I didn't know how to properly draw the 555 timer, so I used a switch for pin 3 (Out)
I finally figured out this circuit with two LED's, a 555 timer, two 300ohm resistors, and one NPN transistor. When (Out) is high the bottom LED will turn on, and when it's low the top LED will turn on.

I tried looking up how much current the 555 timers can take and found it was only about 100ma. I want to run 20 leds, 10 on and 10 off depending on the output of the 555 timer.

10 x 20ma = 200 ma, so I added the TIP41c NPN to hopefully help, but the TOP LED worries me because it attaches back to the 555 Timer for ground. (I think) Won't that send too much current to the 555 timer?

No other variation I've tried with one NPN transistor would create a toggle flash between the LED's. Would I need a second transistor to save the 555 timer?

Thanks !

 

You can use a single transistor. I'd use a MOSFET, a special kind of transistor that makes an excellent switch, but unless you need more than 1A or so for all your LEDs, a BJT ("normal" transistor) will be fine.

I don't quite understand your question about the TOP LED. It has a resistor to limit the current, right? The current of a single LED is no problem for the 555.

Oh, and your circuit will work better if the transistor controls the return to ground for the LEDs. The emitter should connect to ground and the collector to the "bottom" of the LEDs. You can switch the high side as you've shown, but you'd need a PNP for that.

 

You can put both leds on pin 3 like this, alter the resistors for brightness.

 

You can put both leds on pin 3 like this, alter the resistors for brightness. View attachment 117861

Right Dodgydave, that works great, but I want to increase the load and my worry was killing the 555 timer, so I ran it through a transistor. Do you have a circuit diagram for something with a draw of say 500ma?

I always get two schematics when I try to figure this problem out. The one you showed, which toggles the LED's like I want, and then one with a transistor to handled higher current, but then it no longer has the alternating toggle.

Example 1:

Example 2:


But I never find an example that combines the toggle of the first example and the higher current allowance of the second example.

 

Search for Push Pull output, using transistors. IGNORE THE RESISTOR, put the centre of the transistors to the leds with base resistors try 1k to start with.

 

Ok Dodgydave, that is a pretty cool circuit. If I use two transistors there is probably a few ways to do it. your example will work for me, thanks.

 

Note the two transistors are not identical. Maybe that was obvious to you but it's an important point that's easy to overlook.

 

Nowhere do you say what the circuit operating voltage is. If it is greater than 6 V, then you can put multiple LEDs in series rather than all of them in parallel. This will cut the total LED current proportionally. For example, with 10 parallel LEDs, 20 mA x 10 = 200 mA. With 5 parallel strings of 2 LEDs in series, 20 mA x 5 = 100 mA. Running on 9 V you could have three strings of 3 LEDs in series and one single LED, for 80 mA. The circuit in post #3 can do this without external transistors.

ak

 

View attachment 117853
I didn't know how to properly draw the 555 timer, so I used a switch for pin 3 (Out)
I finally figured out this circuit with two LED's, a 555 timer, two 300ohm resistors, and one NPN transistor. When (Out) is high the bottom LED will turn on, and when it's low the top LED will turn on.

I tried looking up how much current the 555 timers can take and found it was only about 100ma. I want to run 20 leds, 10 on and 10 off depending on the output of the 555 timer.

10 x 20ma = 200 ma, so I added the TIP41c NPN to hopefully help, but the TOP LED worries me because it attaches back to the 555 Timer for ground. (I think) Won't that send too much current to the 555 timer?

No other variation I've tried with one NPN transistor would create a toggle flash between the LED's. Would I need a second transistor to save the 555 timer?

Thanks !

From your post it appears you're wanting to run two arrays of 10 parallel LEDs, one array to turn ON when the 555's output is high and turn OFF when the 555's output is low, and the other array to turn ON when the 555's output is low and OFF when the 555's output is high.

You can achieve that, using complementary transistors, like this:


I've assumed a single 9 volt supply for the LEDs and the 555, white LEDs with a forward voltage of 3.5 volts and a forward current of about 20 milliamperes, and a separate ballast resistor for each LED. YMMV.

 

A bipolar 555 output does not swing to the positive rail, so the 3906 never completely turns off. Also, with such low base resistors there will be significant time during each edge when both transistors are on. I don't think the simulation captures accurately the cross-conduction current spike. Shouldn't it be 38 mA?

ak

 

A bipolar 555 output does not swing to the positive rail, so the 3906 never completely turns off. Also, with such low base resistors there will be significant time during each edge when both transistors are on. I don't think the simulation captures accurately the cross-conduction current spike. Shouldn't it be 38 mA?
ak

Very nice catch, but there's no 3906 in there if you're talking to me.

If you are, [talking to me] how would you fix my error?

 

View attachment 117853
I didn't know how to properly draw the 555 timer, so I used a switch for pin 3 (Out)
I finally figured out this circuit with two LED's, a 555 timer, two 300ohm resistors, and one NPN transistor. When (Out) is high the bottom LED will turn on, and when it's low the top LED will turn on.

I tried looking up how much current the 555 timers can take and found it was only about 100ma. I want to run 20 leds, 10 on and 10 off depending on the output of the 555 timer.

10 x 20ma = 200 ma, so I added the TIP41c NPN to hopefully help, but the TOP LED worries me because it attaches back to the 555 Timer for ground. (I think) Won't that send too much current to the 555 timer?

No other variation I've tried with one NPN transistor would create a toggle flash between the LED's. Would I need a second transistor to save the 555 timer?

Thanks !

For white or blue LEDs - that's about 34V, which is outside the 555 supply rails. Red LEDs would be at least the limit.

You will probably need to drive 2 separate single ended output buffers. If the current is within the 200mA limit for a bipolar 555 - you can use positive and negative grounded base output stages to handle the LED chain voltage. That will of course need an extra negative rail below GND - it would be easier to use 2 identical grounded base stages and feed one of them via an inverting transistor stage.

Grounded gate MOSFETs are easier to bias - but P-channel are harder to find if you go for the split rail option.

 

Very nice catch, but there's no 3906 in there if you're talking to me.
If you are, [talking to me] how would you fix my error?

Your 2907 - too many schematics, not enough brain...
If you change to emitter followers, the output circuit in post #3 with the LEDs connected as in post #5.
If you want to keep the saturated switches, then the 555 drives two zeners, one in series with each base, such that the sum of the zeners is greater than Vcc. That way, at no time in the 555 transition is there a conducting path through both bases. It is easier to visualize if you imagine a slow triangle wave rather than a square wave driving the output stage. Another restriction - each zener plus one Vbe is less than Vcc.

If you scale things correctly, it goes something like this:
0% to 25% Vcc - PNP is on
25% to 75% Vcc - neither transistor is on
75% to 100% Vcc - NPN is on

ak

 

Your 2907 - too many schematics, not enough brain...

Well that's certainly insulting but, considering that it was you who made the error, understandable. And, basically, if no one posts schematics all we're left with is thousands and thousands of words trying to explain La Giaconda to the blind.

If you change to emitter followers, the output circuit in post #3 with the LEDs connected as in post #5.

That's not even a complete sentence, yet it smacks of even more condescension since If that's what I wanted to do then that's what I would have done without your imprimatur.

If you want to keep the saturated switches, then the 555 drives two zeners, one in series with each base, such that the sum of the zeners is greater than Vcc. That way, at no time in the 555 transition is there a conducting path through both bases. It is easier to visualize if you imagine a slow triangle wave rather than a square wave driving the output stage. Another restriction - each zener plus one Vbe is less than Vcc.

If you scale things correctly, it goes something like this:
0% to 25% Vcc - PNP is on
25% to 75% Vcc - neither transistor is on
75% to 100% Vcc - NPN is on

It seems to me that you've missed the dead time between when the NPN is OFF and the PNP is ON, and doing it your way will result in a great deal of dead time between transitions, which isn't what the OP asked for.

But perhaps I'm mistaken.

In order to resolve the conflict can you post an LTspice schematic or circuit list supporting your position, please?

 

Well that's certainly insulting but, considering that it was you who made the error, understandable. And, basically, if no one posts schematics all we're left with is thousands and thousands of words trying to explain La Giaconda to the blind.



That's not even a complete sentence, yet it smacks of even more condescension since If that's what I wanted to do then that's what I would have done without your imprimatur.



It seems to me that you've missed the dead time between when the NPN is OFF and the PNP is ON, and doing it your way will result in a great deal of dead time between transitions, which isn't what the OP asked for.

But perhaps I'm mistaken.

In order to resolve the conflict can you post an LTspice schematic or circuit list supporting your position, please?

I like collecting schematics - but I draw the line at demanding them of people.

 

Well that's certainly insulting

Hold on there, sparky. I was talking about ***my*** brain. I am an expert on its limitations. I'm active on six fora and am tracking dozens of threads. Lotsa parts...

trying to explain La Giaconda to the blind.

When I was in school I worked as a reader for blind EE students. I literally explained color to a blind man, twice. It is challenging, but not impossible. It is far harder to explain red to a colorblind man.

That's not even a complete sentence, yet it smacks of even more condescension

No, it doesn't, and the fact that you chose to interpret it that way is telling. Again, you are completely misinterpreting the tone and intent of what I wrote. Yes, when I'm slamming out a response I drop into a terse writing style. But all of the content usually is correct and connected. And as you can see from my posting background, I am not contentious or condescending by nature. There are three specific exceptions, and you are not one of them.

It seems to me that you've missed the dead time between when the NPN is OFF and the PNP is ON, and doing it your way will result in a great deal of dead time between transitions, which isn't what the OP asked for.

Given that a 555 transitions in a us or so, that dead time is not perceivable. My point, however poorly expressed, is that I don't think there is any dead time in your circuit. With Vcc at 9 V, imagine the 555 output the moment it is at 4.5 V and calculate the two transistor base currents. Like the dead time, any dual conduction is very brief, but I think it is there.

ak

 

Hold on there, sparky. I was talking about ***my*** brain. I am an expert on its limitations. I'm active on six fora and am tracking dozens of threads. Lotsa parts...

When I was in school I worked as a reader for blind EE students. I literally explained color to a blind man, twice. It is challenging, but not impossible. It is far harder to explain red to a colorblind man.

No, it doesn't, and the fact that you chose to interpret it that way is telling. Again, you are completely misinterpreting the tone and intent of what I wrote. Yes, when I'm slamming out a response I drop into a terse writing style. But all of the content usually is correct and connected. And as you can see from my posting background, I am not contentious or condescending by nature. There are three specific exceptions, and you
are not one of them.
Given that a 555 transitions in a us or so, that dead time is not perceivable. My point, however poorly expressed, is that I don't think there is any dead time in your circuit. With Vcc at 9 V, imagine the 555 output the moment it is at 4.5 V and calculate the two transistor base currents. Like the dead time, any dual conduction is very brief, but I think it is there.
ak

Point taken, and I apologize for the unwarranted overreaction.

 

I like collecting schematics - but I draw the line at demanding them of people.

"Draw the line" ? That's punnishingly funny

I think I've not demanded anything of anyone here with respect to posting schematics, although I've requested that schematics be posted, along with wish lists, in order to better serve a client's needs.

 

Hello everyone.

I think I got a cool circuit, I'm interested to know if I messed it up. This circuit should put 100% of each toggled load on the battery and the transistor and none on the 555 timer.

Assumption 1: The switch is the output line from a 555 timer.
Assumption 2: the LED's & resistors represent a load > 200ma or higher than the maximum threshold of the 555 timer.

To me this looks like it should toggle high powered loads without hurting the 555 timer. Anyone disagree?

Also, I'm not sure if I need a zenor diode or if a regular diode would work, and I'm not sure if I would need a resistor between the 555 timer output line and the transistor base, but this toggle works on the falsad.com circuit builder. I haven't tested in reality yet.

(Previous circuits provided here would have worked for me as well, but I wanted to see if I could create the circuit using one NPN transistor instead of multiple transistors.)



Thanks !

 

I just did a test on the circuit created site, apparently the resistor on the transistor base is not neccessary, and a regular diode will work instead of a Zenner.