Help newbie with flashing led project please

AnalogKid

Joined Aug 1, 2013
9,610
Old-style schematics had a little half-circle hump when two wires crossed without connecting. Contemporary schematics have a connection dot to show connection. In my schematic you can see this at the top of R3 versus the top of C2. In a well-drawn schematic, two lines never cross and connect; all connections should be T junctions with a dot. In the schematic in post #19, there would be no ambiguity if the ground symbol were moved a bit to the left or right.

ak
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
Once again, AnalogKid has an elegant solution, Below is a schematic of a more conventional solution,

The 555 circuit on the left is a 555 monostable circuit, with an output of a twenty second high signal. Once twenty seconds has elapsed, the signal goes low until it is triggered again.

The 555 circuit on the right is a 555 astable circuit, that runs a little over 10Hz with a 52% duty cycle. This is what flashes the LED and is triggered by the 555 monostable circuit.

Besides a 556, only a handful of passive components - resistors and capacitors - are needed.

View attachment 120893
In that diagram, wherever two wires cross each other they are not connected with just one exception, the ground connection to the left hand '555. That exception, circled in red below, is connected.
View attachment 121287

Hi folks, so I finally got the components and breadboarded the circuit shown in the diagram above. Problem is, it's not working. I know that the LED is in the right direction, and I even tried flipping the direction of the capacitors, just in case. But still, no avail. However, if disconnect pin 1 from ground on either of the 555's, then the LED starts blinking! But nothing happens if I push the button. Any ideas what this might mean?

Thanks!
 

djsfantasi

Joined Apr 11, 2010
8,059
Hi folks, so I finally got the components and breadboarded the circuit shown in the diagram above. Problem is, it's not working. I know that the LED is in the right direction, and I even tried flipping the direction of the capacitors, just in case. But still, no avail. However, if disconnect pin 1 from ground on either of the 555's, then the LED starts blinking! But nothing happens if I push the button. Any ideas what this might mean?

Thanks!
A couple of checks. But first, just to understand what you've done: did you use a 556?

One thing I don't understand is your statement that if you remove ground from either of the 555s, the LED blinks. Can you be more specific? If you remove the ground from the second/right 555, I'd expect nothing from the LED. To understand better, where is the LED cathode connected?

I'd remove the connection from pin 3 of the first/left 555 and measure the voltage from pin 3 to ground. Then make a second measurement from the same place after pushing the button. It should go high for about 20 seconds.

Please perform these tests and post your results.
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
A couple of checks. But first, just to understand what you've done: did you use a 556?

One thing I don't understand is your statement that if you remove ground from either of the 555s, the LED blinks. Can you be more specific? If you remove the ground from the second/right 555, I'd expect nothing from the LED. To understand better, where is the LED cathode connected?

I'd remove the connection from pin 3 of the first/left 555 and measure the voltage from pin 3 to ground. Then make a second measurement from the same place after pushing the button. It should go high for about 20 seconds.

Please perform these tests and post your results.
Hi,

Ok, so I checked more carefully. I mistated earlier. When I disconnect pin 1 from the left 555, the LED starts blinking. Nothing happens when I disconnect pin 1 from the rigth 555. I am not using a 556. I am using two 555s (LM555CN)

The LED cathode is connected to ground

I removed pin 3 from the left 555, and when I did, the LED started blinking. I checked voltage between the output of pin three (left 555) and ground, and found ~20 mv at all times, whether or not the button was pressed.
 

djsfantasi

Joined Apr 11, 2010
8,059
When you say "at all times", you mean when you press the button or not?

This is closer to what I expect. The one shot which turns on the flashing LED appears to not be working. I'll check my schematic later. While I do that, can you double check the resistor and capacitor values in the 555 circuit on the left?

Edit: I see you included "whether or not" at the end of your post. Missed it the first time!
 

djsfantasi

Joined Apr 11, 2010
8,059
Yep, I found a mistake. Pins 6 & 7 should be connected together. NOT on opposite sides on the 390K resistor.

BlinkingLED.PNG
 
Last edited:

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
Yep, I found a mistake. Pins 6 & 7 should be connected together. NOT on opposite sides on the 390K resistor.

View attachment 121888
Cool! I now connected pins 6 and 7, as your new schematic shows. The LED is off until I press the button. Once I press the button, the LED starts blinking, but it remains blinking. I waited a few minutes to see if it would turn off, but it kept on going. hmmm....
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
Once again, disconnect pin 3 from the leftmost 555 and check the voltages as before.
Ok, here's what I get:

If I have not pushed the button, when I disconnect pin 3, the led starts blinking fast, and the voltage from pin 3 to ground reads around a few mV. Then, when I re-plug pin 3, the led stops blinking.

If I push the button, the voltage reads at 4.2V.
 

djsfantasi

Joined Apr 11, 2010
8,059
Ok. don't replug pin 3

You've measured the voltage from pin 3 tonground without pushing the button. Now make the same measure when pushing the button. See it it changed after 20-30 seconds.

Also, just curious. When you push the button, it's a momentary push. Right?
 

mtonge

Joined Apr 19, 2016
93
I don't understand why the output of the first timer (pin 3) is connected to the reset (pin 4) of the second. The reset pin is normally held 'high'. Wouldn't the second timer be on while the first timer is off? I have set up circuits using the output of a mono-stable timer to power an astable timer; timer 1 output (pin 3) to timer 2 Vcc (pin 8).
 

djsfantasi

Joined Apr 11, 2010
8,059
The normal state of pin 3 on the first timer (monostable) is low. When it is connected to pin 4 (reset) of the second 555 (astable) it is prevented from oscillating. When the button is pressed, it creates a 30 second high signal on pin 3, allowing the astable to flash.
 

mtonge

Joined Apr 19, 2016
93
Thanks. As usual, I have been doing things the hard way. From the 555 Timer Tutorials, "...the reset, when taken to ground, inhibits all device functioning". I need to re-draw my schematics.
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
Ok. don't replug pin 3

You've measured the voltage from pin 3 tonground without pushing the button. Now make the same measure when pushing the button. See it it changed after 20-30 seconds.

Also, just curious. When you push the button, it's a momentary push. Right?

Hi. I'll check again when I get back home tonight, but when I tried this morning, I did the measure both when I did not push the button and also when I pushed the button. In the latter case, the voltage from pin 3 was 4.2V. I will check to see if the voltage remains at 4.2V after 30 seconds.

And yes, it is just a momentary push of the button.

Thanks!
 

djsfantasi

Joined Apr 11, 2010
8,059
And one more thing. Have you checked the values of the resistors and capacitor on the monostable (left) 555?

The resistor connected to the pushbutton should be 10,000 Ω. Or 10K Ω

The resistor connected to pins 6&7 is 390,000 Ω or 390K Ω.

And the capacitor should be 47μF
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
And one more thing. Have you checked the values of the resistors and capacitor on the monostable (left) 555?

The resistor connected to the pushbutton should be 10,000 Ω. Or 10K Ω

The resistor connected to pins 6&7 is 390,000 Ω or 390K Ω.

And the capacitor should be 47μF
I did check again and pin 3 remains at 4.2V for a long time (well over two minutes) after I push the button. The interesting thing is that whether or not I push button, the led starts flashing when I disconnect pin 3.

Regarding the capacitor and resistor values:

Ok, this is weird. First, the normal stuff: the 47µF capacitor is 47 µF. The resistor connected to the pushbotton is 10KOhms.
But, the resistor connect to pins 6 and 7 (and to the 47 µF capacitor displays the following:

when unplugged from anything, it's around 0.4 MOhms
when plugged into the circuit (but not yet connected to the 47 µF capacitor), it is 0.4 MOhms when the circuit is unplugged. When I power the circuit in this configuration (but do not press the button), I get no reading (0 MOhms).

when plugged into the circuit (AND connected to the 47 µF capacitor), it is also 0.4 MOhms if I make sure that the capacitor is discharged. When I power the circuit in this configuration (but do not press the button), I once again get no reading. (0 MOhms). (Incidentally, the voltage across the resistor is 4.92V and 0 V across the capacitor. Once I push the button, the voltage is 3.25 across the resistor and 1.5V across the capacitor.)

why???
 

Thread Starter

Eldarnoe

Joined Feb 18, 2017
12
I've tried three different resistors, and they all end up having the same behavior. I never heard of a resistor being affected by a capacitor. Any ideas?
 

djsfantasi

Joined Apr 11, 2010
8,059
It's hard to tell by your description what is connected to what and where. Or where specifically you are taking these measurements.

If you take the resistor totally out of the circuit? I'd guess it's measured resistance would be around 0.4MΩ. That's because a 390kΩ resistor is 0.39MΩ.

If you were to connect the resistor in series with the capacitor (observing polarity on the capacitor if its polarized.) Ground the capacitors free end. Connect your volt meter to ground and to the joint between the cap and resistor. Then connect 5V to the free end of the resistor.

Watch the meter. Voltage should slowly rise over about 20 seconds.
If it does, the capacitor us likely good. If not, it may need to be replaced.

Im not sure what it is that makes you think the resistor is being affected by the capacitor. The voltage across is affected, but not the resistor. Read up on RC (resistor/capacitor) networks.
 
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