TTL headaches: driving a BC bc557b

[csdave]

sorry I mixed two topics in the same thread.

regarding the 2n2222 I was wondering if 100ohms would harm or if it would just be overkill.

regarding the bc547, I am trying to see if my circuit to discharge the capacitor works and if my analysis is correct.

 

[Audioguru]

A 160 ohm or 150 ohm base resistor will be fine for any passing 2N2222 transistor. 100 ohms will not harm it.
Your two BC547 transistors will discharge the 555's timing capacitor.

 

[csdave]

Indeed right, except that I didn't consider that when the bottom BC547 is off and the top one is on, the capacitor would quickly charge through the base-collector junction! Indeed not what I want.

I instead put back the 557 with a resistor on the collector to limit the current (even though I am not sure it is necessary. I am opening another thread on this. )

---------- Post added at 02:31 ---------- Previous post was at 02:03 ----------

this solution, **broken link removed**, should do what I want and at the same time guarantee that the power dissipation in the transistor never exceed 500mW and that the current never goes beyond 100mA. Actually it should be somewhere around 75mA when the capacitor is fully charged. At the same time, discharge should be fast enough for my purposes.

WDYT?

 

[Audioguru]

I instead put back the 557 with a resistor on the collector to limit the current (even though I am not sure it is necessary.

You do not have a resistor on the collector. Instead the 40 ohm resistor is on the emitter.
The LS-TTL gate's output high is about +3.4V so the emitter goes up to about +4.0V.
When the transistor conducts the voltage across the 40 ohms is about 2.9V which is a current of 2.9/40= 73mA.
The maximum dissipation in the transistior is 0.9V x 73mA= 66mW.