URGENT HELP NEEDED: 555, transistor, monostable

SgtWookie

Joined Jul 17, 2007
22,230
The reason that your transistor doesn't seem to be working properly is because there is no resistance in the base circuit.

The capacitor C2 charges via R7 while the output of the 555 timer is high. When your selected time-out occurs, the output of the 555 goes low, near 0v.

C2 now discharges through the base-emitter junction of the NPN transistor. Since there is no resistance between the capacitor and the base, very high current flows from C2 through the base-emitter junction to the 555 for a brief moment or two. It may be so brief that you never even see LED3 or hear B1 (the buzzer) sound.

The cure is to put a resistor between C2 and the base of the NPN transistor. 3.3k should give you about a 1/2 second light and buzz. If you need to increase the time that the buzzer sounds/LED3 is on, you will need to increase the size of C2.
 

Thread Starter

gunexer

Joined Mar 30, 2009
12
yeah, infact, before you guys told me to switch around the positions, I tried replacing the S3 with the resistor and it didn't work with croc clips, I tried it the other way around and it works. So thanks guys, through this I understand more the 'behind the scenes' functionality in the 555. :)
 
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