Why is the base current much higher than the max output high current of a Cmos 555 that has a low supply voltage?
Long Duration LED Flashers
- Thread starter Wendy
- Start date
Point well taken!
Since the 555 CMOS version can sink about 10x the current that it can source, he might be better off using a PNP transistor to source more current for the NPN's base.
The LMC555 is spec'd with a 1.5V supply.
It has a minimum output high current of only 0.25mA with a max loss of 0.5V.
Its minimum output low current is 1mA with a max loss of 0.4V.
The TLC555 is spec'd with a 2.0V supply.
It has a minimum output high current of 0.3mA with a max loss of 0.5V.
Its minimum output low current is 1mA with a max loss of 0.3V.
It will work with a PNP transistor and a 3V supply that drops to 2V.
It has a minimum output high current of only 0.25mA with a max loss of 0.5V.
Its minimum output low current is 1mA with a max loss of 0.4V.
The TLC555 is spec'd with a 2.0V supply.
It has a minimum output high current of 0.3mA with a max loss of 0.5V.
Its minimum output low current is 1mA with a max loss of 0.3V.
It will work with a PNP transistor and a 3V supply that drops to 2V.
I'll grab some true 2N2222A's, I just used what was handy. The TN's are close equivalents if I'm not mistaken. These are really old parts, it is possible I'm being bitten by an oxide layer.
At this point I'm working with currents only to check the parts out for suitability, this is an experiment, not the final design. I didn't know Darlington's saturated at such a high CE voltage, so I learned something again. I think I understand the mechanism, once I had my head around it.
The transistors don't "see the voltage", only the current. I already have a setup for the experiments, so the battery was handy. I want the experiment batteries to start fresh, since I'm going to measure how long they last. I've shorted this 9V once or twice.
I now know it can source 1A, or used to be able to.
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Here are the redrawn schematics, these were done before I read the latest posts, so they aren't final. I'll be running some more experiments with the transistors before I finalize it. I'll go with 1ma for the 3V on the TLC555.
How close do you think TLC555's are to 7555's? I'll buy them from Radio Shack, but I was planning on using some 7555's to do the prototypes. I'll build both of them.
How close do you think TLC555's are to 7555's? I'll buy them from Radio Shack, but I was planning on using some 7555's to do the prototypes. I'll build both of them.
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Well, noticed something else, this flasher has a 1Hz flash rate, which means I need the cap to charge in under 1 second. My original concept (another brain fart) was 9.4 seconds for the TC of R4, R6, and C2. I need around .01 seconds for the TC. I have some Shottkey diodes I was thinking might be useful for this, just have to figure out how to connect them.
Another thought occcured, if the CMOS timer has a limit of 1ma it doesn't really matter what the resistor is, if I try to pull too much it saturates the output.
Another thought occcured, if the CMOS timer has a limit of 1ma it doesn't really matter what the resistor is, if I try to pull too much it saturates the output.
Bill, it's your project. But a quick glance at the datasheets tells me that the ICM7555 can source more than the TLC555 can.
Radio Shack stocks the TLC555, and many n00b's have quick access to a RS, but not a place that sells ICM7555 (unless they order it by mail).
With that in mind, you'd probably be more helpful to the n00b's by using the PNP/NPN arrangement something like I posted above, and Audioguru also suggested.
[eta]
Don't plan on the TLC555 to be able to source more than about 0.3mA when Vdd=3v - you're planning on 3x that much.
However, at that same Vdd, it can sink more than 1mA current, far more than required with the circuit I posted.
Better to go conservative; this will help ensure that the newbies can make it work, even if they use components that aren't quite in spec.
Yes, it's more complicated - and that may be a problem for beginners. It's all too easy to get base/emitter/collector connections swapped around, and then nothing wants to work.
At the same time, one of the first pieces of test equipment that a newbie should pick up is a DMM (digital multimeter, for the newbies) that has a transistor checker built-in. These are available for under $10 almost everywhere. Harbor Freight Tools regularly has sales on cheap meters with these built-in transistor checkers for well under $5 - and they're surprisingly accurate for the price. I pick up several when they're on sale; it's cheaper to buy a whole new meter than replacement batteries or test leads. I just refridgerate the batteries until I need another meter.
Radio Shack stocks the TLC555, and many n00b's have quick access to a RS, but not a place that sells ICM7555 (unless they order it by mail).
With that in mind, you'd probably be more helpful to the n00b's by using the PNP/NPN arrangement something like I posted above, and Audioguru also suggested.
[eta]
Don't plan on the TLC555 to be able to source more than about 0.3mA when Vdd=3v - you're planning on 3x that much.
However, at that same Vdd, it can sink more than 1mA current, far more than required with the circuit I posted.
Better to go conservative; this will help ensure that the newbies can make it work, even if they use components that aren't quite in spec.
Yes, it's more complicated - and that may be a problem for beginners. It's all too easy to get base/emitter/collector connections swapped around, and then nothing wants to work.
At the same time, one of the first pieces of test equipment that a newbie should pick up is a DMM (digital multimeter, for the newbies) that has a transistor checker built-in. These are available for under $10 almost everywhere. Harbor Freight Tools regularly has sales on cheap meters with these built-in transistor checkers for well under $5 - and they're surprisingly accurate for the price. I pick up several when they're on sale; it's cheaper to buy a whole new meter than replacement batteries or test leads. I just refridgerate the batteries until I need another meter.
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OK, you talked me into it. TLC555 it is.
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I may have figured out a work around for the other problem, I'll post it shortly. I was about to leave for work during the previous post. I really want to keep component counts down where and if I can, so I'll be trying several approaches.
Thanks for the help guys.
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Another experiment to try when I get a chance...
#1
Vcc = 8VDC
R1 = target 0.3 ma, 27KΩ .. TP1 =
R2 = target 30 ma, 220Ω ... TP2 =
Q1,2 = 2N2222A ............... TP3 =
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I may have figured out a work around for the other problem, I'll post it shortly. I was about to leave for work during the previous post. I really want to keep component counts down where and if I can, so I'll be trying several approaches.
Thanks for the help guys.
*********************
Another experiment to try when I get a chance...
#1
Vcc = 8VDC
R1 = target 0.3 ma, 27KΩ .. TP1 =
R2 = target 30 ma, 220Ω ... TP2 =
Q1,2 = 2N2222A ............... TP3 =
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Looking at it, this layout will light up the LED when the output of the TLC555 is low, or am I missing something?
I need the LED lite when the output is high.
This might do it for the red LED (the resistor would be adjusted to 20Ω), but I still need to figure out a circuit for the blue.
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Thinking about it, wouldn't this do the same thing, but without the invert?
Hmmm, this looks pretty familiar somehow.
I need the LED lite when the output is high.
This might do it for the red LED (the resistor would be adjusted to 20Ω), but I still need to figure out a circuit for the blue.
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Thinking about it, wouldn't this do the same thing, but without the invert?
Hmmm, this looks pretty familiar somehow.
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Opps. So how about this?
I don't quite like the switching, but the BE voltage should cancel between Q1 and Q2.
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Just occured to me, I'm overcomplicating this, this gets the current down from the source nicely...
For a β of 50 to 300 the input current works out to 22μa to 3.7μa, if the component is really substandard and has a β of 20 then the input current is 55μa, which is good enough.
I don't quite like the switching, but the BE voltage should cancel between Q1 and Q2.
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Just occured to me, I'm overcomplicating this, this gets the current down from the source nicely...
For a β of 50 to 300 the input current works out to 22μa to 3.7μa, if the component is really substandard and has a β of 20 then the input current is 55μa, which is good enough.
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Hi Bill,
The 555 output is normally high during timing capacitor charging up and goes low for a capacitor discharge cycle.
Therefore the output high time is always longer than low time. In your circuit, the LED will be driven ON for a long time(2 seconds) and flashes OFF for a short time, in your case 32ms.
Is this what you have really wanted?
The 555 output is normally high during timing capacitor charging up and goes low for a capacitor discharge cycle.
Therefore the output high time is always longer than low time. In your circuit, the LED will be driven ON for a long time(2 seconds) and flashes OFF for a short time, in your case 32ms.
Is this what you have really wanted?
Nope. Fortunately I've gone through every permutation of the driver, so redrawing it will be a snap.
This design of the 555 oscillator was already taken in the ACC book, so this was a good alternate. One by one I'm getting these designs down.
Thanks.
This design of the 555 oscillator was already taken in the ACC book, so this was a good alternate. One by one I'm getting these designs down.
Thanks.
SgtWookie, I'm sorry I ever doubted you.
Red LED Flasher
Blue LED Flasher with Schottkey Diode
Blue LED Flasher with alternate diode
I'd dearly love to figure out how to get rid of R4 and replace it with a transistor. It offends me that sucker is using juice during the flash cycle. If need be I'll lengthen the duration of the flash, but first things first. I'll probably build the transistor sections to see how well they work.
Red LED Flasher
Blue LED Flasher with Schottkey Diode
Blue LED Flasher with alternate diode
I'd dearly love to figure out how to get rid of R4 and replace it with a transistor. It offends me that sucker is using juice during the flash cycle. If need be I'll lengthen the duration of the flash, but first things first. I'll probably build the transistor sections to see how well they work.
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I'm going to prototype using Wookie's driver to see if it works. It should have a 1ma current out the collector as set by the emitter resistor. Assuming Q1 really sucked and had a beta of 10 it would still only draw .1ma from the base, and 1ma should turn Q2 on pretty well. My current plan is to build the first 2 designs using TLC555s, then use my stock of 7555 for the rest of the experiments.
I bought a 15 pack of PN2222 and another of 2N3904 (I believe that was the P/N) to stick with the Radio Shack parts line. The 1µF 250V cap was huge, so I'll try the tandilidium cap for the prototype too.
I bought a 15 pack of PN2222 and another of 2N3904 (I believe that was the P/N) to stick with the Radio Shack parts line. The 1µF 250V cap was huge, so I'll try the tandilidium cap for the prototype too.
