The base of Q2 needs to be more negative than the emitter by at least 0.7 volts. The signal line can't go below zero volts. When the signal line is high Q1 is On correct? Q1 Emitter will be at about the forward voltage drop of LED 1 and the Q1 base will be well over saturation point. So now I make my signal line Low. That will turn

Then all things considered I would think about an interrupt light beam similar to what we see used on garage doors for safety. The problem I see with for example a sensor like a magnetic proximity sensor or hall effect sensors is they have limited distance detection. This way an off the shelf turn key solution can be used in conjunction with the lights liked to earlier. I would also select something which will function in the environment of the loading dock in question.

Ron

When signal is high Q1, which is an NPN transistor, turns on LED1. When signal is low Q2, a PNP transistor, turns on LED 2. IF signal is left floating out in the ether (not high nor low) then neither transistor is on. Both LED's will be lit but slightly dimmer. Not known if you'd discern the difference between 20 mA bright and 15 mA bright, but at 12 volts with R2 being 470Ω, the red will burn at 20 mA, the green at 18 mA or both in series at 15 mA.

According to your statement that the emitter of Q2 (PNP) must be at a lower voltage than the emitter. It is lower because 12 volts is present (minus forward voltage of the LED) at the emitter via the LED. Some current will flow from the emitter to the base, which is what turns a PNP transistor ON, conducting between emitter and collector - to ground. Voltage at the emitter of Q2 should be around 8 volts positive.

 

I think we've gotten off on arguing different methods of establishing a red or green light. The TS wants to back a trailer up to a loading dock and have a green light turn red when the trailer is in place. The units he linked us to have numerous options. He certainly could build a circuit if he wanted to - but there's already product he can buy. Since he's already planning on buying the lamp unit - spending a very little more will get him the red and green switching he is after. He has his choices of 12VDC, 24VDC (DC is assumed) or 110 VAC. He can get them with or without power supplies. With or without light switching circuitry. Flashing and steady (I think) are also available. I've seen the same thing at many loading docks. I've seen green, yellow and red units where yellow indicates to the driver he's almost there.

The TS has options. In according with his asking how to build a circuit - that's been answered by several different people. Your approach may very well be viable. Draw a picture and post it. That way I don't get confused with what you're saying.

I think, am not sure, but can you put a reed switch with a magnet - say the north pole - behind a reed switch and hold it closed. When another north pole magnet approaches the two opposing poles cancel each other out and the reed switch opens? Maybe I'll give that a try some time. If so that would be a sweet door open alarm for my freezer. When closed two magnets cancel each other out and the reed switch remains open, not sounding an alarm. When one magnet is pulled away the reed closes and sounds a door open alarm. With an adjustable proximity the sensitivity of the alarm could be adjusted. I may just revisit this idea some day. I once tried using a gate to invert the signal but that drained my battery for some reason.

In your last paragraph you described exactly how most normally closed reed switches work. I did not see where the TS mentioned backing a trailer in and changing a light color. That scheme would require a very accurate alignment for most of the reed switches that I am aware of. And the original post showed a flipflop driving LEDs and the description was alternating switching based on application of a magnet. It seems that the targeted operation may be a bit different after 60 comments.

 

There's a certain level of reasonableness. I'd never build my own heart defibrillator. Sure, I could probably build one - given enough time to study up on it, do the necessary engineering and such. But when it comes to something that needs to be reliable - sometimes going with a professionally designed, engineered and built system is the better way. Besides, the TS is thinking about using one of those units anyway. He can simply buy the lights with no circuitry OR he could buy the lights with all the circuitry needed. I build for hobby sake. He's building for the sake of a business shipping / receiving dock. I'm sure the truck drivers would appreciate a system that reliably indicates when they're in proper position. So why build five units (I think he said 5) when he can buy five units with the electronics already in them? All he needs to decide is whether to power with 12VDC, 24VDC or 110 VAC. If you're going that far - why not just buy the unit you need for your purpose?!

There is NOTHING I've ever built that someone's life or property may depend on my "thing" working properly every time.

I have done quite a few of them, maybe not life-critical in all instances, but certainly if some portions failed there could be injuries. And several were definitely dependent on correct operation for everybody involved to be safe. You certainly do not want a crash sled taking off at the wrong time, nor do you want a machine starting it's cycle while an operator has their hands inside. So the functionality of a whole lot of the things that I designed was a rather safety critical requirement. That is part of why I find it important to have enough information. The main reason to make a critical decision without adequate information is to avoid certain death.

 

I have done quite a few of them, maybe not life-critical in all instances, but certainly if some portions failed there could be injuries. And several were definitely dependent on correct operation for everybody involved to be safe. You certainly do not want a crash sled taking off at the wrong time, nor do you want a machine starting it's cycle while an operator has their hands inside. So the functionality of a whole lot of the things that I designed was a rather safety critical requirement. That is part of why I find it important to have enough information. The main reason to make a critical decision without adequate information is to avoid certain death.

Now that you mention it the original thread starter never did share exactly what the project was going to be. I looked at his link and at some point made a dumb assumption. Somehow I got loading dock in my mind which is foolish. Maybe if the thread starter returns he or she can define exactly what their goal is.

Ron

 

Then all things considered I would think about an interrupt light beam similar to what we see used on garage doors for safety. The problem I see with for example a sensor like a magnetic proximity sensor or hall effect sensors is they have limited distance detection.

That's how all of them work that I've seen. With a magnetic switch of what ever type, what happens when another company make a delivery? No magnet no light.

 

That's how all of them work that I've seen. With a magnetic switch of what ever type, what happens when another company make a delivery? No magnet no light.

It would work with a magnetic proximity switch, which uses an AC magnetic field to detect any kind of metal. The reason for thinking "loading dock" was the link to a catalog page of loading doc warning lights. What I got from that is that it was far more than one LED required to provide enough light. I am not certain where the stack of transistors came from, but it was the wrong way to go for many reasons.