I made this circuit, with the intention of it lighting and then fading out the led after very briefly closing the switch:

enter image description here

This is intended for a CR2032 coin cell battery power source and the led is a normal 20ma through hole red LED, probably Vf =~ 2v.

Ignoring the one resistor in series with the power supply, what function describes how the transistor and the values of capacitor and resistor determine the led brightness and how long it takes to fade out? Like if I wanted to make it fade out 50% more quickly or more slowly, how would I change the resistor and/or capacitor?

Does the characteristics of the transistor matter?

  • \$\begingroup\$ You don't mention the value of the voltage source, nor the rated (or maximum continuous) current through the LED. We need those values before being able to say anything about the transistor and its base resistor. \$\endgroup\$
    – HarryH
    Jun 12, 2018 at 3:07
  • \$\begingroup\$ updated: it's a 20ma red led, voltage = 3V coin cell \$\endgroup\$
    – J Halcres
    Jun 12, 2018 at 4:13

1 Answer 1


Its a filter with an RC time constant

\$ R*C = \tau\$

the R or the C part can be changed to change the time constant. If you want to increase the time constant then increase R or increase C (or both).

In this case you will change the bias current of the transistor, so only change the capacitor.

  • \$\begingroup\$ are there any characteristics of the transistor that matter? \$\endgroup\$
    – J Halcres
    Jun 12, 2018 at 2:03
  • \$\begingroup\$ Yeah, edited, if you like the answer upvote \$\endgroup\$
    – Voltage Spike
    Jun 12, 2018 at 3:03

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