Adding a buzzer to an existing led diode

Thread Starter

broadways

Joined Jan 24, 2022
23
Hello friends,

Long time lurker here and finally decide to join forum.

I have a set top box. It has a 3mm green led(on front panel) that lights up when I connect the dish coax cable.

I measured the voltage across the led diode. It has 0v when dish cable is disconnected and about 1.2v(max) when I connect the dish cable.

I would like to add a buzzer to this diode. It should buzz when the green signal led comes on.

I have a few of those tiny buzzers from old pentium 4 motherboard PCs. I connected it across the led but only managed to get a low humming sound(I guess it lacks a signal generator?).

I want to use the voltage from the led diode as a trigger and an external 5-12v power supply to induce a loud 1200hz signal into the buzzer. I have a square wave generator which showed that the buzzer was its loudest at 1.2khz.

Any help would be greatly appreciated

Thank you
 

Reloadron

Joined Jan 15, 2015
6,507
I would just find an off the shelf suitable buzzer and drive it with a simple single transistor like a 2N2222. Keep in mind anytime the green LED is lit the buzzer is On. That the condition you want?

Ron
 

Yaakov

Joined Jan 27, 2019
5,476
I would just find an off the shelf suitable buzzer and drive it with a simple single transistor like a 2N2222. Keep in mind anytime the green LED is lit the buzzer is On. That the condition you want?

Ron
I wish the TS had provided information on what he is trying to accomplish with this arrangement so it would be easter to help. On its face, it's very confusing. It would seem having an audible annunciator that signaled what seems to be an ongoing normal condition is... not sensible. I can only assume it's something else and he intends to include a switch so he doesn't have the buzzer competing with the TV audio.

My first impulse was to recommend an optoisolator given that it is effectively a second LED and provides isolation to protect the set top box from whatever he builds on the other side. My second was to wait and see if anything more substantial appeared in the thread.
 

MisterBill2

Joined Jan 23, 2018
11,556
Hello friends,

Long time lurker here and finally decide to join forum.

I have a set top box. It has a 3mm green led(on front panel) that lights up when I connect the dish coax cable.

I measured the voltage across the led diode. It has 0v when dish cable is disconnected and about 1.2v(max) when I connect the dish cable.

I would like to add a buzzer to this diode. It should buzz when the green signal led comes on.

I have a few of those tiny buzzers from old pentium 4 motherboard PCs. I connected it across the led but only managed to get a low humming sound(I guess it lacks a signal generator?).

I want to use the voltage from the led diode as a trigger and an external 5-12v power supply to induce a loud 1200hz signal into the buzzer. I have a square wave generator which showed that the buzzer was its loudest at 1.2khz.

Any help would be greatly appreciated

Thank you
Since you are already opening the box, the simple approach will be to find the current limiting resistor for that green LED and connect the electronic buzzer to the other side of the resistor, which should give you five volts and plenty of sound. That is a much simpler way to do it.
 

Yaakov

Joined Jan 27, 2019
5,476
Hello friends,

Long time lurker here and finally decide to join forum.

I have a set top box. It has a 3mm green led(on front panel) that lights up when I connect the dish coax cable.

I measured the voltage across the led diode. It has 0v when dish cable is disconnected and about 1.2v(max) when I connect the dish cable.

I would like to add a buzzer to this diode. It should buzz when the green signal led comes on.

I have a few of those tiny buzzers from old pentium 4 motherboard PCs. I connected it across the led but only managed to get a low humming sound(I guess it lacks a signal generator?).

I want to use the voltage from the led diode as a trigger and an external 5-12v power supply to induce a loud 1200hz signal into the buzzer. I have a square wave generator which showed that the buzzer was its loudest at 1.2khz.

Any help would be greatly appreciated

Thank you
By the way, welcome to AAC. Glad you decided to join.
 

Thread Starter

broadways

Joined Jan 24, 2022
23
I actually use the box as a satellite finder to rotate the satellite dish. When I'm on the roof, due to the sunlight, it is hard to see the lcd screen. Plus when I'm rotating the dish, my eyes are primarily on the dish mechanism so I'm looking at the lcd screen and then at the dish back and forth.

A buzzer would have been very helpful in this case.

I have an option of using an 817 optocuplar but I'm looking at other options also. The 555 circuit allows me to change the frequency (tone) of the buzzer(by changing the value of the capacitor). It also allows me to change the intensity.

I have an 5v electromagnetic speaker. It is bare and has no amplification or oscillation circuit inside it. When I connect it across the led, there is just a slight buzzing noise.

I don't know how to use the 2n222 transistor. I thought a coil was used to amplify the voltage and the transistor simply converted DC to ac?
 

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MisterBill2

Joined Jan 23, 2018
11,556
I actually use the box as a satellite finder to rotate the satellite dish. When I'm on the roof, due to the sunlight, it is hard to see the lcd screen. Plus when I'm rotating the dish, my eyes are primarily on the dish mechanism so I'm looking at the lcd screen and then at the dish back and forth.

A buzzer would have been very helpful in this case.

I have an option of using an 817 optocuplar but I'm looking at other options also. The 555 circuit allows me to change the frequency (tone) of the buzzer(by changing the value of the capacitor). It also allows me to change the intensity.

I have an 5v electromagnetic speaker. It is bare and has no amplification or oscillation circuit inside it. When I connect it across the led, there is just a slight buzzing noise.

I don't know how to use the 2n222 transistor. I thought a coil was used to amplify the voltage and the transistor simply converted DC to ac?
THAT is why I suggested what I did. Actually, you can make a simple oscillator and driver for that EM speaker using a CD4049 hex inverter, and then the remaining four inverters as a bridge-output speaker amp. It will run very well on the 5 volts that are feeding the LED and it's dropping resistor. You do need to trace back so that you can locate the other side of that resistor.
 

k1ng 1337

Joined Sep 11, 2020
413
Note there are passive and active buzzers. Passive requires an AC signal across the bare piezoelectric element such as a 555 square wave where getting sufficient volume can be a pain. Active buzzer has the circuitry inside the package and only requires a DC input voltage of 5V and can be quite loud (and annoying). Placing (or replacing) an active buzzer in parallel with the LED and resistor could work if the voltage across them matches the buzzers needs as it doesn't need much current. Both types of buzzer are dirt cheap. You can test them by putting a DC voltage across, the ones that buzz are active.
 

Thread Starter

broadways

Joined Jan 24, 2022
23
k1ng 1337,

I did reading on buzzers and found out the difference between active and passive buzzers.

I will have to go to the hardware store and grab an active buzzer. Quick google search reveals that there are 3v, 5v and 12v types. If I get a 3v type, that is still 2v high than what I need. Can I use a coil or a transistor to amplify the 1v to 3v?
 

AnalogKid

Joined Aug 1, 2013
9,931
1.5 V seems a bit low for a green LED. And, there is nothing to suggest that either end of the LED is connected directly to either Vcc or GND, so you have to treat the 1.5 V signal as floating. This presents a grounding problem for the beeper circuit.

On the assumption that this circuit is not on 24/7, rather than use an opto-coupler for ground isolatiion I suggest battery power. This eliminates a lot of issues. A 1-transistor PNP circuit can disable the 555 Reset input, allowing the 555 to drive the speaker. 2N4403, 2N3906, 2N2907.

ak
 

MisterBill2

Joined Jan 23, 2018
11,556
1.5 V seems a bit low for a green LED. And, there is nothing to suggest that either end of the LED is connected directly to either Vcc or GND, so you have to treat the 1.5 V signal as floating. This presents a grounding problem for the beeper circuit.

On the assumption that this circuit is not on 24/7, rather than use an opto-coupler for ground isolatiion I suggest battery power. This eliminates a lot of issues. A 1-transistor PNP circuit can disable the 555 Reset input, allowing the 555 to drive the speaker. 2N4403, 2N3906, 2N2907.

ak
You can continue to ignore the fact that if the logic is 5 volts then there is a resistor limiting the LED current, and on the other side of that resistor is a higher voltage and probably a higher available current. And the 4049 can provide a bridged output giving double the supply voltage to drive that piezo sounder. And the oscillator needs only one resistor and one capacitor for the whole package to drive the sounder with a nice square wave.
 

AnalogKid

Joined Aug 1, 2013
9,931
You can continue to ignore
Calm down there, Sparky. I'm not ignoring anything, but I'm also not making any assumptions. All we have from the TS is a signal that goes from 0 V to 1.5 V, no indication if either end of the signal is at GND or Vcc, and an initial suggestion of a 555 circuit.

As is well-documented, I'm not one to push a 555 into all, or nearly any, applications. But it fits this one, in that a 50% square wave output also takes only one resistor and one capacitor (assuming the decoupling capacitor is a constant across all possible circuits), and can drive just about anything reasonable with its extra-beefy output stage. One drawback is its relatively high static current, but I covered that.

Gotta run to get a replacement airbag (product recall). More later.

ak

And -
... the fact that if ...
Um ... no.

If it is a fact, there is no "if". If there is an "if", it is not a fact.
 
Last edited:

k1ng 1337

Joined Sep 11, 2020
413
k1ng 1337,

I did reading on buzzers and found out the difference between active and passive buzzers.

I will have to go to the hardware store and grab an active buzzer. Quick google search reveals that there are 3v, 5v and 12v types. If I get a 3v type, that is still 2v high than what I need. Can I use a coil or a transistor to amplify the 1v to 3v?
I wouldn't rush out to buy anything just yet, make sure you understand first.

What is the model of your device? Maybe someone can pull up a schematic which will help a lot.

If not you should first perform nodal analysis on the nodes where you intend to connect your external circuit, it's quite possible the system is complex which requires a complex solution.

Is there a resistor in series with the LED? If so that would suggest the positive terminal (to the LED) is a voltage source where current is limited by the resistor. In this case, the voltage is the independent variable and current is the dependant variable.

If there is no resistor then that would suggest it's a current source in which current is limited upstream. In this case current is the independent variable and voltage is the dependant variable.

It's worth noting in both cases that an LED will always have a corresponding voltage/current characteristic where if the voltage is set to 3V (for example) then the current will be 20mA, likewise a current source of 20mA will yield a voltage of 3V.

My suggestion was meant to get you thinking about options, as one user pointed out: It's possible the LED is not directly connect to VCC or GND and may be floating which complicates things.

Don't be afraid to ask more questions, everyone wants to help just be prepared to put in some work otherwise we are just guessing.
 

dl324

Joined Mar 30, 2015
14,303
I measured the voltage across the led diode. It has 0v when dish cable is disconnected and about 1.2v(max) when I connect the dish cable.
1.2V is low for a green LED. It might not be on constantly when it appears to be on.

I'd suggest using a comparator to drive a buzzer that only requires DC to buzz. That would give you precise control of the voltage that triggers the buzzer. You'll need a transistor if the buzzer requires more current than the comparator can sink.
 

LesJones

Joined Jan 8, 2017
3,701
I think the LED only indicates that the LNB is drawing current. I don't think it has anything to do with the strength of the received signal. The normal way to position the dish is to use a satellite signal strength meter which is connected between the LNB and the coax that goes down to the satellite receiver. The signal on the coax is NOT at the received frequency from the satellite. The LNB mixes the received frequency with a fixed frequency from an oscillator. The signal down the coax is at frequencies around 1 Ghz. The signal strength meter is like a wide band receiver that will indicate the strength of any signal being received. I think the TS needs to confirm that the light only comes on when the dish is pointing at the satellite.

Les.
 

AnalogKid

Joined Aug 1, 2013
9,931
Here is a first pass at a schematic. The enable circuit for the 555 looks a bit complex. That's because until we know more about the true nature of the control signal, it covers several bases regarding what might be coming from the cable box. Note that this is a concept schematic for discussion, not a final design. For example -

1. 1.5 V is enough to turn on Q1, which enables the 555, which drives a speaker.

2. I added in C2 on the chance that the cable box LED is being driven by a square wave or PWM

3. The circuit power source must be completely floating from any power system in the cable box.

4. If the beeper is a DC device (like a magnetic buzzer or some piezo devices), then U1 can be eliminated. In this case, Q1 drives the buzzer directly. For this, R2 might have to be decreases a bit.

ak
!Cable-Box-Alarm-1-c.gif
 

k1ng 1337

Joined Sep 11, 2020
413
I think the LED only indicates that the LNB is drawing current. I don't think it has anything to do with the strength of the received signal. The normal way to position the dish is to use a satellite signal strength meter which is connected between the LNB and the coax that goes down to the satellite receiver. The signal on the coax is NOT at the received frequency from the satellite. The LNB mixes the received frequency with a fixed frequency from an oscillator. The signal down the coax is at frequencies around 1 Ghz. The signal strength meter is like a wide band receiver that will indicate the strength of any signal being received. I think the TS needs to confirm that the light only comes on when the dish is pointing at the satellite.

Les.
I was wondering how TS plans to tune to a satellite with just an LED or buzzer.. I used a deflection meter as you described to dial in and even then the slightest movement took it out of alignment. Eventually we gave up for lack of specialized equipment. At the time we were trying to intercept and decode with codes found on the internet from insiders working at the sat companies. We did have some success but it was a huge hassle and paying a subscription was worth it. This was 10 years ago.. I wonder how technology in this department has changed.
 

Thread Starter

broadways

Joined Jan 24, 2022
23
I found an easy way to accomplish what I want using an 817 optocuplar.

For test:
I fed leg1 and leg2 of 817 with 1.2v using bench power supply. I connected buzzer to another 5v supply, as shown in the picture. It started buzzing perfectly as soon as I feed 1.2v. Awesome.

Now on the set top box side, the connections are:
Pin 1 of 817 is connected to anode of led
Pin 2 of 817 is connected to cathode of led
Buzzer connected using 5v line from USB port as shown in picture.

But something weird happened. The buzzer started making a weird buzzing noise like the one you hear at the overhead transmission lines(50hz?).

I disconnected the 5v line and connected it to the 12v line and it gave the same noise, just louder this time.

Anyone knows what's going on?

k1ng 1337,
The led positive comes from a chip(fd650). There is no resistor for this led.
 

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