Electronics 2000

Hi

I haven't done electronics since GCSE (long time ago!!) but I figured I'd give it a go again. I got this circuit for a sound trigger and I wired it all together on my breadboard and it didn't work. So I've gone back to basics but I'm not sure this is right.

I have a 9V battery coming into a 330Ohm resistor into an LED with 20ma current. It doesn't say what voltage it's at so I took a guess at 3.2V as that seems the norm. I think the resistor is ample for this. Connected it all up and it works except that the LED is flashing. Is that right? I would've thought it would be on permanently.

All help appreciated. I will undoubtedly be asking more questions

Drew

There are flashing LEDs available, these have a built-in IC to flash them, normally at about 3Hz. If you look though the LED sideways you should see a small black 'blob' - this is the circuitry. It's a while since I used one, but from what I remember they don't need a series resistor (unlike a standard LED).

A quick search on Farnell (http://uk.farnell.com/kingbright/l-56bh ... dp/1142543) seems to suggest a forward voltage of up to 12V.

For a standard LED (red / amber / green non-flashing), forward voltage is 1.8-2V and current is 20mA. The led series resistor calculator on this site will also give you a suitable resistor value, as will my Electronics Assistant software. However, for the flashing LED you have this is not necessary.

Hey Simon

Yeah I see the little black blob. That makes sense. I hooked one of these up straight to the 9V power and it just blew up. The resistor sorted that though. Could the fact that this is a flashing LED with different requirements cause my circuit to fail?

What's the best way to troubleshoot my circuit? Multimeter? Post a pic of the schematic and my breadboard layout?

Thanks
Drew

The flashing LED you are using must be different - most seem not to need a series resistor. Depending on the function of this LED in the circuit it might be the cause of the problems. Best would be to post the schematic and layout, and I'll try to suggest some fault finding steps.

The LED will come on when the sound is triggered. This in turn triggers the flash (it detects the optic change) so I'll need to replace the LED to a permanently on one.

Here is the schematic I'm trying to follow

http://img.ironthumb.co.uk/external/soundtrigger.png

Here is my breadboard from the top

http://img.ironthumb.co.uk/external/shot1.jpg

And from the front

http://img.ironthumb.co.uk/external/shot2.jpg

Hopefully you can see all my connections.

Once this is working, I can adapt it by adding a couple more resistors and a couple of 1N4148's as shown here

http://img.ironthumb.co.uk/external/soundtrigger2.png

Hi,

Sorry for not replying sooner. I don't seem to be able to get to all the images, but I do have the one with the schematic. Start by checking again that what you have on breadboard really does match the schematic - it might help to 'reverse engineer' it by drawing out the schematic from what's on the stripboard and comparing it to the original. Also if you haven't already you definitely need to swap the LED for a standard (non-flashing) one. Check the transistors, diodes and LED are the right way around - transistor diagrams sometimes show the layout from the top, sometimes from the bottom, so it is quite easy to get these wrong!

After that you will need a multimeter - firstly check that you are getting a voltage out of the left-hand side of the circuit as shown on the schematic - i.e. where the 3 metre wire would connect. Making a sound into the mic should give you a voltage here (this . If you don't get an output try again with the receiver (right-hand) side disconnected, in case a fault in this is dragging the voltage down.

This should indicate which side of the circuit is a fault - or at least if the transmitter part works. You need to fix any faults with this side of the circuit before looking at the receiver.

Thanks Simon

I now have standard LED's and a multimeter.

All components are giving me a reading however the LED is 0.01. The 4.7uF cap jst before it gives me about 7V and drops as I keep connected to it. None of the other caps do this, they have a fixed voltage.

I took out the mic and just have the wire going to ground so I know there's no problem there.

I tried changing the 1M resistor to something smaller but that didn't make a difference. All my components are the right way round.

Just stumped. A lot of people have commented where I got this circuit from and they have it working fine, so I presume the issue is with me!! lol