Easy electronics project question

I have a bunch of sensors, and when they activate, they have 2 I/O pins that give off 7V. I am using the 7Vs to power an LED. I would like to also have a buzzer as well as an LED in case the lighting is not ideal to see the LEDs when the sensor gets tripped. My problem is that, I do not want ALL of the indicator LEDs to light up when I attach the buzzer, the LEDs all need to be independent.

How can I have the individual LEDs for each sensor light up independently when the sensor is tripped, and also get the buzzer to sound, without buying 8 different buzzers? I’m sure there is a simple solution but it’s late at night and my brain is mush right now. I’m wondering if I need a relay of some kind. The simplest/cheapest option will of course be ideal. :smack:

edit: I know this would be simple with a microcontroller, but I’d rather keep this discrete for now.

I think you just need a few transistors…

A diode on each leg will do it. (if you have BJT transistors but not diodes, you could use them.)

In TTL logic, its called dot OR gate. (TTL doesnt need diodes because transistors are diode based.)

> [DIODE]] > …the buzzer>. GND
> Resistor > LED > GND.

You can repeat the left hand side for all the sensors… And have only one buzzer.
You need the resistor with the LED to current limit … (The buzzer may need a current limit system too ?)


I might be missing something, but why can’t you just wire the buzzer into the each Sensor?

At them moment you have a setup like this:

			        - Sensor+			+ Sensor-
				|	 |			|	 |
				|	 |			|	 |
				|	 |			|	 |
				|	 |			|	 |
				 \	/			 \	 /
				  \    /		          \     /
				  -LED+			          +LED-

What you should be able to do is:

			+ Sensor-
			| \_________________________
			|  	 -_________         |
			|	 |         -  BUZZER+
			|	 |         |        |
			|	 |         |        |
			\	/          |        |
			 \     /	   |        |
			  +LED-            |       |
                                           |        |
				           |        |
				           |        |
                      			   |        |
				          /        |
				        /        /
				      /        /
 				    /         /
				   /         /
				  /        /
				|	+
				|	 |
				|	 |
				|	 |
				\	/
				 \     /	

The sensor is just acting a a switch. When it’s closed, current should run to both the LED and the buzzer, activating both. Since it’s all wired through the sensors and any inactive sensor will be an open switch, current can’t run back from the buzzer to any inactive LED. If for some reason that is a problem, eg to much current when two sensors are activated at once, you should be able to wire is a diode to ensure a one way flow.

You only need 1.5 volts to power an LED so feed the individual sensor outputs to the anodes of the appropriate LED and tie all the LED cathodes together and feed that into the buzzer and the other side of the buzzer to return/ground. Might want a resistor in there to fine tune the current, but if the buzzer can work with 5.5 volts, you should be good. If any LED is activated its current will be directed through the buzzer. Or as already mentioned above, isolate everything with transistors.

Really need a bit more information in order to properly describe a solution. Most importantly,need the specification of the buzzer. Then, what LEDs are being used, and what is being done about current limiting in the LEDs. Knowing something about the sensor outputs would useful too. Max current for one.

The most generic solution is the diode OR. But it may reasonably need a transistor to drive the buzzer.

I read the OP four times but am still very confused. How many sensors are there? How many LEDs are there? How many buzzers are there? What do you want to happen when one sensor is activated? What do you want to happen when more than one one sensor is activated?

He has n sensors with n LEDs, presumably each one labeled. When one sensor trips, the corresponding LED lights up, but no others, to let him know that that sensor is sensing. If multiple sensors go off at once, all of the corresponding LEDs light up, but no others. He now wants to add a single buzzer, so that when any sensor trips, he hears the buzzer, and knows to look at the LEDs to see which sensor did it.

This is a much clearer way of putting what I meant, thanks! I will check the responses later to see if any of them will work for me.

Edit: in this application, only one sensor should ever go off at a time, if that makes any difference.

So can’t you just put the buzzer across all the common legs of the LEDs? They’re already diodes, so any single LED lighting up will activate the buzzer (assuming it passes enough current, etc.)

For the buzzer, what voltage (or voltage range) does it operate at? Do you know how much current it draws?

For the 7 V output on the sensor, would you happen to know the max current it can produce?

Sorry for the late reply, I’ve been super slammed at work lately and haven’t had time to follow up on this side project.
Crafter_Man**, I will have to call the sensor manufacturer monday to ask them about the maximum amperage on the I/O pins, the manual does not make it clear. The voltage on the IO pins is 7V and I have to use a 220ohm resistor for my 2V LEDs. Assuming the buzzer draw is not a problem for the I/O pins what would you recommend to isolate the sensor outputs?

edit: The buzzer operates on 4V+ just fine, and is fine at 7V. The draw on it is so low that my test power supply is not even picking it up, it’s quite a small buzzer.

I like the idea proposed by others to “OR” the signals with diodes. So here is something I scratched up. Will it do what you want?

It seems to me that the diode ‘OR’ described above would work. Assuming only one sensor will be active at a given time. You will just need to size the resistor appropriately. Each sensor output drives an LED anode. The cathodes of the LEDS are all tied together and fed through a single resistor which then leads to the buzzer. Put a 4v zener diode across the buzzer to control its voltage. So now the resistor is dropping just 1v instead of 5v so a 47 ohm should do the job.

My schematic has the buzzer operating at around 6 V. If it indeed needs to be operated closer to 4 V, then I could add a regulator or do it with multiple diode drops.

In my schematic I decided to “play it safe” and isolate the LEDs from each other. The disadvantage is that it requires more components.

Oh, and if it’s not already obvious, my schematic shows just three sensors. Adding more sensors should be obvious.

Right. If the buzzer needs no more than 4V, then I was thinking of something along these lines, using the LEDs themselves for the diode OR logic.

That’s a nice, simple circuit. Much lower part count than mine. But there are some *potential *problems:

  1. If one sensor is putting out 7 V, and another is at 0 V, then you would be putting a reverse bias of 5 V across the LED for the sensor that is at 0 V. LEDs are* very* sensitive (and very unhappy) to reverse bias voltages. Of course, this assumes a sensor that is “off” is at ground potential. It may be at high impedance when it is off, which means this may be a non-issue.

  2. I don’t like using a common resistor for multiple LEDs. The more LEDs that are on, the dimmer they all get. In other words, the current through each LED will be inconsistent. And because the LEDs are not perfectly matched, some will be brighter than others.

  3. Using a zener diode as a shunt voltage regulator can work well for very light loads. But I am a bit nervous using one for a buzzer, especially when we have no idea what the current draw is.

Yeah, this is why we have design reviews. :smiley:

  1. I guess I was assuming that the input was like a switch, either sourcing 7 volts or open, i.e. as high impedance as we can get. But, you’re right, we need more information.

  2. I remembered reading that no more than one sensor would be active at a time. Going back to #10, the OP wrote: *" in this application, only one sensor should ever go off at a time, if that makes any difference."
    *Guess it hinges on the meaning of “should.”

  3. In #13, the OP said:* “The buzzer operates on 4V+ just fine, and is fine at 7V. The draw on it is so low that my test power supply is not even picking it up, it’s quite a small buzzer.”
    *I guess we can lose the zener as the buzzer is happy at all voltages up to 7V.Whether or not it requires its own driver circuitry depends on how much it actually needs.Can you tell my background is in large scale manufacturing? As in many thousands of units per month?

We come from opposite corners. :wink: I work in the areas of failure analysis, so “reliability” is at the forefront for me. :slight_smile: I tend to look at every little detail, for better or for worse. In my head I keep thinking, "What could happen? What don’t I know? How can it fail?