Problems with 555 simulator

Thread Starter

DK3250

Joined Jul 15, 2020
13
I'm still new and I'm still a beginner...
As an exercise I decided to build a simple timer - somewhat similar to 555.
My diagram is here:
555.jpgThe green block is my private "the 555 timer". It is expected to be in astable mode (output on/of periodically).
For the two comparisors I use a chip LM339AN.
The Flip-Flop I've made from two NOR-gates. To the left in the picture below.
Problem is that is doesn't oscillate. All I see is a permanent signal on the Q output (connected to an LED and via a 220 ohm resistor to ground).
IMG_1139.jpg
In my trouble-shooting, I have partly disassembled the circuit, disconnecting the flip-flop.
At position A and B in the diagram I measure 5V and 3.3 V - as expected.
At position C, I only measure 1 V - this is a surprise to me; I expected 5 V. Don't think this is the only problem, however.
I have also tried installing two pull-down resistors at the flip-flop input - no changes..
If I move the transistor input from one side of the flip-flop to the other, the flip-flop changes state and becomes stable in this new position.

Oh, - a final remark of frustration.
Every time I try to build something electronic - even if I just copy something directly from YouTube - I always end up in hours of troubleshooting. Things doesn't work, ever, until hours of measurements and desperation has passed.
If it is only logic (SR-latch, binary adder) it is easier - but as soon as I add a transistor or a capacitor to a circuit Mother Nature turns angry.
It reminds me of the time when I learned to program - reading a book or watching the internet, I understood everything. Trying to code 5 lines myself and I was stuck. I did learn to code - and I will learn to build electronic circuits;but, Jesus, it is difficult and time consuming.

Back to my timer - all constructive input are welcome :)
 

dl324

Joined Mar 30, 2015
11,264
The comparator outputs need pull-up resistors.

What is the part number for the NOR gates? It looks like HC02. If it's CMOS, it's unlikely that it could source enough gate to damage the transistor, but you should still use a base resistor.

Oh, - a final remark of frustration.
Every time I try to build something electronic - even if I just copy something directly from YouTube - I always end up in hours of troubleshooting. Things doesn't work, ever, until hours of measurements and desperation has passed.
There's a lot of junk on YouTube.

I saw a guy trying to replace the driver side headlight on a Ford Explorer. He took out the battery and battery tray, had to use all kinds of socket wrench extensions. All he had to do was lift up a slide (maybe 2) to remove the molded piece holding in the bulb. It's about a 5 minute job that requires no tools.

He probably thought he'd impress people with his knowledge, but he just showed everyone how stupid he was.
 

Thread Starter

DK3250

Joined Jul 15, 2020
13
Thanks, dl324, for your reply.

Yes, the NOR gates are 74HC02.
I have placed a resistor before the transistor base - 100 k, maybe too much?
Also, I have changed the capacitor to 33 microFarrad and installed a LED on both output of the RS-latch - now I see a quick blink on one side before it becomes stationary on the other.

Pull-up resistor on the comparators? Won't that bring both R- and S- input permanently high? Please explain how I should install the pull-ups.
 

dl324

Joined Mar 30, 2015
11,264
I have placed a resistor before the transistor base - 100 k, maybe too much?
It depends on the maximum current you want the transistor to sink. The base current needs to be at least a tenth of that.

National Semiconductor used 100 ohms. but they don't give any information about the discharge transistor (other than its polarity).
Pull-up resistor on the comparators? Won't that bring both R- and S- input permanently high?
That depends on the outputs of the comparators.
Please explain how I should install the pull-ups.
Between the output and power supply.
 

Thread Starter

DK3250

Joined Jul 15, 2020
13
Eureka - You're a wizard! My respect!
It works now, the updated diagram:555-2.jpgI'm not sure I fully understand how the pull-up works, however.
I would expect them to bring +5 V to the R- and S- input.
But apparently only when the comparitor output is high - Hm, this I have to mull about...
 

crutschow

Joined Mar 14, 2008
25,264
It works now, the updated diagram:
The 100k resistor should be in series with the base of the transistor, not the collector, as you stated in post #3.

For higher speed operation you may want to reduce the size of the internal 100k resistors to 10k.
 

Thread Starter

DK3250

Joined Jul 15, 2020
13
The 100k resistor should be in series with the base of the transistor, not the collector, as you stated in post #3.
I did this first (resistor in series with base, no resistor on emitter - [my diagram is wrong - the resistor is on the emitter]) - it did not work.
Just now made a re-test: It still don't work.

Maybe it has to do with the size of resistors.
I think it is two different mechanisms here: A resistor to the transistor base will protect the transistor, but not otherwise affect the circuit. A resistor to the emitter reduces the speed of capacitor discharge and somehow stabilizes the circuit (?).

For higher speed operation you may want to reduce the size of the internal 100k resistors to 10k.
The resistor in scope for the discussion above...? Or the pull-ups?
 

dl324

Joined Mar 30, 2015
11,264
updated diagram
Now you're showing the Q# output being made available. You can't load an output and expect to still be able to use it to drive other logic.

We know the resistors on the comparator outputs are pull-ups, but you should still show them being connected to the power supply.

The resistor on the emitter is in the wrong place. That will limit the discharge current to such a low value that it will affect timing. It should be on the base and the value should be a lot less than 100k.
 

crutschow

Joined Mar 14, 2008
25,264
did this first (resistor in series with base, no resistor on emitter - [my diagram is wrong - the resistor is on the emitter]) - it did not work.
Just now made a re-test: It still don't work.
No the correct location is is series with the base.
The base-emitter junction looks like a forward biased diode and needs a resistor to limit the base-emitter current.

An emitter in the resistor will just slow down the discharge of the capacitor, which you do not want.
It has nothing to do with "stabilizing" the circuit.
 

Thread Starter

DK3250

Joined Jul 15, 2020
13
@dl324 and @crutschow:
Thanks for your input - you are marvelous.

It appeared to me that something was fishy. I went back and checked my components and found that the transistor was malfunctioning. Replaced the transistor with a new one - and, voilà, everything worked as you have described.

So, final diagram (I hope):
555-4.jpgYour help has been fantastic.
 

eetech00

Joined Jun 8, 2013
1,965
@dl324 and @crutschow:
Thanks for your input - you are marvelous.

It appeared to me that something was fishy. I went back and checked my components and found that the transistor was malfunctioning. Replaced the transistor with a new one - and, voilà, everything worked as you have described.

So, final diagram (I hope):
View attachment 213221Your help has been fantastic.
The Q output should connect to a source/sink driver.
 
Top