# LED light activated beep circuit

#### bennu_500

Joined Apr 7, 2020
7
Hello, I wanted to see if I could get some help with a circuit. I am definitely a novice with electronics and reading diagrams but have made some simple circuits before so not a complete beginner.

The problem I am trying to solve is this. I work with a laser engraving machine that has a fairly bright blue LED that lights when the part/cycle is complete. For long cycle times I tend to multitask and do something else on the other side of the room or with my back to the machine. So I often get side tracked and the machine will sit idle until I remember to turn around and see if the complete LED is lit up. The stages of the complete LED are this. When the laser is idle and the door is open (loading/unloading) the LED is off. When you start marking a part and laser is working the LED is off. When the cycle is complete the LED will illuminate and stay lit until the door is opened and then it will turn off.

So my thought was if I could make a little tube/cover to go over the LED with a LDR or something similar to detect when the LED comes on and make a beep. Ideally I would like to replicate the LED outside the cover so as not to lose the visual indication as well, but could live without it. I found an interesting circuit someone posted awhile back for an alarm to indicate a garage door was open. But not sure how to properly adapt from the reed switch to the LDR or some sort of light sensing component. Title and link to the afore mentioned circuit below.

I have a fairly good stockpile of electronic components and breadboards as I keep thretening I am going to learn one day and try to incent myself by purchasing parts and pieces here and there. I also have a fairly good collection of wall warts of various voltages and a cheap variable power supply to be able to test. I have a good assortment of buzzers and speakers so if there is a preferred type to allow for a simple circuit I may have one. My preference is to create something that can detect the LED rather than hacking into the machines electronics and possibly shorting/damaging the machine as it would be a very pricey repair. It could also void any warranty/service or impact resale later on.

Feel free to ask me any questions and thank you in advance for your valued expertise.

Help on a Simple LED and Short Beep Circuit | All About Circuits

#### Tonyr1084

Joined Sep 24, 2015
7,199
We need to start with knowing what kind of buzzer(s) you have. If it's a piezo buzzer then you'll need a circuit to produce the buzz. Depending on the size and intended frequency of the piezo the matching circuit will need to reproduce the correct frequency.

On my freezer I have a simple timer and microswitch. When you open the freezer door, after approximately 30 to 40 seconds it switches a "SonAlert" which has its own circuitry internally. All I need to do is provide, in this case, 9 volts. Close the door and the timer resets. If you have something that sounds when power is applied then you're a lot further along in the project than having to build an oscillator circuit for a piezo.

If you have a mechanical buzzer, they draw more current than the other alerts mentioned. They also produce a lot of back EMF, which can fry small electronic components. But they're not beyond using. There are many ways to approach this project.

One approach would be to take the drive voltage that lights the LED and connect it to a MOSFET of the right sort. Use that to switch both the buzzer AND a mast mounted flashing light. But before we can start offering advice we need to understand what you have access to. Wall warts are good, whether you use 5VDC or 12VDC. You can use 12VAC but now you have to build a rectification circuit along with filters so that it doesn't produce electronic noise that can interfere with other electronics.

Really, we're flying blind so far. Help us help you.

#### SamR

Joined Mar 19, 2019
4,491
There are 2 types of "buzzers". Active (like the sonalert @Tonyr1084 mentioned) which makes noise when power is applied and passive which requires a frequency signal to drive it in order to make noise sort of like a small speaker. Now the LED is current driven component, but at what voltage? These are some of the details needed to help you. Can you draw the circuit that provides the signal for the LED with particulars such as voltage and any resistors or other components that may be involved along with their values?

#### bennu_500

Joined Apr 7, 2020
7
Sorry, I did not give a lot of info about what components I had as I figured let people suggest some common ones and if I did not have it I would just order one. If a sonalert buzzer makes the circuit easier to design and function I can order one that someone recommends. But I grabbed a few noise makers from a box and this is what they are.
KC-1206 ~ Piezo Buzzer, 12mm diameter, 6.5mm lead spacing, 5V 50mA 2.5kHz
Radio Shack 273-059 ~ 3-20V DC
EFM-290EDFW ~ Buzzers 9 V 20mA 3.2kHz 85dB @ 9V,
SD153BH ~ 150 Ohm Plastic Telephone Speaker

I have a large selection of LEDs, but let's say a standard Cree blue LED. I think about 3.3v and 20mA to power it if I remember correctly.

I have a good selection of caps, resistors, transistors, diodes, relays, voltage regulators and other odds and ends. But am willing to pick up any parts suggested for the simplest or at least most applicable design.

As far as integrating into the electronics of the laser this is not possible. For me anyhow. If I mess that up I am on the hook for about $20K to replace it. Hence why I wanted to make some sort of magnetic cavity with a light sensing component inside that I could adhere over the Indicator LED. It would seal out all ambient light. and when the LED illuminated it would trigger the circuit I am looking to make. I am a machinist by trade so I can figure out some sort of part to house the light detecting component and the new external LED to emulate what the original indicator LED of the laser is doing. So it would be a completely independent system. I can power with batteries or any required voltage from a wide array of wall chargers I have. Voltages of 3, 5, 6, 9, 12, 18, 24, 32, to name a few. Probably most are 500mA or better. #### ThePanMan Joined Mar 13, 2020 520 You mentioned a tube and an LDR. The problem with that is first - you have to make it light leak proof so that nothing else triggers it. Second is that you no longer have the advantage of using that indicator. But once you decide to go that route, a photo diode or photo transistor would work better than an LDR. And as @Tonyr1084 mentioned, a mast with a flashing light on top so it can be seen anywhere from in the shop. The SonAlert is a good suggestion too. I've used them before. Built an automated rocket launcher. Set the timer and when it gets down to 10 seconds it starts beeping once every second for about a quarter second each time. That way I had time to get out to the recovery field before the launch. My kids loved it. Tony: Can you get a link to the SonAlert you used? The TS might have something similar OR may need to order one. #### Reloadron Joined Jan 15, 2015 7,080 During my years before retirement the Mallory Sonalert was my go to device for countless alarms. I just looked them up and about had a coronary. They are great but great comes at a price. How loud depends on your ambient background noise. Something like this might work for you or similar. Generally alert devices like this come with a wide voltage range. Note in the link they mention 100 db and at 12 VDC but the units works from 3 to 24 volts and don't expect much at the low end. Yes, you could use a small LDR (Light Dependent Resistor and a few parts. The merit to just monitoring the LED is the circuit would be external to the machine. A handful of parts, actually a small handful. Drive the alert using a dimple 2N2222 transistor since it only draws 15 mA of current. Power everything using an inexpensive 12 Volt wall wart. You can buy a pile of cheap LDRs for about$6 on Amazon. Glue the LDR in a plastic straw (McDonalds or similar) to slip over your existing LED. This is a read on some basic simple circuits.

There are other options also.

Ron

#### BobTPH

Joined Jun 5, 2013
6,078
KC-1206 ~ Piezo Buzzer, 12mm diameter, 6.5mm lead spacing, 5V 50mA 2.5kHz
This one sounds promising. Connect it to 5V and see what you think.

The next two might work as well, try them wirh their specified voltage. I think all three will make sound on their own. The telephone speaker will not.

#### SamR

Joined Mar 19, 2019
4,491
What I was hoping I could get an answer to was IF the circuit had enough power before the current limiting resistor for the LED to power the alarm buzzer or even a switching transistor to separately power one. Possibly use a mosfet transistor to switch power to an active buzzer.

Joined Jan 15, 2015
7,080
What I was hoping I could get an answer to was IF the circuit had enough power before the current limiting resistor for the LED to power the alarm buzzer or even a switching transistor to separately power one. Possibly use a mosfet transistor to switch power to an active buzzer.
That has potential also. Hey, if you don't mind opening your control box there are other ways to go about it.

Ron

#### WBahn

Joined Mar 31, 2012
27,880
That has potential also. Hey, if you don't mind opening your control box there are other ways to go about it.

Ron
This was tucked in the TS's second post: "As far as integrating into the electronics of the laser this is not possible. For me anyhow. If I mess that up I am on the hook for about \$20K to replace it."

#### bennu_500

Joined Apr 7, 2020
7
Ok, I tested a 5V YXDZ buzzer and that makes plenty of noise for my purposes and I like the size.

Yes, cracking into the laser and integrating it would be too risky for me to attempt. It also won't be worth the time. It is in a very solid metal enclosure and the LED is located in the furthest spot from the access panel. Even if the actual board is near the access, there are enough LEDs in the same spot that I would need to probably access behind them to trace out which wires go to what. I would also need to modify the case to add the buzzer or at least pass wires through to a buzzer. I feel that is more effort/risk than the reward.

The LED is a 10mm (.394") LED with a 16mm (.630") metal bezel ring around it. There is enough space between multiple LEDs that I am confident I can come up with some sort of little attachable enclosure that will be able to seal out all ambient light. To see a pic of the laser/LED you can use this link. laser.jpg (640×480) (infinitycustomfab.com)

Thanks for all the input to date. I start another job very soon where I will actually have to start learning electronics (will still be using the laser) and this site seems like a great resource for questions. Very helpful community. I have certainly been on forums for other interests where people post/comment on threads with unhelpful information or criticism for no reason.

Joined Jan 15, 2015
7,080
Oyay, then I would do it using the link in post #6 just a few parts and replace the bulb with your buzzer. Make sure you get the polarity correct. I would use a 2N2222 for the transistor. I would also replace the 10K fixed resistor with a 10K pot so you can adjust the On/Off light level necessary to get your buzzer On/Off.

Ron

#### Audioguru again

Joined Oct 21, 2019
5,416
I think it is a bad idea to use a high frequency piezo beeper for a task because then if you become accustomed to hearing it you might ignore a same sounding smoke or CO detector.

I would use a dee-doo-dee-doo two-frequencies sound or the slowly ramping up and down frequencies of a siren, both produced from a medium-frequency speaker not from a high frequency piezo beeper.

At first I thought of using a medium-frequency squarewave buzz but it might not be noticed if there is electric guitar "music" noises being played.

#### bennu_500

Joined Apr 7, 2020
7
Looking at the circuits suggested in post #6 it seems the buzzer will be going off the whole time the complete LED is on? This will be a problem as if I run a 3 minute part on the way out the door to lunch the buzzer will be going off until I get back and open the enclosure door which extinguishes the LED. I would like the buzzer to just buzz for .5-1 seconds or so. It is just so if I am not staring at it I have an audible que that it is ready to be changed over.

The conditions of the laser/complete LED are 3.
1. door open to load/unload, LED = Off.
2. door closed and laser is working, LED = Off.
3. door closed and laser completed, LED = ON.
then when you open the door you are back at condition 1.

In my original post I was wondering what people thought about the garage door alarm that someone created. That thread linked below. Seems like it has very similar requirements only they used a reed switch to activate. I would just need to replace the reed switch with the light detecting LED functionality. I think the second version of circuit in that thread (one with two 555 timers) was so they could add reminder delay in the event they were working in the garage and would not have to listen to the alarm the whole time the garage door was open, but would not have to disable the alarm and then negate the whole system if they forgot to re-enable it. Seems that functionality would be ok for me as well. But technically all I need is to beep the alarm one time when the light goes on. Then don't beep again until the light is illuminated again.

Help on a Simple LED and Short Beep Circuit | All About Circuits

#### SamR

Joined Mar 19, 2019
4,491
Now you've added a 555 timer to the mix which will probably require a switching transistor to supply power to the buzzer depending on the power needs of the buzzer... All doable but will require a small PCB and enclosure which the buzzer can be mounted on. Using a potentiometer instead of a fixed resistor for the 555's timing resistor would give you the ability to adjust the buzzer time on.

#### Tonyr1084

Joined Sep 24, 2015
7,199
Tony: Can you get a link to the SonAlert you used?

This unit will beep for approximately 1/2 second and beep every second for I think six or eight beeps, then the rate will double and the time frame cut in half. After another six or eight beeps it again doubles, getting more and more urgent every six to eight beeps until it reaches its max rate, whatever that may be. I've never stopped to check it out.

#### Tonyr1084

Joined Sep 24, 2015
7,199
Mine is connected to an Op-Amp and a fairly long duration RC circuit. It's Monostable (if that's the right term) where it switches on after the RC times out. When the switch cuts out there's a high resistance across the capacitor to drain it off so that next time the freezer door is opened it's ready to signal a long duration door open alarm. As you can see from the photo it can operate on voltages from 6 to 16 volts. I'm running mine on 9V. Out of frame is a microswitch that keeps the battery disconnected at all times except when the door is opened. So there's no drain on the battery until you open the door. And if you open it, grab something, and close the door, the alarm never sounds. It's just a little obnoxious when you're putting food into the freezer. If you have the door open for longer than 30 to 45 seconds it starts screaming at you. Close the door for a minute and you get another 30 to 45 seconds before it starts screaming again.

If I wanted to modify my circuit I could arrange it where the alarm sounds the moment the door is open and then after a few seconds it shuts off. And yes, like SamR said, it's going to take a 555 timer.
Now you've added a 555 timer to the mix which will probably require a switching transistor to supply power to the buzzer depending on the power needs of the buzzer... All doable but will require a small PCB and enclosure which the buzzer can be mounted on. Using a potentiometer instead of a fixed resistor for the 555's timing resistor would give you the ability to adjust the buzzer time on.
Yes, I have a 2n7000 MOSFET to do the switching. From the picture you can see the alert draws 40mA Max. The Op-Amp can not output that much current.

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#### Tonyr1084

Joined Sep 24, 2015
7,199
Somewhere I have the component values but off hand I'm not spotting them. Here's the layout of the circuit board. R1, 2 and 3 are standing on end. The wire coming off the top of R1 & R2 connect to pin 2 of the Op-Amp. R3 (top) connects to pin 3 and C1 positive lead. Pins 2 & 3 are the inverting and non-inverting inputs to the op-amp, pin 1 is the output. Pin 5 is held to ground, pin 6 is tied to the second op-amp output (pin 7). C2 is on the order of something like 0.1µF

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