I have no picture to show what my circuit is like, but I'll try to draw it and explain here as best as possible. The circuit contains a NE555 timer, working in the astable mode with a frequency of 50Khz, 10nF charging/descharging cap, 230 ohm discharging resistor and 2300 +230 ohm charging resistor. It's powered by a single DC supply 15V. So far so good. Powered by the same supply (15V), I have a flip flop circuit, which has no other connection with the 555 timer other than power supply ( which means they share the same ground and same +Vcc, and are not related in any other condition). This flip flop circuit is made by transistors, and it's as follows : enter image description here

The output is taken from the pnp collector, and the 11k resistor receives a pulse high to turn on, and to be turned off I connect by hand the ground through a wire into the npn's base. The problem is: when the 555 timer is not connected to the 15V power supply, the flip-flop circuit works well as intended, but when the 555 timer is connected, the flip flop circuit goes wild and even by applying a ground voltage to the npn's base it doesn't seem to be turned off as the voltage in the pnp's collector stays high(about 14.5V). What could be taking place here? I really thought of many things but none seem to make sense once they are not connected in any situation but through the same ground and Vcc levels.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Are the 555 and your flip-flop circuits bypassed reasonably close to their power and ground connections? You should have a 100nF disk capacitor with reasonably short paths (no more than an inch) to power and ground; one for the 555 and one for the flip-flop. \$\endgroup\$ – TimWescott Oct 28 '18 at 5:09
  • \$\begingroup\$ I have a 100nF right by the 555 circuit. The other doesn't have, though why would that be cause? I really don't understand \$\endgroup\$ – Iron Maiden Oct 28 '18 at 5:24
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    \$\begingroup\$ The short, practical answer, based on years of my own experience and that of every other circuit designer I know is because Weird Things happen if you don't bypass the power rails. The longer answer is that switching noise from the 555 could be getting onto the power rails, and into the flip flop. \$\endgroup\$ – TimWescott Oct 28 '18 at 5:53
  • \$\begingroup\$ Another possibility to consider, since you have at least some local bypassing on the board, is that your 555 output is coupling into your flip-flop. Those breadboards are great, but there's a considerable amount of capacitive coupling between the rows, particularly if you have the style (now long obsolete, I think) that sports an aluminum backplane that is electrically floating. \$\endgroup\$ – TimWescott Oct 28 '18 at 5:55
  • \$\begingroup\$ @TimWescott I'm gonna try and add a capacitor in the flip flop rail as well and see what happens. I can't do it today, but as soon as I get it done I'll let you know here. But one thing I've not understood yet is how can my output be coupling into my flip flop? They are not connected, I don't understand it utterly, though it seems to make some sense in my mind. \$\endgroup\$ – Iron Maiden Oct 28 '18 at 6:13

when the NPN is turned on the PNP turns on when the PNP turns on it feeds 1.5mA current to the NPN base.

To turn the NPN off you'll need to remove enough of that 1.5mA to get the base voltage below 0.6v

With that 11K there you're going to need -15V or more to manage that.

  • \$\begingroup\$ Thank you for the answer. Actually I apply 0 volts straight to the npn base \$\endgroup\$ – Iron Maiden Oct 28 '18 at 4:32
  • \$\begingroup\$ then it must a bad connection on that breadboard - a wire that plugged in but not making contact. \$\endgroup\$ – Jasen Oct 28 '18 at 4:34
  • \$\begingroup\$ I looked all over the circuit searching for a bad connection and found nothing. As I plug the wire to power up the 555, this problem arises, as I plug it off, it's gone. \$\endgroup\$ – Iron Maiden Oct 28 '18 at 4:40
  • \$\begingroup\$ deleted by analogsystemsrf \$\endgroup\$ – analogsystemsrf Oct 28 '18 at 11:39

Charge transfer thru the collector-base capacitances may be adequate to trigger your positive-feedback "Latch". Power your Latch thru RC lowpass: 100 ohms and 0.1uF (10uS time constant).

In other words, provide a local battery for the Latch, with the 100 ohms ensuring some substantial isolation between the 555 VDD and your Latch's VDD.


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