I'm looking at the datasheet / Application Notes for the 558 Quad Timer.
Specifically, Figure 5.a Monostable Operation (One Shot)

I want to drive a LED with the output. I see from the notes on data sheet that the out pin is, "Note: 3 The NE558 output structure is open-collector, which requires a pull up resistor to Vcc to sink current. The output is normally LOW sinking current."

I have connected the -ve side of the LED to OUTPUT A, pin 1 on the 558, the LED +tive side connects to Vcc via a 220ohm resistor. Is my output correctly configured?

From TIMING A pin 2, I have a 10M ohm resistor connected to Vcc and a 0.01uF capacitor between pin 2 and gnd. I'm looking for an on time of 0.1sec for the LED.
Is my timing correctly configured?

Lastly the TRIGGER A, pin 3. I'm using a SPST Normally Open push button wires as follows. One side of the push button is connected to gnd. The other is connected to pin 3. Pin 3 is also connected to Vcc via a 10k ohm resistor (Pull Up Resistor). Is my trigger correctly configured?

Pin 5 is connected to Vcc and pin 12 to gnd.
When powered up the LED remained off regardless of the state of the button. As well the chip quickly became very hot. Fortunately, i have two spare 558 chips. Any assistance would be greatly appreciated.

  • 1
    \$\begingroup\$ Welcome to EE.SE. A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$ – Transistor Feb 16 at 8:51
  • \$\begingroup\$ Is Vcc=5V? /........... \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 at 8:53
  • \$\begingroup\$ Vcc is 5volts The power supply can provide 2.5amps. \$\endgroup\$ – Mark Hache Feb 16 at 8:54
  • \$\begingroup\$ This is a great piece of ic. Why is it obsolete??? Where do buy them? \$\endgroup\$ – Fredled Feb 16 at 21:48
  • \$\begingroup\$ @Fredled [link] (secure.sayal.com/zinc/zinc_SEARCH.asp?txtSEARCH=147964) Sayal is an electronics part shop in the Toronto area. \$\endgroup\$ – Mark Hache Feb 16 at 22:27

Reset pin, pin 13 must be tied high to enable trigger input.

Output pin is normally driven low and is pulled high when timer is triggered. This means that your LED should be normally on and blink off when the timer is triggered.

10M is a very high resistance value. I'd try lowering to 100K and use 1uF cap.

Can't see why it is getting hot. Carefully check your connections.

  • \$\begingroup\$ Ultimately I'm trying to create a debounced switch. I want the LED to be normally off and to lit when the push button is pressed. As suggested, I've tied the Reset to Vcc. As well, I've reconfigured the output. Pin 1 is now connected to Vcc via a 10K resistor. The LED +ve lead is connected between Pin 1 and the 10K resistor. The LED -ve lead is connected to gnd via a 150 ohm resistor. The push button now lights the LED for 0.1 sec. However, bouncing is occurring occasionally both when pressed and released, and when pressed and held beyond 0.1 sec. \$\endgroup\$ – Mark Hache Feb 19 at 7:18

The 10K resistor in series with the LED is too high a value, it only gives 0.3mA of LED current which is why I'd expect the LED to be very dim. Try 300R which will give 10mA LED current if you replace the 150R resistor with a short. You don't need the 150R resistor because it is effectively in series with the other LED resistor when the LED is lit.

The timer's output pulse is 100ms. Any classical switch bounce will be long gone by the time the output pulse ends, bouncing lasting only say 20ms or so. Also if the 558 timer is like the classic 555 then it is non retrigerable which means that the output pulse will not be extended by any transitions of the trigger input during the output pulse interval.

I think what you are seeing is the switch rocking and making and breaking contact whilst you have your finger on it after the 100ms output pulse has ended. Break and make again can also happen when a push-to-make switch is released.

What I would suggest is buying a better quality 'click action' push-to-make switch which should get around the problem. I have used this solution in the past.

Just for completeness I'll just add that if it had been necessary to debounce your existing ptm switch you could do it with a series capacitor-resistor combination added across the switch.

C = 2.2uF or 4.7uF. R = 150R

The series resistor limits the switch current (capacitor discharge current) when the switch closes therefore preventing burning of the switch contacts.


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