Newbie help - converting quick LED pulse to audio

Thread Starter

newbiefail

Joined Dec 7, 2013
12
ok,

White wire is now on the 'other' side of the onboard LED.
Device powered up with 5V
Resting - Black (0V) and White Wire 'other' = 4.9V
LED ON - Black (0V) and White Wire 'other' = 4.62V? I used a question mark here as I was sort of getting an average of this. Sometimes the reading didnt change from the 4.9v, but when it did, it usually went to 4.6v. Perhaps the LED flash is too fast for the DMM to recognise?
 

tbinder3

Joined Jun 30, 2013
30
ok,

White wire is now on the 'other' side of the onboard LED.
Device powered up with 5V
Resting - Black (0V) and White Wire 'other' = 4.9V
LED ON - Black (0V) and White Wire 'other' = 4.62V? I used a question mark here as I was sort of getting an average of this. Sometimes the reading didnt change from the 4.9v, but when it did, it usually went to 4.6v. Perhaps the LED flash is too fast for the DMM to recognise?
Oscilloscope would be nice to see something like that, DMM, not so much...
 

Alec_t

Joined Sep 17, 2013
14,280
Perhaps the LED flash is too fast for the DMM to recognise?
Yes it is; hence the 'sort of average'. But with the white wire in its new position it looks as though it's being switched low (i.e. probably to near 3V) briefly. That will suffice to trigger an audio circuit. I'll post a revised suggested circuit shortly.
 

Thread Starter

newbiefail

Joined Dec 7, 2013
12
Thanks Alec!!

Must be close now... the circuit works but it only comes on for the same amount of time as the onboard LED flash - its as if the 555 isnt 'timing'.

I modified your drawing as indicated to try and get a longer on time, but to no avail.
 

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Alec_t

Joined Sep 17, 2013
14,280
Strange. Simulation shows that with the values you're now using for R4/C4 the buzzer/LED should be on for ~1 sec. Presumably 1 sec is > the onboard flash time?
Check that Vcc (pin 8) of the 555 doesn't dip when the buzzer sounds.
 

Thread Starter

newbiefail

Joined Dec 7, 2013
12
hrmm.. still testing and writing down my progress...
now the timer seems to work perfectly for 1 second. but the strange thing is that its not liking the trigger input from the onboard LED.

If i trigger using some random earth/ground, the LED goes on for 1 second and then goes off - perfect. But if I connect the trigger to the onboard LED, the onboard LED turns on, like there is some feedback or +ve signal going out from the trigger...
ill keep testing! Might even go buy a few more 555's. This 555 has been through some brutal testing from my electronic trial and error!

I might also set it up one step at a time and test the connections with the DMM.
 

Sensacell

Joined Jun 19, 2012
3,432
The first step is getting a clean signal from the board.

The LED on the board is most likely driven from a digital output through a resistor. Locate the resistor by tracing the connection from the LED, one side of the LED will connect to GND or Vcc, while the other goes to a resistor that then connects to the digital output, (the LED and resistor can also be reversed) Just using the signal from the LED itself may provide a low signal voltage that makes things harder.
 

Thread Starter

newbiefail

Joined Dec 7, 2013
12
ITS WORKS!!!!!!!!!!!! :D
Im so happy right now!!
Thankyou so much Alec t and MikeML!

I rebuilt the circuit on the other end of the breadboard, took my time following the circuit and it works! :)

Last night, I couldnt figure out why the onboard LED stayed on with the new circuit. Turns out, the RFID reader needs to have the SD Card inserted!
Good failsafe really!

So now its a matter of reproducing this circuit 6 times! hahah
Also need to sort out the plastic mounting case and how the peripherals will plug in. Ill take a few more photos in the coming days to show the progress.

Thankyou again so much - there is no way I could have done this without your help! :)
 

Alec_t

Joined Sep 17, 2013
14,280
Glad you got it working. I was going to suggest removing R1 to cure the 'LED stays on' glitch; but it looks like that won't now be necessary.
 
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