LM555 for Air Cannon

[4pyros]

Ron; I finaly got a few uninteruped mins to try and relearn transistor stuff. Looking at the base resistor (R4) for the hold transistor (T2) that is now a 2N3904 using a fomula from your link RB=0.2XRLXhFE. 1st need hFE witch changes with Ic. Asuming the reset pin of IC1 has vary littel effect and with (R5) pullup (load resistor) of 1K ohms Ic is about 12ma giving me a hFE of 100 according to the 2N3904 data sheet.

So RB=0.2X1000X100=20,000 ohms or 20Kohms

and so to use a bass resistor of 10K ohms RL should be 100 ohms?

Am I close? Andy

 

[4pyros]

Can someone cheack my last post please? Andy

 

[ronv]

If your load is 1k (12 ma) your base drive should be about 1.2 ma (10K). Usually a gain of 10 is used to assure the transistor is in saturation (turned on hard) in a switching application. If you use Hfe the voltage from collector to emitter (when on) will rise. See Vce sat in the data sheet vs Hfe.

 

[4pyros]

Thanks ronv. Andy