1
\$\begingroup\$

I want to simulate a push button with a little circuit. I'm using an IC 555 as a timer. The button is in another circuit (different device), pin A has 2.4 V and pin B is GND, it does activate if I manually short them for like 2 seconds. Since pin A always has 2.4 V, when the internal IC of the other device detects it goes for a small period of time to 0 V (shorted), it does activate.

I'm trying to turn the timer (555) on as soon as it powers ON (using R1 and C1). I also added a resistor to discharge the capacitor C1. I played with different values to force pin 3 (555 output) to be as close to "0", but sometimes it turns on, then turns off.

The button must be 0 V to activate, cannot be stuck at 0 V because it will turn ON then OFF again in a cycle. If the capacitor very slowly charges from 0 V to say 5 V it does work, but since it must be between 1.5 s and 2.5 s, it will turns off, because after about 2 seconds the capacitor is being charge, at about 0.8 V (so it sees it like a small activation).

In summary, all I need is from pin 3 in the "555" be 0 V for about 2 seconds and then V: VCC i.e. 5 V, but suddenly not gradually.

Is it really possible or do I need many more components?

enter image description here

\$\endgroup\$
5
  • 1
    \$\begingroup\$ It is possible as the 555 has been used as a power-on reset circuit for 4 or 5 decades. But it will likely fry your mystery circuit by driving 5V to the 2.4V button sensing circuit, whose operation is a mystery. If it is a power bank, it may even multiplex the button with LEDs. You likely need to know if that is OK or if it fries the circuit, in order to know if it needs more components or not to be safe. \$\endgroup\$ Commented Oct 29 at 7:12
  • 2
    \$\begingroup\$ Why do you short the output with a switch ? \$\endgroup\$ Commented Oct 29 at 7:55
  • 1
    \$\begingroup\$ @Antonio, this appears to be a follow on to electronics.stackexchange.com/questions/757122/…. The switch is a microphone power-on/off button, but you are correct that if the button itself is pressed then the 555 output is short-circuited. \$\endgroup\$ Commented Oct 29 at 9:06
  • \$\begingroup\$ Why do I short the output with a switch? the switch must be 0V for 2 seconds aprox., With a right combination of resistor and capacitor I can achieve about that time, but I need pin 3 (555) to be 0V then voltage VCC. At last I will or might be using a relay normally open. So before 2 seconds, it's open = 0V then 5V. But I guess it could be overkill for something "basic" like that. \$\endgroup\$ Commented Oct 29 at 9:17
  • \$\begingroup\$ We don't know that the microphone can tolerate +5 V from the 555 when its output is high. Your other question has a comment from you stating that the open circuit voltage across the switch is 2.8 V. The +5 V high signal from the 555 will almost certainly cause a problem and possibly destroy the microphone. (All of that information should have been added to the question, not hidden in the comments.) \$\endgroup\$ Commented Oct 29 at 9:48

2 Answers 2

Reset to default
1
\$\begingroup\$

If you need pin A to be pulled to 0 V for about 2 seconds at power-up and then return abruptly to its normal ~2.4 V, don’t try to use a slowly charging capacitor as the “button simulator” alone: that produces a slow voltage ramp and causes the other device to misinterpret intermediate voltages. The cleanest, simplest approach is to have the 555 produce a single rectangular pulse (monostable) at power-on, and use that pulse to drive a switch (transistor or MOSFET) that pulls pin A to ground while the pulse is present.

555 Monostable

Reference: Electronics Tutorials - 555 Timer

\$\endgroup\$
2
  • 1
    \$\begingroup\$ It makes sense. I'll give it a try and post results, thank you. \$\endgroup\$ Commented Oct 29 at 9:58
  • 1
    \$\begingroup\$ It did indeed worked. Using an S8050 to pin A. Thanks again Alinik and the rest of you guys! \$\endgroup\$ Commented Oct 30 at 5:50
1
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.