Need some advice about this circuit

[Trevor Rymell]

Could someone tell me what the LED D13 is supposed to do in this circuit? It's a cheap FM radio kit I bought just for something to build. It works fine as a radio except this LED doesn't seem to do anything. I was careful to get the polarity correct. The silk screen printing clearly indicated the "+" lead - the longer of the two. It looks like it's part of the 4-digit LED display circuit. Sadly, I have little expertise with schematics. In the parts list it's listed as "LED white to Blue" with no model number.

I'm not sure if this is a fault with the design but the 4-digit frequency display causes an objectionable buzzing in the audio until it turns off automatically 5/6 seconds after selecting a station. It only comes on when being tuned.

There are complete assembly instructions here in case anyone wants to see the thing in detail

**broken link removed**

Thanks for reading.
Trevor

 

[ronsimpson]

LED13 indicates there is power.

 

[Trevor Rymell]

I wondered if it was just that but there's no voltage across the two pads so I guess I must have done something wrong in the assembly. Ah well.

 

[throbscottle]

There should be a voltage, as you have R1 in series with it. However they've put R1 at 10k, which seems very high for powering an LED (unless it's one of the ultra low current ones, I don't know what those will actually run on). Try changing it to 1k or lower. Although it seems like a pointless LED anyway if I'm honest!

 

[Trevor Rymell]

Thank you throbscottle. Yes, totally pointless. Unless they put it in because the 4-digit display can't stay on due to the RF noise the circuit creates in the audio. Pretty crappy design I'm thinking.

You're a Roald Dahl fan, I take it.