# The base resistor calculation for NPN transistor

Thread Starter

#### Delian Diver

Joined Jul 11, 2017
13
I want to make a random gate signal generator for my modular synth. I looked at different designs before making it myself and found that there are a lot of output buffers variations. So I lost myself in getting to know what is the better way to implement this OUTPUT AND LED BUFFER.
So the basic concern is to output the GATE signal from CD4051 to the sequencer in random manner and to visualize the current OUTPUT with LED.
CD4051 is powered from +12V.
The OUTPUT GATE has to be +5V (divider?)
So what is the better way to implement the BUFFER and how to calculate a perfect values for the RB, RG and RLED in this task?

#### ericgibbs

Joined Jan 29, 2010
11,156
hi DD,
What is RG output connected too.?
Also details of the LED would help.

E

Thread Starter

#### Delian Diver

Joined Jul 11, 2017
13
hi DD,
What is RG output connected too.?
Also details of the LED would help.

E
The RG resistor is GATE output and common value here for the SERGE SYNTH format is 330 Ohm.
The LED i'm going to use is common 5V red LED.

Thread Starter

#### Delian Diver

Joined Jul 11, 2017
13
Also there is this similar by the function circuit but it is different in connecting the LOAD. What do I have to know about the way I connect the buffer (collector or emitter) and what is the best way?

#### ericgibbs

Joined Jan 29, 2010
11,156
OK,
What voltage is powering the CD4051.
I am assuming that the LED current is approx 20mA, also a RED LED has a forward voltage drop of 2V, not 5V.
E

#### ericgibbs

Joined Jan 29, 2010
11,156
hi,
If you connected as per drawing in post #4, the outputs would be inverted compared to the original circuit
E

Thread Starter

#### Delian Diver

Joined Jul 11, 2017
13
OK,
What voltage is powering the CD4051.
I am assuming that the LED current is approx 20mA, also a RED LED has a forward voltage drop of 2V, not 5V.
E
CD4051 is powered from +12V.
Yes! 2V

#### Dodgydave

Joined Jun 22, 2012
9,686
Then your transistor Emitter will output 11.3V , assuming your led takes 10mA , then R49 can be 1k, 2N3904 has an hfe of 300, so the Rb resistor can be 10K,

Last edited:

#### ericgibbs

Joined Jan 29, 2010
11,156
OK,
So do you want the LED to light and the Gate output to go High to approx 12V, when the output of the 4051 is High ie: 12V.?
As per your original circuit
E

Thread Starter

#### Delian Diver

Joined Jul 11, 2017
13
OK,
So do you want the LED to light and the Gate output to go High to approx 12V, when the output of the 4051 is High ie: 12V.?
As per your original circuit
E
Exactly! But the GATE output should be slightly more that 5V (around 5.1V or 5.2V).

#### ericgibbs

Joined Jan 29, 2010
11,156
Hi,
This should be close to what you are asking.
The resistors on the out put may need a little adjustment, due to the Gate loading.
E

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