# Diagram Questions

Discussion in 'General Electronics Chat' started by dannyv573, Jan 2, 2014.

1. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
0
I have one of the Engineer's Mini-Notebooks. It's the 555 Timer IC Circuits book. I don't under stand the Diagrams in the book. I attached a picture from page 8. How is the power hooked up? How do I hook up a light to this to have it light up? I've made the circuit but I don't know what to do with pin 8 where it says +5 to +15v and what to do with pin 3.

• ###### fmmiii 555 timer touch-activated switch.png
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2. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
"+5 to +15V" indicates that the 555 IC can be powered by +5 to +15 volts DC. The power supply minus connects to ground, and the plus connects to pin 4,8.

Pin 3 (the output) can provide enough current and voltage to drive an LED.
But, an LED requires a current limiting resistor in series with it.
The resistor can be around 1000 ohms for this circuit.

The brightness of the LED will vary based on the current supplied to the LED.
This is determined by the resistor value, the output voltage from PIN 3, and the voltage dropped across the LED.
Google LED's, then come back with any questions.

Bill Marsden's the 555 guru:

BTW:
C1 also show's a range of values. This will determine how long the output remains HIGH after the finger is removed.
(DELAY in the diagram)

Last edited: Jan 2, 2014
3. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
0
Thanks for the info. So if I wanted to connect a LED to pin 3, where do I connect the other end of the LED? Using the diagram attached before.

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
See the link above, figure 4.2.
It shows two ways to connect the LED. Vcc is the plus voltage connection.

5. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
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I'm new to this. I don't understand the similarity between my picture and the link given. They don't look the same. I used Circuit Wizard to construct the diagram from the picture I attached before. When I connect the LED, it leaves it on regardless the connection of pins 2 and 5. It just won't work.

6. ### sheldons Active Member

Oct 26, 2011
616
101
here are a few schematics you can play with and a data sheet...

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7. ### wayneh Expert

Sep 9, 2010
11,892
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Pay attention to the pin numbers of the 555 versus the drawing. It can be confusing, and it's very easy to mix them up. It's almost guaranteed, so you have to check and re-check. Everyone here knows what I'm talking about.

A good photo of your build might be very helpful in diagnosing your circuit.

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
The positive leg of led goes to pin 3. The negative leg of led goes to pin 1.

Like tubeguy said, pin 3 might provide too much voltage and too much current to led. So you need following pieces of information:
1) led forward voltage
2) pin 3 voltage
3) pin 3 current

Using these three pieces of information you can calculate the value of a resistor that will protect your led.

The final setup for led will be this:
Pin 3 connected to positive leg of led, negative leg of led connected to one end of resistor, the other end of resistor connected to Pin 1.

9. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,567
1,063
No. You must have a series resistor or you will fry the LED !

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
lol
read the rest of the post.

11. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
0
Ok... I haven't read the pdf files yet but I will. The rest that has been posted doesn't help. I only need to make the diagram work that's in the picture I attached at the beginning of this thread. I've connected everything correctly. I connected a LED to pin 3 and a resister to pin 1 and the LED. The LED stays on without connecting or touching pins 2 and 5. This is supposed to be a touch activated switch. Something is not right.

12. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
You are "the expert on the scene". Either something is wired wrong or something is busted.

13. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
0
I've disconnected and reconnected the diagram several times. If the wires are wrong then the diagram is too.

14. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
The interesting thing is that LED is ON. So you wired LED right, but! You also have Pin 3 supplying the voltage and current. I mean normally Pin 3 is OFF. So. Why is Pin 3 ON? Something is turning it ON when it should be OFF. Or. The chip is busted.

15. ### dannyv573 Thread Starter New Member

Jan 2, 2014
13
0
I believe that the diagram is wrong. Reason being is that when I use the Circuit Wizard on my PC, the LED stays on as well.

16. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
Ok. That is good. We now have a physical prototype and a simulation producing the same result. At least we now know that the chip is not busted.

17. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,567
1,063
The circuit shown has pin 2 'floating', so it may be picking up interference causing the 555 to re-trigger. Try connecting a 'pull-up' resistor (say ~ 100k) from pin 2 to pin 8.

18. ### sheldons Active Member

Oct 26, 2011
616
101
if you ground pin 2,the 555 pin 3 goes hi for an amount of time determined by the timing components....if you add a pull up resistor from pin 2 to supply,put a 1N4148 diode in parrallel with it ,add a capacitor one side to pin two the other side to supply via a resistor and ground that side of the capacitor you then have an edge triggered 555-see diagram....your 555 may be picking up noise.

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19. ### inwo Well-Known Member

Nov 7, 2013
2,420
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Works with 20meg pull up on pin 2.
Or 330pf from pin 2 to V+. (low duty cycle)

As per above suggestion.
Didn't try edge trigger. Looked to be too low impedance for touch switch.
May work though.

Wonder if a certain version is needed.
555cn here.

20. ### tracecom AAC Fanatic!

Apr 16, 2010
3,867
1,392
If pin 1 of the 555 is properly connected to ground, then connecting the cathode of the LED (through a current-limiting resistor) to pin 1 is the same as connecting it to ground. However, for clarity to those without a lot of experience, it seems better to me to specify a ground connection instead of a connection to pin 1.