Using an LED to trigger an audible alert

[ei8ht]

I have a barcode printer with a verifier attatched. When the verifier senses a bad label, it throws the printer into an error condition, and an error LED blinks. I would like to use this blinking LED signal to trigger an audible alert.&nbsp;&nbsp;I have traced the line that carries the flashing voltage, which in a normal state (non-error) is 3.5VDC. When the error light is flashing, the voltage is alternating between 2.5vdc and 3.5vdc, with the light coming on at peak. I measured from line to chassis ground. This isnt what i expected to find, but why it works that way doesnt really matter if i'm correct in how i want to use it.<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;I intend to have the buzzer on its own self-powered circuit. I have a 27VDC or 5VDC auxiliary power supply to use for the buzzer. My plan is to use the 3.5VDC to bias a transistor at the Collector, and use the drop in voltage as the trigger to close the Base-Emitter switch, turning on the buzzer. Can anyone tell me if i have the configuration correct, and if not, please tell me the right arrangement. Also, what transistor and/or buzzer do you recommend for those operating ranges?<br>

 

[lkm]

Hi,<br><br>If I face your problem, I'll try to detect the blinking signal first. You can use differential circuit, build by a cap and an operation amplifier with some basic resistors network, to detect it. The output of the detector circuit will be triangle or square wave. You can build the integral circuit to hold the error wave and trigger your buzzer circuit.<br><br>I hope it can help.<br>

 

[13]

I would definitely vote for using the LED voltage signal rather than the blinking signal. What about driving a solid state relay with the voltage signal and switch the power to the alarm using your auxilliary power supply?
A piezo alarm through McMaster-Carr would do the job and they have a wide selection of power inputs. [sig][/sig]

 

[ei8ht]

Thanks 13, Thats exactly what i would like to do, except, my specialty is in computer upgrade/repair, and i have no idea which parts to try to order. I have a McMaster-Carr account, if you could give me a rough idea of the model number or specs on the components i need, like the relay and piezo, i would be eternally grateful. [sig][/sig]

 

[13]

For the piezo alarm look at part#5670T31 its a 3-28VDC .02Amp alarm. it probably is reallllly annoying. but it will get someone's attention. use the voltage to tune it to a reasonable volume.

as for the relay..im not sure yet. to use the voltage signal going to the led, first i would trace this wire back to its origination and measure it there. you need to know what voltage you are dealing with. plus the key here is to find a relay that will work at that voltage, and also take the cycling. some relays cant cycle that much. how fast is the led blinking? can a relay work? im not sure

hope this helps [sig][/sig]

 

[13]

look for a relay in allied electronics catalog-->

http://www.alliedelec.com

[sig][/sig]