# Simple Door Interlock & Light Diagram

Discussion in 'The Projects Forum' started by suprageezer, Jul 10, 2008.

1. ### suprageezer Thread Starter New Member

May 26, 2007
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First let me say I am not and engineer just a techie so any help would be greatly appreciated. Currently I we have a small clean room and leading into that a small gowning room. I need to make a simple green light red light interlock system so that when someone is about to enter the gowning room and the door to the clean room is open, at the gowning room entrance it will a red light. If boths doors are closed it shows green light on both entrences. We dont need anything mechanical just a simple light system. Is there somewhere I could get a simple diagram so I can fabricate it myself? A diagram is all I would need then I could select the parts needed to make the system.

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Here's one way:

3. ### SgtWookie Expert

Jul 17, 2007
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See the attached. S1 and S2 should be magnetic reed switches.

The 40106 is a CMOS CD40106B Schmitt-trigger hex inverter. All of the gates are used in two parallel groups of three, for one to prevent oscillations, and also to multiply the output current of the device.

R2 keeps the input to the left group high, which keeps their outputs low, which forces the 2nd group to have their outputs high. This causes the green LEDs to light. R1 is the current limiting resistor; it's value will need to be calculated from the Vf of the LEDs, the LED's current, and the supply voltage, Vdd.

If one of the switches closes, the inputs to the first group are pulled low, causing their outputs to go high along with the inputs to the 2nd group. The outputs of the 2nd group inverts the high to a low. Now both red LEDs are lit.

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4. ### suprageezer Thread Starter New Member

May 26, 2007
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Thank you so Much ThingMaker and SgtWookie, your help is Greatly Appreciated.
Rick

5. ### suprageezer Thread Starter New Member

May 26, 2007
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Hello Again,
I tried my best with the schematic but I just can't seem to understand it. So I need some help again, this time I've posted a small pic of what I need to wire up.
Thanks Again

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6. ### SgtWookie Expert

Jul 17, 2007
22,182
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OK, both of the solutions previously posted by Thingmaker3 and myself use LEDs, or Light Emitting Diodes.

Light Emitting Diodes are much more efficient, will last far longer, and be much more reliable than the incandescent bulbs that you are suggesting to use.

But if you are still determined to use incandescent bulbs, we can likely work something out.

Tell us what you want to do.

7. ### suprageezer Thread Starter New Member

May 26, 2007
8
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We are a very small medical manufacturer. We have a small cleanroom, and a small room we call the Gowning room. One door goes into the gowning room, and the other door is from the gowning room into the clean room. The clean room is filtered air and keeps pressure in the room so when I door is opened the air blows OUT and never sucks in. Currently sometimes both doors get opened at the same time. So what I need is a simple 12dvc light system. I will be using two cheap trailer running lights next to each of the doors. One light is green and the other is red. The system I would like is when both doors are closed both greens are on, when one door is open the other door light switches from green to red to indicate the other door is open so the operator needs to wait until the that door is closed and the light returns back to green.

8. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Revisiting the Wookie's first schematic in this thread, and contemplating additional gates for more source and sink current, I wonder if throwing a single diode (not LED) in series with a relay coil might suffice...

9. ### SgtWookie Expert

Jul 17, 2007
22,182
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OK, if you want to use the two cheap trailer running lights, you can.

Forget the magnetic switches though - you can't use those then. You'll need heavier duty switches. The magnetic switches wouldn't work well with relays either, as the relays would be energized all the time the doors were closed.

One is a pushbutton type that would need to have a bracket made for it out of some aluminum angle or the like. The other is a roller type that would also need some kind of bracket made for it. With either type, there is only one per package.
You don't have to get them there, but you will definitely need a pair of switches that are SPDT and are spring-loaded.

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10. ### suprageezer Thread Starter New Member

May 26, 2007
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Wookie, Thank You Very Much thats exactly what I was hoping for I can make that easily.
Thank You
Rick

11. ### SgtWookie Expert

Jul 17, 2007
22,182
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You're welcome

On the pushbutton switches, the common terminal is the center terminal.
On the roller actuated switches, the common terminal is the one off by itself.

You want the NO terminal to be connected to the green light. This is because when the door is closed, the common to NO connection will be made, and the NC connection will be open.