# electrical novice

Discussion in 'General Electronics Chat' started by ant0889, Jan 26, 2013.

1. ### ant0889 Thread Starter New Member

Jan 26, 2013
5
1
Hello everyone,

I dont know if im in the right place but i just want to say hello to everone on the forum.

electronics besides automotive circuits are a complete mystery to me.

im wanting to make, what i think is, a simple circuit. i want to have a single LED, that when i apply 12v to it will come on for 10 seconds roughly, then go off and stay off till i remove the 12v. once the 12v is applied again, the process starts over again.

im creating this for a my gilfriends ARC reactor in her iron man costume shes making, full pepakura suit

i know this will be below you and dull for most of you, but any advice will be greatly appreciated.

thankyou

Anthony

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2. ### #12 Expert

Nov 30, 2010
17,956
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It's called a "one shot" and it's done with a 555 timer chip. We do this kind of thing almost every day here. Look at the top of the page and sort through the circuit books to find 555 circuits (Volume 6, chapter 8), then report back here to sort out the details.

ps, this site does not allow discussions about cars, trucks, motorcycles. Something about liability issues. Be careful not to bring automotive words into the discussion.

3. ### ant0889 Thread Starter New Member

Jan 26, 2013
5
1
that great thanks ill have a read! oh really.. oops ok ill try not to mention it, however its kind of all i know about electronics, so will probably end up making a comparison here or there

4. ### #12 Expert

Nov 30, 2010
17,956
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ps again...if the LED is small enough, there are simpler ways to do this. Bring data about the LED(s). That means voltage and current ratings. This might turn out really simple.

5. ### ant0889 Thread Starter New Member

Jan 26, 2013
5
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hello,

the only info i have on the packet is that one blue 3mm led has a forward current of 20Ma (typical) 30Ma (max), a forward voltage of 3.2-3.8v and a wavelength of 465 Nm.

ive tried in vain to read the information about the 555 ic, however even thats above me.

i recommend really dumbing things down for me... im sorry to be a pain.

thankyou
anthony

6. ### Brownout Well-Known Member

Jan 10, 2012
2,374
999
A standard one-shot won't work. You'll need to be cleaver enough to activate the trigger upon connection of the working voltage. You might get lucky and the circuit will come up in the triggered state, buy we try not to rely on luck. You'll need to modify the basic one shot with networks on the 'reset' and 'trigger' connections to correctly squence these inputs. Here is the correct sequency:

1) Activate reset
2) Trigger input stabilized
3) De-activate reset
4) Activate trigger.

This can be accomplished using RC network on these inputs. The RC netwok connected to reset should bring this pin up from ground slowly enough to allow the trigger voltage to be stable. The RC network conected to trigger should bring the voltage down slowly enough that the pin remains high until the timer comes out of reset.

absf likes this.
7. ### #12 Expert

Nov 30, 2010
17,956
9,418
Someone want to check my work? I always have to re-learn the 555 when I do a circuit.

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• ###### ne555.pdf
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absf likes this.
8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Put it in LTSpice, simulates fine.

Changing R1 from 1M to 630k results in an LED time on of about 10.4 seconds after power on.

Blue Line is LED on/off, LED on for first 17 seconds or so. Simulation runs for 60 seconds, Red line is the 1M/15μF charge curve, green is Reset voltage line

LTSpice IV .asc file attached. The wiring could use some neating up.

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• ###### 555-1shot.asc
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Last edited: Jan 26, 2013
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9. ### #12 Expert

Nov 30, 2010
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Thanks. I drew the grounds oddly to remind myself to mention that the power supply capacitors must be attached close to the chip.

I also chose the same value of capacitor for as many things as I could in order to keep the parts list short.

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I zoomed in to capture this trace which shows why the reset RC circuit is needed for the 1 shot to work.

I do the same with capacitors, though often use 22uF, it's the best tradeoff between low value while retaining low ESR. Smaller Electrolytics tend to have relatively high ESR, especially if rated for > 10V.

Reset pin voltage - Blue
LED Voltage - Red (High once $\overline{RESET}$ passes 0.7V to enable internal transistor)
Thres/Dis RC Line Voltage - Green

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Last edited: Jan 27, 2013
11. ### ant0889 Thread Starter New Member

Jan 26, 2013
5
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hello,
thankyou 'thatoneguy' and '#12' for your help.. its been invaluable!

im having difficulty finding a place to purchase the parts.

from the above i guess i would be needing ...

1 x NE555 IC
1x 15k (15 000) resistor
1x 470 resistor
1x 630k (630 000) resistor
3 x 15uf capacitor
2 x 0.1 uf capacitor

is this right?

thankyou
Anthony

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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Radio shack should have all the components you need, but I'd suggest ordering online from either mouser or digi-key or a dealer more local to you (unsure of your location), they are usually far cheaper, even with shipping, than a Radio Shack provides, components are usually a higher quality as well.

A resistor assortment may be a better choice, especially if you can only find 22uF caps and not 15uF, or if you want to change the LED or timing a bit.

0.1uF caps - Ceramic / MLCC
15uF caps - Electrolytic, pay attention to polarity when using!
Resistors - assorted, 1/4W would be fine.
NE555 timer
Wire for final assembly and connections, 24 gauge stranded if it will be flexed otherwise solid core of same gauge.

13. ### #12 Expert

Nov 30, 2010
17,956
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Both of the RC circuits are designed to make a time delay. The 1 meg and 15 uf will work better to get 10 seconds with a 630k and 15 uf, or with 1 meg and 10 uf. Point is, it's negotiable. The power supply filter can be 10 uf and up, so 10uf, 15uf, 22uf, will all work just fine there.

The 15k resistor is not negotiable to a higher value because of the needs of the NE555 chip for current on that pin. You can use a lower value if you like, but you must increase the capacitor proportionately to keep the reset pin "on" long enough to reset the timer.

14. ### sheldons Well-Known Member

Oct 26, 2011
616
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just a bit of extra info for your 555 ,heres a data sheet

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15. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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The time decided for the LED to turn off is when the RC circuit on the inputs reaches 2/3rds of Vcc, or 8V.

You can calculate your own delay for that (currently 10.4s with 630k and 15uF)

Last edited: Jan 28, 2013
16. ### #12 Expert

Nov 30, 2010
17,956
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Did I get that 6 hundred something k wrong? Not paying attention very well today. No big deal. Nobody in their right mind would put a stop watch on Iron Man.

17. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Nope, I did, I'll correct it.

18. ### ant0889 Thread Starter New Member

Jan 26, 2013
5
1
Hello again,
Thankyou both 'thatoneguy' and '#12' for all your assitance.
I'm hoping that I may be able to pick your brains again.

I've made the circuit as per the diagram of 'thatoneguy'... its not pretty but it follows the plan. I've used 1 sided prototype board and some thin multi stand cable. Also used a chip holder so i can swap chips in and out.

ive tested the circuit with a 9V battery and its not turning off... the blue LED just stays on. its been 20 minutes now and its still on haha... could i have mis soldered something?

pics below .....

19. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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Use a DMM to check resistance/capacitance between power and ground (whichever is applicable on the schematic), same for those on feedback, from the IC pins to output wires. Do that without power, first without the IC installed, the next one with the IC.

Double check to be sure you didn't reverse left and right on the IC Pins from the bottom side (Common Mistake).

I cannot guess anything from the wiring, you should try some veroboard, or smaller wire (such as wire-wrap wire) for interconnects if you don't have a board with plated through holes and connected bus lines.

Question: Are you using rosin core solder, or "Plumbing" Solder? The green on the PCB looks similar to the corrosion which happens when acid core solder is used.

20. ### #12 Expert

Nov 30, 2010
17,956
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Good point. Enough leakage current through that green stuff and the the timer capacitor will never fill up, thus the light will never go off.