# RC timer not reaching desired voltage

I'm trying to make a circuit that will light the led with progressive intensity one second after the circuit is powered. For that I created an RC net with tau equal to 1 sec. At 1 second I expect the Vb to reach 63% of the dc voltage(1.2v) which is ~0.7v, that will forward bias the transistor and the led will start lighting. The problem I got is that the voltage at the test point gets stuck at 555 mv when I add the transistor. I'd expect it to eventually reach the 1.2v. Moreover, this doesn't happen if I reduce the RC values to, i.e 1K and 1mf, for maintaining the tau constant.

Why the voltage at the base of the transistor doesn't reach the 1.2v? • How much base current does your transistor need to light the LED? How much current can it get through a 1M resistor? Do you see the problem? – Dave Tweed Jun 8 at 11:04
• I thought that the current at the led was determined by the 5v and the 470 ohms resistor, which gives 10mA. – perencia Jun 8 at 11:11
• Well, yes, assuming that the transistor has minimal voltage drop. But what does it take to achieve that? – Dave Tweed Jun 8 at 11:14
• You seem to have a fundamental misunderstanding about how bipolar transistors work. You need significant current flowing into the base terminal, and a 1 megohm resistor probably will not provide enough current. – Elliot Alderson Jun 8 at 12:22

## 1 Answer

To get the BJT to work properly you need enough current to flow through the base terminal, and in your case due to the high resistance you are only getting: $$I= V/R$$ $$I = 1.2/10^6 = 1.2 \mu A$$

Thus the base current is too low to function. While when you reduce the resistance to 1K ohms only, the base current will be 1.2mA which is sufficient enough.