Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Connection of the transistor in the discharge path betn 7 & 6 of 555

  1. Jul 19, 2013 #1
    In an automatic school bell circuit, The outputs from 3 to 11 are connected to the base of an NPN (BC547) transistor through 1K resistance. A diode is connected in forward bias in each of those outputs so that only one output sends high pulse to the base of the transistor. When the high pulse is activating the transistor and makes it On it gives the discharging path of the 555 timer. The transistor is connected in between pin 7 and the shorted node of 6 & 2. There are two resistances in series with the emitter and the collector is connected directly to pin 7. Pin 7 is connected to 12V source through 10K resistance. I think the transistor connection will be reverse i.e. emitter will be connected to 7 instead of the collector. Otherwise the transistor will not make the discharge path when the 555 timer is in astable mode.Am I write? But practically my idea does not work. What is the problem? pls help.
  2. jcsd
  3. Jul 20, 2013 #2


    User Avatar
    Science Advisor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook