What is the purpose of the two 1N4148 diodes used in this circuit?
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Edit:Source page of the circuit..
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\$\begingroup\$ Sure ...i don't know what this circuit do... I found it while searching about it circuit ^^ \$\endgroup\$– armmaCommented Jun 16, 2018 at 22:00
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1 Answer
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When pin 3 of the timer is low, the voltage at the base of the transistor is about 1.4 V below Vcc, and therefore the voltage across the 180 \$\Omega\$ resistor is around 0.7 V. So the current through the transistor and LED's is about 3.9 mA, regardless of variations in supply voltage (which I guess is a 9 V battery, and can vary a lot). Hence the LED's should stay at the same brightness for a wide range of supply voltage.
In a few words - it's a current source circuit. (Good one to memorize, it comes up a lot. The other common way is to use a zener instead of the two diodes.)
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1\$\begingroup\$ +1 A variation is to replace the two series diodes (~1.4V) with a red LED (~1.8V), which gives you an indicator LED for free. \$\endgroup\$ Commented Jun 17, 2018 at 13:23