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Circuit works with any LED, s long as one is red.

Looeewoo

Jan 4, 2024
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Jan 4, 2024
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I have put together the circuit illustrated, the author states that it should work with any colour of LED.
If I use a red LED, it works. Any other apart from orange fails to light.
BUT - if I insert any other colour of LED(s) in parallel to the red LED, they all flash in unison.
Can anyone explain why this should be?
Changing R values varies the flash rate, but makes no apparent difference to which LEDs are useable.
T1 is BC547
R1 is 3K ohm
R2 is 100 ohm
C1 is 470 uF
input is 12V (max 2A).

 

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AnalogKid

Jun 10, 2015
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Here is a link to what probably is the origin for this circuit.


You might be right on the hairy edge of your particular transistor's avalanche voltage. Power the circuit with two 9 V batteries in series and see if the LED sensitivity remains.

ak
 

Harald Kapp

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Nov 17, 2011
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I agree: This circuit is sensitive to the transistor parameters. Since red LEDs have the lowest pass voltage, the remaining voltage may suffice to make the LED blink, but the higher pass voltage of yellow or green LEDs may stop the circuit from flashing.
Follow @AnalogKid 's tip.
 

Looeewoo

Jan 4, 2024
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Jan 4, 2024
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I swapped out the BC547 for a BC548 and lo and behold, it worked. I used a 220uF capacitor to get the flash rate `i needed.

Thanks all for your help.
 
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