Halls sensor? Reed Switch?

Thread Starter

Mumeral

Joined Jan 28, 2020
12
I am trying to setup a PCB to use a magnet for the indicator. I'm going to use a 12v 0.3a red/green LED stop light and MAYBE a flipflop relay. I am not sure which to use, and I'm just a hobbyist at best.Circuit.jpg
Something like this.
 

Tonyr1084

Joined Sep 24, 2015
5,956
Welcome to AAC. There are many ways to achieve this. Your diagram is helpful but we need to see how you're using your switch (reed & magnet). If you're using a hall effect transistor that's a little different than a reed switch.

[edit] do you want the light to change every time you (per se') push a button? Or do you want them to be "Red" when the switch is pressed and green when it is not?
 

MaxHeadRoom

Joined Jul 18, 2013
23,083
There are also the Fairchild SS400 type sensors with different characteristics/options when using a small magnet..
Latch/ unlatch etc.
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,956
[edit] I have removed my drawings because they are in error and will not work. In addition to removing my drawings I'm also removing all comments from this single post.
 
Last edited:

Thread Starter

Mumeral

Joined Jan 28, 2020
12
How would it look like with the Reed Switch? Are there specific parts I should use? I may get a PCB made just to make my life easier.
 

Tonyr1084

Joined Sep 24, 2015
5,956
Your reed switch will provide power (I assume) to the signal input of the inverter. When powered the green LED will be lit. However, when you remove the magnet - power is removed leaving the signal floating, not powered, not grounded. A pull down resistor will pull the signal down sufficiently. As for values of components - that depends on what voltages you're working with and what power source you have.
 

Thread Starter

Mumeral

Joined Jan 28, 2020
12
I have no parts, starting from scratch with recommendations from people far more experienced then I am. Only thing I have in mind is the light
 

ebeowulf17

Joined Aug 12, 2014
3,276
There are also the Fairchild SS400 type sensors with different characteristics/options when using a small magnet..
Latch/ unlatch etc.
Max.
Does Fairchild own Honeywell now? I've had good luck with the Honeywell SS495 and a few of the other analog out varieties.

I can't keep up with all the mergers and acquisitions - seems like every time I try to recommend a product it's under a different name than the last time I checked!
 

ebeowulf17

Joined Aug 12, 2014
3,276
The other big question here is how accurate and precise the trip point needs to be.

If a magnet is near the sensor, the light turns green. If a magnet is far away from the sensor, the light turns red. How different are the near and far distances in your scenario?
 

Thread Starter

Mumeral

Joined Jan 28, 2020
12
As long as the magnet is probably within 2-3 inches is fine. I'll probably wanna use a small magnet for convenience
 

MisterBill2

Joined Jan 23, 2018
8,709
I think that there are also 3-terminal reed switches, which would make the scheme much simpler.
What is the application of this creation? Certainly it would be more than just lighting LEDs. But if not, the simplest circuit would use one IC, a CD4049 hex inverter, and one resistor, to operate the two LEDs. A 20K resistor pulling up the input of inverter #1, with the reed switch pulling it down to zero volts when it is closed. The output of inverter #1 feeding the input of inverters #2, and #3 and #4. The outputs of #3 and #4 in parallel feed the cathode of one LED, the anode of that tied to V+. The output of Inverter#1 also feeds the inputs of inverters #5 and #6. The outputs of inverters #5 and #6 in parallel feed the cathode of the other LED, with the anode tied to V+. The V+ will need to be about 12 volts to provide the proper drive to the LEDs.
While the circuit sounds complex the wiring is very simple and the parts count is abut as low as it can be. The inverters serve as current drivers and so there should be no need for an LED current limiting resistor. The inverters are used to pull down instead of source because they have a much greater sink capability than heir source ability.
 

MisterBill2

Joined Jan 23, 2018
8,709
OK on the loading dock style of lights, that is a lot different than just an LED. Is there an option of mains power for the light? ALL of the loading dock lights that I have seen are quitre bright so that truck drivers can see them from a distance.
 

Tonyr1084

Joined Sep 24, 2015
5,956
View attachment 197810

Huh? This circuit will result in the instant destruction of the transistors- shorting the power supply to 2X Vbe.
Please explain how you feel this will result in self destruction. I see a high input turning on the lower transistor while holding the upper transistor off, causing a low at the output. The next scenario, a low at the input will turn the upper transistor on and turn the lower transistor off, causing a high at the output. Am I wrong? If so - I can take criticism. If I learn from my mistakes - I learn something. And I certainly don't purport to know "everything". In fact, there is more I don't know than what I DO know.

In a thread I posted recently I used the opposite setup with an emitter follower that worked. Unfortunately in my circuit, the device the buffer was powering has an internal short, causing excessive amounts of current to be drawn, which resulted in the destruction of a PNP transistor. Not being the most "highly skilled" person on this website, I make mistakes. Sometimes I give bad advice. When I learn it I amend my comments - or remove them altogether. So if you can teach me something then please - by all means, teach away.

If one is concerned about the circuit self destructing - then just use a CD4049 buffer. They can give either a high or low output, which can toggle the LED's as the thread starter is seeking to do. Only, with a 4049, there are six in a package and only one would be needed - provided (and I don't know this) they can handle the current for the LED's.
 

Tonyr1084

Joined Sep 24, 2015
5,956
@MisterBill2 Here's what I saw on the page the TS provided as far as power supplies:

Stop And Go Light Technical Specifications
Housing: Safety Yellow Polypropylene
Power Source: 12 Or 24 volt DC operation or 115 volt AC operation
Lens diameter: 4 1/4″
Light Set Dimensions: 11 3/8″ H x 6 3/8″ W x 3 3/4″ D
Retrofit Kit Dimensions: 4.125″ H x 4.12″ W x 1.125″ D


I also read that they can be flashing design or steady design. They can even be automotive bulbs. So LED's are not specifically a necessity. But there are different models which may simplify what the TS wants to achieve. Here's one example:

Model SG20
This model is equipped with an internal flasher and selector switch. The switch will cause either the RED or GREEN lamp to signal alternately. The user is only responsible for providing a power source.


Why re-invent the wheel? The SG20 seems to have all the capabilities the TS wants.
 
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