# What is the purpose of the two 1N4148 diodes used in this circuit?

What is the purpose of the two 1N4148 diodes used in this circuit?

Regards

Edit:Source page of the circuit..

• Sure ...i don't know what this circuit do... I found it while searching about it circuit ^^ – armma Jun 16 '18 at 22:00

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.)

• +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. – Spehro Pefhany Jun 17 '18 at 13:23