through R5 to turn on the transistor. I think it'll work AND turn off LED1 at the same time, or at least make LED1 so dim it could be considered off.

If LED1 doesn't turn off, then R5's resistance can be upped to 4.7k, which will only require 2.1mA source current from the TLC555's output. This will also force the transistor well into saturation for the load of LED2, which should be about 16mA. He could actually go up to 6.2k and still be within the forced beta rule (Ib=Ic/10)
 

SgtWookie

Joined Jul 17, 2007
22,230
Why can't the TLC555 flash 2 LED's by itself? Just askin'.
Because at the voltage you're operating it with (about 10.7v), it can sink probably around 50mA current, which is more than enough to turn on LED1, but it can only source about 5mA current, which isn't enough to both turn off LED1 and turn on LED2.
 

SgtWookie

Joined Jul 17, 2007
22,230
Oh, P.S. Back to the original. The blinking LED stays on Twice as long as off.
That's because R1=R2.

C1 charges via R1 and R2. So roughly, Thigh = .7 x (R1 + R2) x C1
C1 discharges via R2 to pin 7 of the 555. So roughly, Tlow = .7 x R2 x C1

With the original configuration of R1, R2 and C1, the output will always be high more than 50% of the time.

R1 cannot be less than about 100 Ohms per volt of the supply, or the 555 will burn up. When C1 is discharging, pin 7 is shorting to ground. The only thing between pin 7 and Vcc is R1. If R1 is 100 Ohms per volt of Vcc, in your case Vcc is 10.7v, so 100 x 10.7= 1,070 Ohms, there will be a 10mA current through R1 when C1 is being discharged.

This is why I suggested that you decrease C1 to 1uF, put R1 to 10k, and R2 to 100k. Your on and off times would be much closer to the same; it would be 11/10 instead of 2/1.
 
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