I am currently toying around with electronics and read Make:Electronics which is a very fun book.

In one experiment (19) I came to a deviation in behaviour of my circuit compared to the expected behaviour described in the book.

The following image contains two different 555 IC configurations (sorry for the schematic, I am a beginner)


Here is the schematic schema

The left one is a bistable configuration (with the omitted reset button) that is responsible to start the left 555 IC and therefore trigger the red LED.

The right one is in an astable configuration that, when powered, causes the green LED to periodically blink. The period is very long and implemented with a 470k resistor combined with a 3.3microfarad capacitor.

Both parts are basically copied from the book and behave predictably when executed independent of each other (by removing the one and then the other IC).

The above configuration however has some oddity for me, which I do not understand.

When powered, the green LED begins their cycle but when the green LED is off and I start the bistable 555 IC, the red LED is turned on. And this is the part I do not get. I would expect that the red LED stays on indefinitely but the next time the astable IC turns the green LED back on, the red one is turned off. And even vice versa, when the green LED is on, pressing the push button on the bistable one, turns the green LED off and the red on.

My assumption is some voltage drop but I do not see what could cause that. I suspected the discharge pin 7 but since there is a 10k resistor between VCC and pin 7 I am not sure what actually happens.

Any hint would be wonderful.

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    \$\begingroup\$ Technically it is not a "schematic" at all. It is a (rather less useful) fritz diagram. And it seems you have left some inputs floating (not connected to anything) on the 555. \$\endgroup\$ – Edgar Brown Nov 28 '18 at 18:50
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    \$\begingroup\$ @Samuel, those Fritzing drawings are only wiring diagrams and are often referred to here as "cartoons". They are of little use to engineers as they don't show the schema. Fritzing should allow you to generate a schematic from that but you'll need to tidy the schematic up. Put the positive rail at the top, use ground/GND symbols for anything connected to the supply common and try and make the logic flow from left to right. \$\endgroup\$ – Transistor Nov 28 '18 at 18:57
  • \$\begingroup\$ Fritzing diagrams are good for showing that bypass caps are in the wrong place, though, which matters a lot on those cheezy protoboards. \$\endgroup\$ – TimWescott Nov 28 '18 at 19:18
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    \$\begingroup\$ You have no bypass caps, which means that signal is getting onto the power rails and into the other chips (555 timer chips are particularly sensitive to glitches on the power rails). Put 100nF caps right at the chips; I usually put it right next to pins 1 & 8 or pins 4&5. Run short wires to the VCC and GND pins of the chips. Then put a 10uF or so cap across the power rails for the whole board. Report back on how things work. \$\endgroup\$ – TimWescott Nov 28 '18 at 19:21
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    \$\begingroup\$ You should always bypass every IC in a circuit. For large ICs with multiple power pins, you should bypass every set of power pins -- such ICs usually have the power pins in pairs. If you're doing something analog, you should bypass each stage. When I was a TA for circuits labs, I would refuse to answer questions about any circuit that did not have bypass caps -- and putting in bypass caps fixed the obvious problems about 60% of the time. It's just something you always do, like putting your pants on before you walk out the door. \$\endgroup\$ – TimWescott Nov 29 '18 at 21:44

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