In essense, whatI have are four 555s (2x556) One is a 0.5 s monostable,the other a 147 Hz tone generator. The other is a (30 s) monostable, and a 0.3 s astable.

If I run the 0.3 s astable on its own, (disconnect diodes) it runs fine. I have the reset tied to ground via a relay coil which disables the astable. Place VCC on the high side of relay, and it is enabled as planned.

The other part of the circuit was originally a 30 s timer, but it would cut off after about 1.5 s despite a 150 kΩ and 470 uF cap.

So I changed the circuit a little so the threshold was grounded via 100 kΩ. Now when triggered, it should stay on indefinitely.

However, the problem arise trying to connect the two parts.

Again, my theory says that the high output of the monostable should power the relay and present a high to the reset pin.
That's when it chops off at 1.5 s. (Why it is 1.5 s when there are no timing components with that RC period is also a mystery; I have tried 4700 uF as a supply decouple, but it makes no difference.)


simulate this circuit – Schematic created using CircuitLab

The GND to relay top is a protection diode, and the other was suggested to stop the circuit "locking up".

The relay is a FRS-1 with a coil value of 320 Ω.

(Maybe the reset is being pulled low? But I thought the top of the coil should be at the voltage of the 556 output)

** I have shown two 555's in diagram as there is no 556 on Circuit Lab. But the 556 IS two 555 chips anyway !

  • \$\begingroup\$ I'm not sure if the 555's output is strong enough to drive the relay coil... \$\endgroup\$ – aschipfl Jun 6 '19 at 18:09
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    \$\begingroup\$ @aschipfl I'm certain that even if it happens to be, it's not something you should do. Also, having four 555s would under normal circumstances be a clear indication you should have zero 555s and one cheap microcontroller. \$\endgroup\$ – Marcus Müller Jun 6 '19 at 18:18
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    \$\begingroup\$ What is the supply voltage, and how are you providing it? Battery? \$\endgroup\$ – Elliot Alderson Jun 6 '19 at 18:22
  • \$\begingroup\$ also, your modifications (the 100 kΩ resistors) don't appear in your circuit? It'd be much less ambiguous if they did! \$\endgroup\$ – Marcus Müller Jun 6 '19 at 18:25
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    \$\begingroup\$ Your diagram shows a normally-closed pushbutton, so the output of U2 will be high from the get-go. If that's yet another error/omission in your schematic, NOW is the time to go in and edit it so that it reflects reality. There is no excuse for leaving errors in a diagram. \$\endgroup\$ – Dave Tweed Jun 6 '19 at 20:47

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