problem with 555 timer!

Thread Starter


Joined Jan 3, 2007
in basic 555 timer circuits,there are two resistor and 2 capacitor ( one in pin 5), 4 and 8 connect to Vcc,and then they run,simple!

now i want to control it!

i disconnect pin 4 (floating),the 555 no OSC.Reconnect it to Vcc,555 run again

so i used NPN transistor(ex: C1815),C to Vcc,E to 4,B have a resistor and connected to controller.Result : in logic 0 and 1,555 still run.(no problem with transistor)

in fact,if i use PNP transistor ( Ex: A1015),E to Vcc,C to 4,B have a pull-up resistor and connected to controller.There are no problem.Logic 0: 555 run,logic 1:555 off

anybody ask me why ?


Joined Apr 20, 2004

You can do the control with your NPN transistor. Use a resistor of about 1000 ohms to pull up pin 4 (reset). Connect the transistor so the emitter goes to ground, and the collector goes to pin 4. When the transistor is on, the 555 will stop oscillating.

Thread Starter


Joined Jan 3, 2007
ok,i tested before and it run!

so i just want to ask you why i have problem with NPN transistor,because the transistor is off when logic 0,and the timer must be stop! continue to run :(

and that problem not happen when i use PNP transistor


Joined Dec 12, 2004
When you use NPN transistor, the NPN transistor is with common colector, and in this mode think that the base resistor is connected to +Vcc, when the base signal is 1, and therefore as the pin 4 is connected to +Vcc. Or when the base signal is 0, through EC of the NPN transistor however a current flows, reverse current (that is bigger than that when the transistor is with common emitter) , because the emitter of the NPN transistor has not resistor connected to the ground, and so think that the pin 4 is as connected to +Vcc with a resistor with 10M. Place a resistor between the emitter of the NPN transistor and ground to avoid this malfunction.

When you use PNP transistor, the PNP transistor is with common emitter, and when the base signal is 0, the PNP transistor is ON and its collector (the pin 4 of NE555) is on +Vcc. When the control signal is +Vcc the NPN transistor is blocked and therefore the pin 4 is like disconnected.