Fake arming circuit

Discussion in 'The Projects Forum' started by Dh4mp1r, Mar 26, 2013.

1. Dh4mp1r Thread Starter New Member

Mar 26, 2013
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I'm making a mad scientist costume and I need help making my prop work.
I'm trying to recreate the idea behind the old "bad guy" doomsday device.
the box where they flip a bunch of switches, LEDs come on, and it all looks pretty impressive.
Its been years since I designed a circuit and I need help.
I'm trying to have it battery powered. I mostly need help with the math. what resistor values do I need to use, how many batteries, etc. I already have the components, minus the battery case, and I'm pretty confident that I'd burn everything out if I were to assemble it now. the picture is what I'm aiming for. . .

oh, FYI this will do absolutely nothing other than light up. this is a prop, and I'm not some nutjob looking to build a real detonation circuit. well. . . I may be a nutjob. . .

• fake arming circuit.jpg
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Last edited: Mar 26, 2013
2. Dh4mp1r Thread Starter New Member

Mar 26, 2013
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this is the general idea I'm shooting for

3. tubeguy Well-Known Member

Nov 3, 2012
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Good start on the circuit, but for it to work, reverse the diode polarities.
If the arrow symbol at the top is a gnd, it should be on the - side of the battery.

And D6 with switch should connect across bottom as the other LED's and of course, must have a current limiting resistor

Your LED's have a ~ 2.0v forward voltage (Vf) and can run at up to 20ma current comfortably.
The resistor formula is (battery voltage - Vf)/LED current
for a 4.5 v battery:
4.5v - 2.0v = 2.5v
2.5v/.020 = 125 ohms. (A 130 or 150 ohm is fine.)

If the LED's are brighter than needed you can use a larger value resistor to dim them and use less battery current.

Last edited: Mar 26, 2013
4. t06afre AAC Fanatic!

May 11, 2009
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The fireing switch looks kind of wrong to me.

5. WBahn Moderator

Mar 31, 2012
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It would be easier if you described the behavior you want.

In your circuit, D6 can never come one because when SW6 is open the LED is open circuited and when SW6 is closed the LED is short circuited. Either way, no light.

6. Dh4mp1r Thread Starter New Member

Mar 26, 2013
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@tubeguy: that helped! thanks!

@WBahn: I'm looking for the LEDs to come on in sequence when the switches are turned on. but if the switches are turned on out of order, the LEDs don't light up. I'm trying to avoid PC boards and ICs, I know I can do a lot more with those, but I am going for the spare-parts-cobbled-together look. I'm looking for D6 to come on with a PBNO. right now I've revised the diagram, but I'm hoping that you can give me a couple pointers to have D6 come on only after all the switches are on, but still having all the individual switch LEDs on at the same time. . . if that makes any sense. . .

and the revised diagram:

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7. tracecom AAC Fanatic!

Apr 16, 2010
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Starting with the schematic as you have it drawn, here is the way I read the sequence.

SW1 operates, LED 1 lights
SW2 operates, LED 1 stays lit and LED 2 lights
SW3 operates, LEDs 1 and 2 stay lit and LEDs 3 and 4 light
SW4 operates, LEDs 1, 2, and 3 stay lit, LED 4 goes out, and LED 5 lights
SW6 operates, LEDs 1, 2, 3, and 5 stay lit, and LED 6 lights

Is that what you are after?

8. WBahn Moderator

Mar 31, 2012
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In your revised diagram, D6 will light when you press SW6 independent of the state of any other switches.

What is it you are trying to achieve with SW4? Your description doesn't hint at wanting D4 and D5 to behave differently than D1, D2, and D3.

9. Dh4mp1r Thread Starter New Member

Mar 26, 2013
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@Tracecom: Exactly! i couldn't figure out how to say it. . .thanks!

@Wbahn: it's a selector switch, a or b, think stun or kill on a phaser from star trek

10. tracecom AAC Fanatic!

Apr 16, 2010
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And now, you would like to add a feature. You want SW6 to do nothing unless either LED4 or LED5 is lit?

If so, try this.

• fake arming circuit.jpg
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Last edited: Mar 26, 2013
11. WBahn Moderator

Mar 31, 2012
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Gotcha. So what you probably want to do is put your firing branch into the circuit just like it was the next step in your arming tree. The only difference is that you use a push button instead of a toggle switch. This would be in parallel with the selector switch path.

The only thing that is not "nice" about this design is then that if a later arming switch is on, the as soon as the prior switch is turned on both it's LED and the next one (and maybe more) will turn on. That may just be something you have to accept in order to keep it simple.

12. Dh4mp1r Thread Starter New Member

Mar 26, 2013
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@tracecom: Perfect! thank you!

@Wbahn: yes, i saw that, but simple is better than having to deal with ICs or other semicondutors. they're the only component I routinely fry when I try to solder them.

one last question, should I reverse the order of resistor/LED? or is it ok as is?

Last edited: Mar 26, 2013
13. tracecom AAC Fanatic!

Apr 16, 2010
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You are welcome.

A resistor in series with an LED can go on either side of the LED.

Dh4mp1r likes this.