# Multiple switches, LED's, and 555 timer circuit

Discussion in 'The Projects Forum' started by LooneyToonDad, Dec 12, 2010.

Dec 8, 2010
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I am looking for advice, corrections, and or suggestions on a project I'm working on. It is basically a fake missile launch control panel (see picture attached). The main idea of the project is that when I turn a three position key switch from off to on #1, power will flow to a set of 4 yellow LED's, each controlled by another toggle switch. So once power is flowing, I can turn the individual LED's on or off, or "enable the missiles" one by one. When I turn the key switch to on #2, I want a second set of LED's (red) to flash to indicate "Launch", and the yellow ones to turn off.

I am very new to this, so I've tried to read and understand what I could about creating the circuit. I also found "Yenka" software to test the circuit and it seems to work (see attached pdf of schematic). Here are my questions:

1. I ran the 4 yellow LED's in parallel, each with its own resistor, since each one is controlled by a switch. Is this correct?
2. Do the 4 red LED's need to be in parallel also (that's how I did it in Yenka), or should I do them in series?
3. I will use a 555 timer chip to control the blinking. Based on trial and error in Yenka I found a combination of resistors and caps to get the pulse I wanted, but how does one figure out in real world what to use. I've seen the formula, but don't understand how to get that in seconds vs. kHz/Hz. Ultimately I'd like to have a three pulse cycle (on-off-on/off-on-off) within about 1 second. What affects it more, the resistors or the cap?
4. Best battery source? I originally thought a 9V battery (space wise good choice), but after reading some it seems like most prefer AA/AAA in series to make 9V. Which is best, since it will only be on short periods at a time.

As far as parts, I think the diagram has values for the resistors and caps; but for LEDs I was looking at 5mm leds, and it seems like both colors are 1.8-2.2 V, and max 20mA. Eventually they will be mounted on a panel, so I'm not sure if I will use surface mount LEDs or basic LEDs with the plastic holders.
Any thoughts, corrections, suggestions are much appreciated. If I left out any info that might be useful, please let me know, although I'm still in the very early stages and may not quite know exactly what I will have to use. Thanks for helping me learn.
Eric

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2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540

Dec 8, 2010
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Bill,

Thanks for the link. I had actually already read through that a couple of times. Unfortunately, I'm really new to this and some of that is over my head. I saw the parameter equations you had for the timer, but still don't understand how I figure out the frequency per second and/or values of resistors (question 3). I was also looking for some recommendations/suggestions based on the schematic I attached, mainly should the second set of led's be in series/parallel (question 2) (although in series, won't the brightness of each be different?) and will a 9V work or is a series of batteries better (question 4).

Please, I don't want anyone to think "hey, please do this for me". I did a lot of reading on the web and some books (and searching here) to come up with what I have so far, but before I start to buy some parts and test an assembly, I'm kind of looking for a second opinion on will it work, or if there's a better way.

Thanks in advance for any help.

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Check out this handy tool:
http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm
You can download it and use it in the freeware mode for a number of functions.

In the astable design wizard, I plugged in 3Hz for your desired frequency, and then did a bit of adjusting.
R1=47k, R2=220k, C1=1uF; all common values using the traditional 555 astable design.

I generally go for lower values for C1 and larger values of R1/R2. If R1/R2 start getting large (>1 MEG) I'll increase C1.

R1 should never be lower than about 100 Ohms per 1v of the supply; or the current on pin 7 will get too high. So, if you were using a 9v supply, 900 Ohms would be the smallest resistor you could use without risking overheating the 555 timer IC.

AA batteries are a good trade-off for size and current. Radio Shack sells battery holders to enable you to get various voltages out.

5. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
Ok. The 555 can always be running.

The 3 way switch should be selecting either the red group or yellow groups ANODES or positive.

The individual switches will switch the CATHODES or ground. So the red leds and the yellow leds cathodes will all run to the switches that correspond to the row.

The output of the 555 goes to the center of the 3 way KEY.

If it is in the On position, the line is connecting to the YELLOW anodes

If it is in the FIRE position, the line is connecting to the RED anodes.

Nothing will blink until you give it a GROUND with the individual switch.

I just checked out YENKA. neat.

Here is what I am talking about. Replace the 9v in my schematic where it connects to the "select/fire" switch with the 3 pin from the 555.

I attached the yenka file also.

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