555 timer delay before on strange behaviour

Thread Starter

Sergio34

Joined Nov 10, 2018
42
Hello, I have made this delay before on circuit in the past with the 7555 timer (the one that can work from 2 volts) and worked perfectly the first time. Now I made exactly the same thing with regular 555 timer and the led does not turn on unless I touch the output pin 3 with something metal (like multimeter probe) it turns on. Obviously I triple checked the connections on the breadboard like crazy, nothing is loose. I tried a different chip, various values capacitor and resistor combinations. All the same, after the delay led turns very faintly. I use 12V and 820Ω led resistor. Output pin does not go high, only after measuring with the multimeter a few times since I essentially I touch the pin with the metal probe and this somehow triggers it.
 

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Zeeus

Joined Apr 17, 2019
508
Hello, I have made this delay before on circuit in the past with the 7555 timer (the one that can work from 2 volts) and worked perfectly the first time. Now I made exactly the same thing with regular 555 timer and the led does not turn on unless I touch the output pin 3 with something metal (like multimeter probe) it turns on. Obviously I triple checked the connections on the breadboard like crazy, nothing is loose. I tried a different chip, various values capacitor and resistor combinations. All the same, after the delay led turns very faintly. I use 12V and 820Ω led resistor. Output pin does not go high, only after measuring with the multimeter a few times since I essentially I touch the pin with the metal probe and this somehow triggers it.
The circuit in the image looks strange : what do you want the circuit to do? : have the led blink?
Also, what did you do to pin 7?
 

Thread Starter

Sergio34

Joined Nov 10, 2018
42
The circuit in the image looks strange : what do you want the circuit to do? : have the led blink?
Also, what did you do to pin 7?
Delay before on, depending the capacitor and resistor values the led should turn on after a few seconds from when I apply power to it and stay on until power is disconnected. Pin 7 is floating. As I mentioned it worked with the icm7555 timer but not with the regular 555 timer. Should they perform the same?
 

Thread Starter

Sergio34

Joined Nov 10, 2018
42
Hello, I have made this delay before on circuit in the past with the 7555 timer (the one that can work from 2 volts) and worked perfectly the first time. Now I made exactly the same thing with regular 555 timer and the led does not turn on unless I touch the output pin 3 with something metal (like multimeter probe) it turns on. Obviously I triple checked the connections on the breadboard like crazy, nothing is loose. I tried a different chip, various values capacitor and resistor combinations. All the same, after the delay led turns very faintly. I use 12V and 820Ω led resistor. Output pin does not go high, only after measuring with the multimeter a few times since I essentially I touch the pin with the metal probe and this somehow triggers it.
Actually I found the solution from someone who had similar problem in the website I saw the schematic. I am writing it here in case someone has similar problem. This delay before on will work with operating voltages up to 5V , for higher voltages a resistor should be added at pin 2 in series with everything that connects to it. Currently this resistor is 47K but maybe needs experimenting with other values.
 

AlbertHall

Joined Jun 4, 2014
9,310
Actually I found the solution from someone who had similar problem in the website I saw the schematic. I am writing it here in case someone has similar problem. This delay before on will work with operating voltages up to 5V , for higher voltages a resistor should be added at pin 2 in series with everything that connects to it. Currently this resistor is 47K but maybe needs experimenting with other values.
Curious: does anyone know why this should be?
 

Zeeus

Joined Apr 17, 2019
508
Actually I found the solution from someone who had similar problem in the website I saw the schematic. I am writing it here in case someone has similar problem. This delay before on will work with operating voltages up to 5V , for higher voltages a resistor should be added at pin 2 in series with everything that connects to it. Currently this resistor is 47K but maybe needs experimenting with other values.
Nice...I tried it mins ago with 6v and it did not work and what website is this?
 

Dodgydave

Joined Jun 22, 2012
8,829
Hello, I have made this delay before on circuit in the past with the 7555 timer (the one that can work from 2 volts) and worked perfectly the first time. Now I made exactly the same thing with regular 555 timer and the led does not turn on unless I touch the output pin 3 with something metal (like multimeter probe) it turns on. Obviously I triple checked the connections on the breadboard like crazy, nothing is loose. I tried a different chip, various values capacitor and resistor combinations. All the same, after the delay led turns very faintly. I use 12V and 820Ω led resistor. Output pin does not go high, only after measuring with the multimeter a few times since I essentially I touch the pin with the metal probe and this somehow triggers it.
Swap the resistor and capacitor on pins 2,6.
 

crutschow

Joined Mar 14, 2008
24,354
Currently this resistor is 47K but maybe needs experimenting with other values.
That's likely to prevent some parasitic current at the inputs when the full power supply voltage is initially applied to the TRIG and THRS inputs (pins 2 and 6) through the capacitor, which apparently disrupts the circuit.
(My LTspice simulation shows a peak of about 0.9mA into both inputs @ 12V, below.)

I would think that a resistor value of 10kΩ or higher in series with pins 2 and 6 (R1) should be sufficient for that task.

1574794282117.png
 

Thread Starter

Sergio34

Joined Nov 10, 2018
42

Thread Starter

Sergio34

Joined Nov 10, 2018
42
That's likely to prevent some parasitic current at the inputs when the full power supply voltage is initially applied to the TRIG and THRS inputs (pins 2 and 6) through the capacitor, which apparently disrupts the circuit.
(My LTspice simulation shows a peak of about 0.9mA into both inputs @ 12V, below.)

I would think that a resistor value of 10kΩ or higher in series with pins 2 and 6 (R1) should be sufficient for that task.

View attachment 193163
Thanks a lot crutschow, it seems you know very well what are you doing!
 
So this configuration is using the 555 in Schmitt trigger mode with an RC delay?

The hysteresis can be changed using feedback from the output to the CV.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
42
I would like to point out that crutschow's solution is much better if not the best as with the previous one (47K resistor at pin 2) I could not use high value resistors with smaller capacitors, delay was not happening, it worked only with smaller resistor and high capacitance. Now I managed to achieve the desired delay which in my case is a little more than half a minute with 47μF capacitor and 680K resistor.
 

TeeKay6

Joined Apr 20, 2019
572
I would like to point out that crutschow's solution is much better if not the best as with the previous one (47K resistor at pin 2) I could not use high value resistors with smaller capacitors, delay was not happening, it worked only with smaller resistor and high capacitance. Now I managed to achieve the desired delay which in my case is a little more than half a minute with 47μF capacitor and 680K resistor.
@Sergio34
Have you confirmed that the cap (e.g. 47uF) is actually fully discharged to 0V before you re-apply power?
 

Bernard

Joined Aug 7, 2008
5,396
I thought I'd take a look & for the life of it will not misbehave. C from 1uF to 1000 uF & R 10k to 1M, 6V & 12V. IC KA555.
 

TeeKay6

Joined Apr 20, 2019
572
I thought I'd take a look & for the life of it will not misbehave. C from 1uF to 1000 uF & R 10k to 1M, 6V & 12V. IC KA555.
@Bernard
I assume that "take a look" means doing a simulation? Or did you build an actual circuit? I performed many simulations and found no problem using the NE555 model with LTspice.
 
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